All You Need To KNOW About CROWBAR Circuits To FIX Stuff! How crowbar circuit works tutorial

hi guys welcome to learn electronics repair today I want to talk about a circuit that you may come across sometimes especially in things like industrial electronics repair and if you're not familiar with this circus you may be a little bit confused by it and what it does so the circuit is called the Crowbar circuit and there are two ways in which this is generally implemented so I want to demonstrate with this little circuit board as I've built what the Crowbar circuit does what problem it is there to solve and two ways that they implement this now we can test this let's see if our crowbar actually works okay I will say I've not tried this yet so it could all go horribly wrong Anything could happen in the next stop fire yeah but anyway we have a little circuit board here you can see there's an IC socket here but there's nothing plugged in at the moment so you can ignore all this stuff for now we will use it shortly and we have here a positive Supply coming in to a fuse and a rather badly fitting fuse holder I happen to have a line around we have a voltage regulator and this is a 7805 so this is a five volt positive voltage regulator we have two capacitors we have a resistor and an LED so that's what we basically have and there's also a jumper here so as I said we'll ignore this little bit let's draw this on a piece of paper I'll explain what the circuit should do then we can test it and then we can look at a common problem you might have with this type of circus what this circuit is doing it has a 12 volt positive Supply and this comes via the fuse so the fuse is 500 milliamps or half an amp okay and then from here we go to our voltage regulator so with the 7805 this is terminal one this is in you have two more terminals two and three two goes to ground and this is where I put the jumper okay so we have a jumper here connecting to ground and then pin three is the out full load I'm using a resistor which is 100 ohms and I'm using an LED which is also connecting to ground so that's what we actually have on our circuit the 7805 can take inputs up to about 30 volts 35 volts you can look at the data sheet it's something like that and the output voltage will depend on the type so a 7805 we'll give a fixed 5 volt output we could also have 7808 for an 8 volt 7809 7812-7815 and there's a seven eight one eight as well I think that's all of them so depending on which model you fit you will get an output voltage depending on the last two digits so in this case zero five as long as the input is a few volts more so I've got 12 volts coming in and five going out now on our boards there are two capacitors this is the standard where you use these regulators so we have a capacitor on the input and we have a capacitor on the output in particular if you don't fit this capacity to the regulator can become unstable especially when there's very little difference in the voltage between the in and the out it may not regulate the values of these are not critical at all I've used a 100 microphones here and I've used it 1000 here this is 16 volt I think this is a 25 volt but as I say they're not particularly important to the exact values so as I look at this circuit how it works in our little board that we built here is our circuit board again I will just connect it to my bench power supply and you can see the LED has come on so the circuit is active that's with a few voltages so this is ground this is the input voltage and we have 12.29 so just over 12 which is coming through the fuse and go into pin one on the regulator which is the input okay pin two is zero which is also the tab okay and pin three is the output which is exactly five point zero zero so this regulator is going exactly the correct output voltage everything's working fine but what would happen if the regulator went short well if we had a short circuits in the regulator it would feed all the 12 volts through to here the LED would come on very bright but it would probably survive that we can see it five volt Supply this is drawing 10 milliamps so at 12 volts is going to draw about 20 milliamps 12 24 milliamps which is high for an LED but always not maybe for a protracted period but at least it would survive the other thing that could go wrong with this is what happens if it loses the connection to ground from pin two well we can test up by actually removing the jumper which I fitted so this is what the jumper was for so we've now disconnected the ground Supply LED is on very bright let's have a look again so on the inputs we still have 12 volts and on the output we have 8.5 so we now get you know much higher output voltage than we should have because the ground connection from the regulator Has Come Away now if this regulator was handling a lot of power it's not so it's cold but if it was handling a lot of power This Cloud I think it's one amp continuous 1.5 amp Peak and if it was dropping as this is about seven volts across it you can see it one amp is dissipating seven Watts yeah so this would often be mounted on the heatsink and it may not be mounted on the PCB so you'll have wires going to it and it's possible that you could lose the ground connection to it and get an excess easier output voltage now with this circuit it's not really a problem the fuse didn't blow it just drill with a bit more power but it didn't really cause a problem what you need to consider though is what would happen if we had a circuit here which is voltage sensitive some sort of IC for example that will only roll up to a maximum of five volts well that's what this circuit is so let's draw the circuit I'll explain what it does and then let's try this so we put it back at five volts we'll get us working and then we'll take the jumper off again and let's see what happens okay so this is the other circuit I built this uses a IC called the 74 hc-04 and inside this chip they're at six inverters so an inverter is a logic gate sometimes called a not gate and that is a symbol for it and basically with a not go to an inverter if you have a voltage on the input 5 volts you'll have zero on the output if you have zero on the inputs you have five on the output okay so whatever voltage is on the input is inverted but we're using three of them in series like so okay and this is pin one two three four five six okay I'll have a chip a bit of a dodgy six there you go it looks more like a six now okay and I've wired some resistors so from pin one we have a 10K resistor from the Junctions of two and three I have a 1K resistor yeah and from the Junctions of four and five we have an electrolytic capacitor which is one thousand microfarad on what I built and this forms an oscillator these three points are all connected together so this circuit will oscillate and it will oscillate depending on if the value of this capacitor and the resistors that sets the oscillation frequency the output on pin six goes to an LED and that goes to ground okay and then this point here which is the output of that one if you like pin four goes to another LED there's also goes to the ground okay and we have two more connections on this so our chip is a 14 pin chip okay seven on each side we've just discussed one to six okay pin seven is ground these ones 8 to 13 are actually not used we could build another one of these circuits on the other side if we wanted with four LEDs and then this pane 14 is VCC the supply voltage and these chips and this series are meant to work on a maximum of five volts you can get seven four series with different letters here that will work on higher voltages but these ones are five volts so we're going to connect this to our five volt regulator we'll put five volts in we'll watch to see what this actually does with the LEDs and then we're going to remove the jumper which is that's the ground of the regulator which will cause the voltage to go up above eight volts now either this chip will basically pop its clothes and die because it doesn't want the eight volts or something else will limit the currents if it draws water coming to go shorty it made by the fuse I haven't tried it so Anything Could Happen yeah let's see what does happen so as you can see I'm inserting the chip that is the chip which drives this we put the jumper back on let's power our circuit up again and see what happens but you'll see that this led flashes and that one flashes dimly I actually think they should both flash at the same brightness but it's not important depending on this application I'm not too worried why it's not exactly doing that what we can see that this is flashing okay so we know that circuit is operating now let's see what happens again if we remove the ground connection on our regulator okay what will we do well that came on and stayed on okay the other ones now flashing strangely enough so something different is going on what voltage do we have well we have eight volts okay let's put the jumper back on again and now this is on and stayed on so something's happened to our Chip he didn't like that yeah you can see he didn't like it he's no longer functioning as it was we can take the power off we can put it back on again and this is still on okay so the fuse didn't blow but we damaged the ship and we damaged the chip because we had eight and a half volts on here and nothing was pickled drawing a lot of garments but this somewhere inside got fried okay so how can we get around this problem well this is where the Crowbar comes in let's have a look okay here is our circuit again I've just redrawled it to add a few things so same thing 12 volts in through the fuse through the regulator to the output this is our led the blue one and this effectively is our output to our TTL circuit transistor transistor logic this is a 740 onward series that was just seven four zero for these as we know all that consume no more than five volts otherwise they die yeah so how do we protect our search against this sort of problem we can see that the fuse didn't help and if there was a lot of these devices which might draw more current the fuse might blow but for the fuse to blow more than 500 milliamps in east of 53 which and where's that 500 milliamps going into our circuit that doesn't like more than five volts so it's now a kind of a race which is going to die first yeah to get more comments through here to cause us to blow there's got to be more flowing through here and the reason more comment was falling from here is probably because the chips are dead yeah that's why so that fuse isn't going to help us what we can do is put a Zen it's iode now normally a zener diode has a voltage Supply positive a resistor the zero diodes to ground okay and depending on the voltage of the zener we will have a voltage on here between here and ground so if we put a 12 volts in here and we put a 5.1 volt zener here we'll get 5.1 volts if we put 6 volts down here we'll get a six volts okay 7.5 7.5 wherever the standard voltage yes will give us a fixed voltage here and the reason for this resist it is to limit the current that flows to the Xenon diode this isn't usually a few hundred ohms I don't know 270 ohms something like that depends on the circuit and that will limit the current that flows from here because once you apply more than let's say this is a 5.6 volt Zen once you apply more than 5.6 so this this wants to stay at 5.6 if you put 12 on here and there's nothing to limit the currents what will happen is a massive code will flow down through here and this will either blow up or go short or go open yeah probably go short so that's a zener circuit now this property of these going short if there's too much voltage and too much current can help us so from here we can connect as iOS to ground with nothing in the circuit to limit the current so once this loses its grammar connection the voltage goes up this will not allow it to go more than let's say 5.6 volts okay so once the voltage gets to that point this will just go short and this will probably blow the fuse okay if the circuit wasn't fused something might have sat on fire the regulator might blow but this is designed not to particularly save the regulator and certainly not to save the seller Dio this is your crowbar this is like taking a big battery back and dropping the Crowbar across the terminals to shut it down effectively yeah so it's not meant to protect any of this stuff but it is meant to protect the expensive stuff on this end okay so that is a basic crowbar using the zealer diode let's put one on our circuit board and let's try the same thing we just did okay here is our Zenith diode this is a one watt zenf 5.6 volts why there's a crowbar so one end goes to ground and the other end goes to this yellow Y which is the output from the voltage regulator so this shouldn't really do anything particularly well different from what it was doing before as long as the voltage here is 5 volts or less okay I've replaced the jumper so let's switch it on and let's see what it actually does well you can see it's powered up this chip is still faulty we'll change it in the moment and we can see across our crowbar we have 4.99 volts okay so let's replace our chip the one that died okay here is another sacrificial lamb to the slaughter this is actually the last one of these I have I had three what it looks like the other one is 40 anyway so let's hope this crowbar works because I have no more of these yeah talk about pushing it to the Limit yeah but let's have a loop so we'll power this up and this one actually flashes the LEDs properly which is good to see those be some sort of difference between these chips I have it is the same chip by the way 74hc oh this is something for HCT zero four where those are seven four eight Z zero four okay let's measure some voltage again we know we have 12 volts coming in on this okay from ground okay and on our crowbar I was xanna is five yeah what happens now if we lose the ground connection yeah I'll see what it does well the actual fact the LED is handling it yeah so the LED is very hot but it was managing to keep the voltage down and this cap running yeah because you're extremely hot so soon away to that would go short I would say and it would blow the fuse so we can see that actually works but in this going short not only will it by the fuse it may well blow over the voltage regulator as well so can we improve this circuit let's have a look so this is the same circuit we just had without the Crowbar so I'm going to put the Crowbar on I'm going to do it slightly differently so this is our Xenon tires okay 5.6 volt and we're going to put a resistor here it's a good round uh I don't know 400 ohms 100 Ohms a couple of them something like that yeah it's not a critical particularly just limits the comments and from here we're going to connect another component so this is a thigh wrist that also you can control Rectify it one end goes to ground and the other end we're going to connect to here okay so what will happen now if this goes above 5.6 volts this will turn on as it did but you won't be drawing a lot of current getting extremely hot and it probably would eventually burn out it will draw a limited comment because of this resistance it'll put a voltage on the gates of the thyristor or scr so you can control Rectify it they're all the same thing okay thymista SEO I know cathode or cathode you'll see both and when you put a voltage on the gate this will turn on and this will short out the input Supply the 12 volts to ground now you can get thyristors in quite large packages like such little handle I think this one's four amps continuous and 15 amps or 12 amps Peak okay so this should be able to handle what I've covered and certainly enough comments about this fuse okay and the idea of this is that because it's short in the fuse to ground directly you no longer put it any strain on the voltage regulator if there's triggers just a fuse will blow the Zelda will be absolutely fine because the resistance Limits The Commons and the scr or time yesterday was more than happy to take that amount of coverage for as long as it takes to buy the fuse so this circuit you should be able to trigger replace a fuse fix the problem it'll be fine thought comes back up we'll trigger again okay let's wire it off and here is our crowbar circus so this is the 5.6 volts then they're going to the gate of the thyristor the gate also goes to ground via a 100 ohm resistor so that will limit the current flowing through the xenod diode this effective way of his classes has five volts and this is the seven that's up to twelve I have about seven volts across it so at 100 ohms you'll get about seven milliamps that's fine I think if they're coming through there once the voltage on here it goes above 5.6 volts this will put a voltage on the gate switch this on one end of this is going to ground and the other end I'm going to connect to the output from the fuse okay so we'll just tag this last wire on I was hoping to put it on a little pin Adder but never mind so this can go to the output from the fuse and there we go so I do want the fuse to blur with the switches on I don't just want the short the powers of my house although obviously the comment limited would come on the power supply my Advanced power supply anyway I'm going to remove the chip because it's the last good one I have I'd like to test the Crowbar circuit first so we can put the jumper back on here and let's try it and the blue light is on obviously this chip isn't fitted at the moment okay so let's now remove the jumper and see what actually happens okay in fact you've probably saw the fuse boot did you see the fuse blue so this circuit definitely works on the blew the fuse on the over voltage so what we'll try now is we'll replace the fuse okay I've put another fuse and I've put the jumper back on this is now that the chip in so let's see what happens okay so the circuit is working once again let's trigger it okay fuse blue so we'll put the power off we'll put the jumper on for the last time we'll take another fuse okay we'll put this in so we've corrected our overvolts condition we've put the new fuse in was it fast enough to save the chip this is the question and yes it was so we can see our crowbar circuit has done exactly what it should and there you go guys that's all you need to know about crowbar circuits to fix stuff as I mentioned you will find these quite often on Industrial Electronics those sorts of things if they fitted dust onto something like graphics cards vrms we may get a lot less vrms although it may be a little bit more difficult to do this at the where voltages they run up but probably not I guess there's some Engineers I'm not one remember okay so I'm sure the circuit now makes sense to you when you reverse engineering stuff when you find this is that a diode or is it a normal diode and it looks like it should be reverse biased it makes sense it's a zener but there's no resistor in series and now you'll know why because it's a crowbar diode and this whole business with a thyristor strapped across the power you think well what's that foil just shut the power out yeah that's what it's supposed to do Hoover voltage protection crowbar circuits hope you enjoyed that one guys something a little bit different I've done today let me know if you like this sort of video maybe we can do a little bit more of this so please get into the comments below and I look forward to seeing you all on another awaren electronics repair video ciao for now guys

Channel: Learn Electronics Repair

Views: 182,885

Keywords: learn electronics repair, learn electronic repair, electronic repair, school, lessons, course, training, free, fault diagnosis, trouble shooting, troubleshooting, pcb repair, over voltage circuit, overvolts circuit, thyristor, what is a thyristor, thyristor circuit, zener diode, what is a zener, overvoltage protection circuit, overvolts protection circuit

Id: UF9TWAY-mf0

Length: 26min 0sec (1560 seconds)

Published: Sat Feb 18 2023