EEVblog #222 - Lab Power Supply Design - Part 2

now let's take a look at the LT 30 $80 sheet because we might find some practical considerations in here in fact I guarantee we'll find some practical considerations in here that we have to take care of in if we want to actually build this thing as opposed to just doing a conceptual type our top level design schematic that we've been doing up until now and I really like this device because it does just exactly what we want now let's have a look at some of the specs our output current up to 1.1 am so I'm going to use the to-220 package of this that's available in many different packages RDF nad pack and a SOT 2 T 3 version which is really nice or an M soft package but we use the to-220 to really need um 1% our initial accuracy on the set current that's not bad at all and what are the neat things about this is that a single resistor programs the output voltage although we're not going to use that today but we might have to take aspects of that into account as you'll see but it's really neat as like the unlike the lm317 we're going to do use two resistors to actually set the output to voltage this one only needs one I'm going to Y low output voltage noise is only 40 microvolts RMS through in that bandwidth and it's got its support some input voltages up to 36 volts terrific we're not going to go that high today but hey if you're into high voltage power supplies it can do it and the dropout voltage and that will be at full load is only 350 millivolts but we can check the curves on that one as you have to listen 1 millivolt load regulation line regulations also ah now one thing minimum load current there it is not point 5 milliamps we have to worry about that and it's stable with 2.2 mic ceramic output capacitors that's what we need to make this thing stable then is guaranteed over any load you like terrific and of course it's got fallback current limiting and over temperature so it's pretty bulletproof just like the lm317 and there's the typical application which was just spent a long time talking about you've got series pass transistor the error F connected straight through but instead of having an internal voltage reference like the lm317 it's got an internal constant current generator which is actually 10 R 10 micro amps in it it tells you that over here here's the distribution graph anyone noticed you know we've been talking about bell curves recently well there it is again and that is 10 micro amps constant current down through there now although it doesn't tell you this in the datasheet ah basically it implies that because this is only 10 micro amps this constant current generator it's pretty done wimpy right you can force a voltage into the set pin just like you can on the lm317 and it's now it's going to be no big deal at all but got to remember there's an extra 10 micro amps which has to flow out through there and we'll see that that matters later and we'll just have a quick browse through the characteristic spectable here and see if there's anything that takes our fancy shall we now set pin current as we're set is 10 microwaves we don't necessarily need to know about the min max values of that because we're driving it we're not using an external external resistance we're just driving that pin hard so really all we care about is that typical figure which we'll use later in a simple calculation now let's take a look at the output offset voltage and what this is is V out minus V set what that means is the actual output voltage the real output voltage minus what you've actually set what your your set on your adjustment Pottle what your software has set in what you're driving into that V set pin in this case now these specs are Freya a control voltage of a villian of one volt with an output current of 1 1 milliamp now it could be your actual value maybe plus minus 2 millivolts look at that so it may not be spot-on - if you put exactly 1 vault on that V set pin if you drive in that vase in 1 volt then it could be 2 millivolts either side of that not a problem in the case of this pass like 2 millivolts that's neither here nor there but if you design imprecision really precision are applications or precision power supplies then this sort of this thing can matter and you got to take it into account now and you notice this dot next to it there's it's actually wider than that over the full temperature range it tells you up here bingo little track the other spec there is only for an ambient temperature of 25 degrees Celsius and remember this is not the ambient temperature it's going to be the junction temperature of the device itself so it's dissipating all that power and your heat sinks gone up to sixty degrees well your your lab might be 25 degrees but your heat sink and your device is at 60 degrees so just beware and you'll notice that there's two different specs here depending on the different packages and the reason that they have these is because the dye use the silicon die is going to be different in these smaller packages as they'll have a larger die in these larger power-type packages and the spec gets worse plus minus 5 millivolts ah man terrible or plus minus 6 millivolts I have a temperature who cares for our case but you know you can't be aware of that sort of stuff in the output offset voltage is fairly critical for is when you parallel devices up and this is how you go in to increase the output current cuz this device is only rated to just over an AB what if you want two amps or three amps or something like that well you can parallel devices up like this and all you've got to do is include a select series ballast resistant like this and you can do these do this to worm any similar type of our voltage regulator as well now there is an alternative device the LT 30 80 - 1 think it's a bit rarer to get but it actually includes a built-in ballast resistor in there and the output offset voltage a small one like the plus/minus 2 millivolts you are see here quoted for this one or at least at nominal at at room temp and if it's tight like + - 2 millivolts it means that your output our ballast resistor only has to be very small in this case they recommend 10 milli ohms output resistance and it's still going to share you know 80 to 90 percent of the current between two devices or even better than that typically and so you don't even have to actually buy a resistor for that it's good enough to actually use a PCB trace for that if they actually tell you are - they actually recommend that a 10 10 mil width or 1/10 hour with trace on a PCB 20,000 trace typical 1 ounce or two ounce copper 1 ounces your normal weight copper or if you're designing heavy duty power supplies you might have ordered a 2 ounce a copper PCB but you can get your ballast resistors you don't actually have to buy one there's no filling with extra bill of materials item and cost you just include it with a PCB trace brilliant and as long as that value is high enough then you can share the current between the two devices adequate adequately without one device heating up much more than the other device but you can't make it too high because then you get a voltage drop and in this case let's say you've got a two air output and our two ballast resistors are ten millions well it's a total of five milliamps because they're actually in parallel - essentially what they are so it's five millions ballast resistance or output resistance there at 2 amps is going to give us a 10 milli volt drop on the output and that's not too bad at 1 volt that's only 1% so the neat thing about parallel devices like this is that you can actually leave are one of the footprints unpopulated or multiple footprints on your board if you're designing a power supply like this you want to save a bit us to begin with who you're designing a kit or something save a bit of cost you only have one device or if you want two or three or four or more you can actually parallel them up and you can solder in the individual devices as you need them load regulation is going to be excellent from one milliamp up it's fully specified from one milliamp up to one no problems line regulation up to 25 volts input not a problem at one it's specified at 1million load there don't need to worry about that minimum load current now here we go minimum load current very very important we need to take that maximum figure there 500 microamps or half a million as our minimum load current if we don't do that it doesn't tell you what's going to happen there's couple of notes here which we'll read but doesn't tell you just assume that's not going to be stable or it's going to have a larger dropout voltage or sorry it's not going to allow you to go down to a as lower voltage as it could or whatever there's a whole bunch of different reasons not to if you don't meet that a whole bunch of bad things can happen and ruin your day so we have to somehow get a minimum load current over our minimum load current of half a million over our entire voltage range and it's only specified at a VN range of 10 volts if you go higher it needs one milliamp minimum and there it is note three minimum load current yada-yada quiescent current it's a minimum load current required to maintain regulation if you don't mean it ain't gonna regulate and that defeats the whole purpose of a power supply now the dropout voltage of this part is interesting because it specifies it in two different ways there's V control pin dropout voltage and V in dropout folding so if you look back to the circuit here it's got V in and V control normally you tie these twos together and we will in our application here today but if you want a really low dropout voltage of this part like a low input voltage and a minimum input dropout voltage between the input and the output but you happen to have in your circuit somewhere a higher control voltage then you can take it to that and get a lower dropout voltage from in to out by Ty envy control up to a higher voltage up there like that but we're going to time together and also not all parts have the extra V control pin some of the summer some of the packages actually will tie those two pins internally so you don't have it actually have it available so we need to take the worst-case version of that because we're tying them together which will be there it is 1.2 volts or at full that's at 100 milliamps or at full current which you have to take an account it could be as bad as 1.6 so our input voltage has to be at least 1 point 6 volts above our output voltage and that's this voltage here not over here so if our output voltages are 5 volts here we need to have at least 6 point 6 volts or 1 point 6 volts higher here and if we are using a 1 ohm resistor and we're drawing and air you're going to get an extra volt and that so it needs 7 point 6 volts here minimum for a 5 volt output and the current limit here our maximum output voltage a typical one point 4 amps but you might well you might be able to push it that far and you probably can but when you're designing and you want to set your maximum figure the lowest one here is what you're going to use as opposed to say the minimum low current you use the highest value in this case you want to be conservative and use the lowest so depending on the parameter you either have to choose the maximum value or the minimum value will choose one point one X that's what our circuit will be capable of now as far as the output noise goes most linear regulators are pretty darn good and this one's no exception 40 micro volts RMS are for the error amplifier noise now if we have a look at the error amplifier here all of the noise assuming that the input voltages are perfect and there's nothing no noise coming in there then all of the noise is going to be generated by the internal current source and the error amplifier so that's basically all the noise internally is going to be that 40 micro volts RMS so even if we feed force in an absolutely perfect voltage onto here with no noise at all we're still going to get 40 microvolts or there abouts our worst case rms output noise but as you can see the set because this is a direct feedback loop and whatever voltage you put on here comes out here any noise that you put on this set pin is going to come out here as well within limits of bandwidth and all sorts of other things like that so really are the noise limit will depend on this now if you're driving your circuit with a pot like this and you've got this fed to a you know a voltage reference or something like that say a 2.5 volt voltage reference er really quiet low noise voltage reference then your noise has got anyone driving that pin directly then the noise is going to be pretty good but if you're doing a PWM signal and you're feeding that through your RC filter like that and then you're driving that with a buffer obviously and you're driving that into the set pin like that then your noise then any noise that you haven't filled it out here any noise on there is going to make it through to here and it's going to make it through to output so filtering if you're using microcontroller control still great of your pulse width modulation modulated signal is important but you can really up these values you can you know up them as high as you want to really you know absolutely slaughter any noise and just kill it dead it's not that hard just you need to choose high values and if you want to care about the ripple rejection then you have to figure out what 75 DB is for your various input ripple which has specified at half a volt peak-to-peak if you're pairing this thing from a a transformer and a bridge rectifier and a and a filter cap you're gonna get 100 you know this is a full wave one because it's double sixty Hertz so it's a hundred and twenty Hertz ripple there it's specified you know that's pretty good and you can calculate that if you're are you in um a you know a noisy input and you've got ripple but if you using say a battery input or something like that then you don't have to worry about that at all but if you actually want some real figures you can plug those in for 75 Rd be there at the nominal were half volt peak-to-peak and that's going to give you an output noise of less than 100 micro volts so 100 micro volts doesn't sound like much and it's not it's you know it's down in the noise although once again if you've got a really low noise very high spec power supply system then it could matter but just for a lad power supply like us more than good enough order magnitude good enough but that's only 120 Hertz that's for ripple AC mains ripple if you're powering this thing from an AC mains input what if you got a switching frequency of 10 kilohertz or 1 megahertz well where would you get that from well if you're powering this regulator if this regulator has been at the LT 30 eighties being powered from a DC to DC converter well that's going to have a switching frequency and that's going to have output noise and a very high efficiency our switching regulator might be up near a megahertz or something like that in that case look at the look at the rejection there at one megahertz is down to only 20 DB that's a huge drop from 75 DB at 120 Hertz and if we calculate with 20 DB is just like 75 DB there is you know the formula you send it before DB equals 20 log in this case it's going to be V out or our noise or ripple output voltage over our V in in this case which is that given as R naught point 5 volts and if you calculate that and if you change the formula around because DB is already known and you work out what your output ripple output noise is going to be then it's going to be one-tenth of your input noise so if your input is not point 5 volts you're going to get 50 millivolts output noise so it's not down in the micro volts region anymore it's in the tens of millions and that can ruin your day if you're designing precision apps and you've got and you haven't added we filtered your input noise like that now if you were actually using this in its traditional configuration with the external resistor and you were aligned upon the pin set current at 10 micrograms you'll see that that's only nominal at 25 degrees C when you actually go up or even down in temperature like this it does vary a bit you know if you go right up to you a hundred degrees and go up an extra 15 or 1500 volts woohoo but that could be significant and remember when you look at these colors don't fall into the trap of thinking that your product is only operating at ambient temperature at 25 degrees it's not this is the junction temperature of the actual device itself the dye temperature and because this is a power supply that's dissipating power that Junction temperature could easily get up to a hundred degrees depending on how you do your thermal design so if you're designing really precision power supplies you need to take that sort of thing or any power system that dissipates power you were to take these thermal graphs into consideration and the offset voltage once again doesn't really matter for our application because it this is versus load current so the output offset voltage in millivolts is actually going to work drop based on the output load current so the apple load currents one up here at ambient temperature or C it tells you here TG Junction the temperature of the junction not just the ambient temperature because ambient temperature makes no difference to it at all all it cares about is temperature anyway you're almost going to be R naught point 5 millivolts offset there and if your junctions up 125.75 millivolts are offset and if you're designing precision applications that could matter take it into consideration now looking at the minimum load current which is quite important as we said because we have to take this into consideration then ah a input to output differential of 1.5 volts then you know here your minimum locality needs to be point three milliamps n but when it dark Rises your input to output differential rises to a much voltage then um your minimum low current needs to be higher so you might put in half a million that'd be that's what it tells you in the top-level specs but you might say design it for a milliamp just to be on the safe side if you didn't care about wasting that extra half a million now this load transient response here we do want to consider this because this tells us our typical performance when you change your load like this here's the output load current in hundreds of millions so we're doing a 200 milli amp jump in the load current it goes from 50 milliamps up to 250 milliamps and then back down and you can see what the output voltage how it deviates because these regulators aren't perfect okay they have a transient response when your output current suddenly changes and this is the transient response you get if you've only got 2.2 microfarad ceramic art cap you can expect it to change by 50 millivolts output you can expect it to our droop down like that and then then recover like that and if you use a 10 micro farad ceramic you can see that it takes a bit longer to recover and the a drip isn't a that well in this case the rise isn't up quite as much so there you go but that's at a nominal 1.5 volts output baduk the greater the your outputs are the greater the output capacitance on your regulator you better your load transient response can become but just be careful you don't want to put a massive amount of capacitance on the output of a constant current power supply like this because that capacitor can store a lot of energy you know if you put in a I'm going to put in a big 2200 microfarad capacitor that'll really you know give it lots of transient performance well there's a downside to doing that and that's when this thing switches into constant current mode then it can't react because it's still got all this energy in the cap that can get dumped high current into your load because it's not current regulating so you want to keep in a lab power supply like this with a constant current um circuit like this you want to keep the out output capacitance as low as possible just to ensure stability in your regulator although I guess you could say on ok you might determine transient response is a more important thing than switching over into constant current but generally I cannot want to keep that thing the output capacitance low so we're probably going to want to just you know the same value or are twice the recommended value just to be on the safe side that it recommends for stability of this particular regulator and just for a bit of completeness there you've also got line transient response generally not important in a power supply design like this because our input basically this means up line transient line means your input voltage coming into your voltage regulator as opposed to mode transient response which is changing our current on your output so input line transient here the the power supply that's powering your power supply is generally going to be pretty stable and it's not going to change by this sort of our current like this this sort of voltage this is in this case it's got a 3 volt step in your input voltage to your voltage regulator you can see that the output gives a droop like that of 25 millivolts so there you go not important but just thought I'd mention it another thing to consider is the turn-on response of the regulator and that's what happens to the output voltage here when your input voltage ramps up you don't want to overshoot by a massive amount because they can damage you circuitry if it's already hooked up to the power supply so this one looks pretty good it looks pretty well-behaved and will actually when we build this thing up will actually check that now here's an interesting graph you don't normally see these this is a bit rare it's the residual output voltage with less than minimum load so it's basically telling you implying how this device is going to perform if you don't meet that minimum load current requirement and you know that one of the big banner specs one of the big highlight better specs of this arm of this voltage regulator is how low the output which can go it can go all the way down to zero that's what a claim is but only with a minimum amount of output current and that's what it's basically are saying and this is your test resistance here so as your test resistance gets lower and the output current increases then let's say you've got a test resistance up here of 2 K okay let's say 5 volts there we go 5 volts at 2 K and if you've got a 2 K output resistor with an input voltage of 5 volts here when it's trying to set you see the set pin is that grounded here so it's trying to set zero volt output but you don't get 0 volts you get there you go not point 5 5 volts or something like that terrible so you're really saying that big banner spec that it shows on the front page here you know it claims ah output adjustable down to zero volts fantastic yeah here's the devil in the detail it only goes down to zero volts if you've got a zero ohm output resistor effectively so if you've got a 1 K output resistor not on there you're only going to be able to go down to about you know not point two five volts or something like that and herein lies the trap how do we get a minimum output load current here on an adjustable power supply because we just still shot we could just stick a resistor here on our circuit down to ground make that 1k no worries we're easily going to meet a minimum load requirement let's say it's 1 volt it's even at 1 volt we're going to get 1 milliamp that easily meets our minimum load requirement 1/2 a million and we can probably go down to naught point 5 volts output we're going to get naught point 5 milliamps great ok that sounds like a good solution but what if our output voltage is 10 volts well we've got 10 milliamps and we're effectively pissing away 10 milliamps there just on that output resistor and uh-huh here's the thing to consider if you're trying to get mili-amp accuracy over here with your with your constant current setting let's say you want to adjust it in 1 milliamp steps you know your input voltage down here 0 to 1 volt you want to adjust it with your micro in 1 milliamp steps well jeez you've got 10 milliamps 10 volts in fact you've got an output current which flow which is a jug which are changes based on the output voltage so you'll microcontroller over here that's driving all this thing will have to be smart and know well it knows what the output voltage is because it's setting it through here so it will have to know that loads 1k and then take that into account and then compensate by driving instead of driving let's say you wanted to you know adjust this to 5 milli amps and that'll be 5 milli volts well if you're drawing an extra 10 milliamps out of here you've got to actually make because you want 5 milliamps only 5 milliamps max to go into your load so you know that you're going to add say at 10 volts output you're going to have 10 volts coming out of 10 volts on here 10 milliamps going down here you have to actually compensate and set this one to 15 milli volts to know and it gets really what gets quite ugly so if you use a fixed resistor like that right that's not a very elegant solution I don't necessarily like it on a variable power supply like this and by the way another little art trick if you're robbed if you really care about how much current this whole thing jaws you might want to add an LED in there like that and that could be your power LED and so you get two for the price of one so instead of wasting your leather carriage is lighting up a power LED you might get it from the output and you can use it as an output leg or something like that but then it doesn't work down at low voltages if this thing goes down to you know zero volts or one volt or you know it it's ugly anyway ha we got that I reckon we need to find a better solution than the fixed resistor so what do we use for that output character I can't think of anything better than the classic r LM three three for constant current source it's a single resistor it's available in TOI et packets is not bad it goes down to very low currents up to 10 milliamps maximum and so I reckon we set this sucker for a value of 1 million now the LM 3 3 4 has been around since pretty much the dawn of time it's why those classic devices that is still incredibly useful today it operates from 1 volt after 40 volts has got good counter regulation can programmable current from 1 microgram to 10 milliamps to terminal operation 3% initial accuracy if you actually care about the you know the absolute accuracy we're not that fussy at all it could be 10 percent we couldn't care less and and it's available in them you know a Cheeto 92 package or an SR weight so very very usable cheapest chips I love it one thing we do care about though is is minimum operating voltage of not point of typically no point nine volts um up to one milliamp so we plan to operate it at 1 milliamp or there abouts or half a million maybe so um it's unfortunately it's only going to operate better down to about point nine maybe point eight but hey point eight mil you can get our power supply to operate down to say no point eight or nine point nine volts if that's a lot better than the one point two five volts you might get on an lm317 and then you can go right down to zero if you want but then you're reliant upon the load actually providing the minimum current then you can't use like a high impedance load actually below nor point nine volts or or not point eight faults are there abouts and 0.8 volts is a nice figure naught point nine is nice because um for a power supply to go down to is because the a single cell battery a single cell you know D cell or double a alkaline or something like that I'd be basically pretty much dead from naught point down to not point eight volts and 0.9 volts Oh a power supply they can go down that low is pretty good and it can go lower but depending on the load hey that's good enough for me I like it and it's really easy to use it's only a three pin device and has to have a single set resistor like this voltage in like this I said this will be connected to ground down here and this will be our output voltage and we'll have a single resistor like this at what value does it need to be well you can go through all sorts of formulas and take into account bias currents and stuff like that but yeah we can cheat and duck do the look at this our graph here our set sixty eight ohms and bingo look it settles at 1 billion so it looks like it's 68 ohms will give us 1 million 68 ohms is a nice e 12 resistor value I love it and as you can see it operates down to art no point eight false no problems it drops off a little bit ah it's probably drops oh yeah it probably dyes it half at noir point eight volts that's as probably as low as it's going to go because um we are operating this thing at well sorry we want a minimum load current of half-a-million so really it's going to operate down to you know there it is - milli amp is somewhere in there around about point eight on the graph so if we set it to one hey we're going to be happy you know we could go up to here and then get some extra voltage margin you know if we are set was up there at you know five milliamps we can set it higher like that but and get a bit more margin for our output voltage but I don't think we need to do that and if your output current doesn't happen to fall on one of these characteristic our curves on this graph you know if you were two milliamps or something you will have to use these foils and you'll have to take the bias current ratio into account which changes with your output current and that's the ratio that you can plug into various formulas down here to calculate your resistor value now you remember this 10 micrograms current we've talked about quite a few times from this set pin on this sir LT 30 80 doesn't just magically disappear when you drive that pin it's got to flow out there it's a constant current generator so it's got to flow out of that pin into that resistor and assuming that we're in constant voltage mode all this stuff is vanished it's got a good flow out of here as well and if we got say trying to set one volt on their input here sure we'll get a volt here but then it's going to be there's going to be an offset error there due to that 10 micro amps current it doesn't sound like much but Ohm's law do the math 10 microwaves through 2k there is 20 millivolts once again doesn't sound like much but if you're trying to set 1 volt on the output there that's a 2% error horrible don't want that so what do we do about it now there's two things we can do about it first one the obvious one of course is to lower these values until it gets to a point where you don't care anymore and that's a perfectly reasonable design technique but in this case well we don't want to lower it too far you know we could harvest for use 470 ohms or 220 ohms or something like that but ultimately when you short out this op-amp and high voltages you're gonna have a lot of excess current flowing out of that and are you just wasting pissing that count away so I don't really like that solution at all so the next way to do it is to put it in the feedback loop of this op-amp and compensate for it and the way you do that is to break into here so instead of that output going directly to the buffer there it's a pretty standard technique if you want to increase the output impedance of your buffer or your amplifier or whatever is to simply stick that in the feedback loop there like that bingo you've instantly gotten rid of that resistor and the op-amp compensates for it remember juda op-amp action this voltage here will be equal to this voltage here it'll do whatever it needs on the output to make that voltage the same so you've effectively eliminated the output voltage drop across that resistor but you still have the advantage that it's protecting the output so when you short this you're not going to short directly the output transistors of this op-amp beautiful but that still leaves this resistor appear yeah well we it turns out we can actually extend that you can put all of this inside that feedback loop up here so we can connect that directly to the source pin so effectively ah the voltage once again due to op action that if you set one volt here you're going to get one volt there and one volt on that pin and it's going to compensate for the drop through these resistors so these resistors can now be almost any value you like within certain limits of course ah and it doesn't matter the op amp is going to take care of it for you magic so that's what we're going to do and because you may this voltage er set here is going to come from our reference voltage you may want to use a 2.5 volt reference voltage or something like that to match your op amps of your um microcontroller or something like that so you may not be using like I say a 10 volt reference so you're probably going to need I can't get rid of that there we go probably going to need some gain in there as well no problems at all it works exactly the same not just in the buffer configuration but also in the gain configuration too you put all your stuff you want to get rid of your feedback loop beautiful oh there you have it after all that design effort I'm pretty darn happy with this design I think it's going to be the one I probably build up once again you can add in a second device up here parallel them like that we've mentioned stuff like that to increase your output current and I might do that and another thing you might want to do is replace this crusty differential ampere with a proper instrumentation amplifier like I mentioned before like an ad 620 oh that's pretty expensive and at the low cost ad 63 or one of the high side current monitor chips monitor amplifier chips you can get specifically for high side current sensing like this because it's going to be okay at 1 ohm like that you know just using a fairly jellybean sort of low-end precision op amp that has you know 500 800 micro volts or something like that you can get down to milli and accuracy on this type of thing but if you go down if you drop that if you want to reduce your voltage drop across there and use point one ohms or something like that uh your general purpose op amps even your precision ones are gonna cut the mustard too much and you may as well go to a proper instrumentation app or something like that um now you can do a low side current sensing as well but as I mentioned before you're going to get voltage drop across there if we include the resistor on the ground return path the current shunt resistor on that ground return path from the output here the voltage drop there there are ways around that but odds Dickey I don't like it stick with the high side current monitor and we're sweet so I like that let's build it up said that last time but this time I think I really mean it

Channel: EEVblog

Views: 234,573

Rating: 4.9352379 out of 5

Keywords: power, supply, linear, bench, lab, laboratory, switchmode, lt3080, voltage, constant, current, regulated, design, circuit, lm317, lm334, source, shunt, resistor, capacitor, feedback, opamp, construction, diy, project, inverting, compensation, high, side, low, drop, measurement, open source hardware, open hardware, oshw

Id: 6Otr1I0OR18

Length: 38min 24sec (2304 seconds)

Published: Mon Nov 28 2011