DIY Precision milliohm meter by popular request with circuit diagram.

hi guys and girls on youtube and welcome to our channel now for everybody who's been watching my channel over the last couple of weeks you'll notice i posted a couple of videos um demonstrating how easy it is to find a short circuit on a circuit board with a million meter now i've been inundated with requests for the schematic diagram for this um for this meter now i didn't actually design this um i made it from a magazine all i've done is modified to suit my requirement and it's taken me quite a long time to actually find the schematic because i've got thousands and thousands of magazines spread over different places so for everybody who's been waiting very patiently and just stick with me and watch this video um so before we move on to the meter let's just have a quick chat first right so this is my homemade million meter um now you could probably build this there's not a lot to it you could probably build it for about 20 pound and if you don't want to build one you could just buy one if you've plenty of money and you could just buy one um here i've got a radio spares one so let's just tip it out press the button that brings it on as you can see we've got four terminals on the front um exactly the same as mine right so this particular one it's a radio spares component one uh that's the one there it comes in a nice fancy carrying case and this will set you back 373 pound 12 plus vap which is including the vat if you're not read that registered like me which is 447 pounds 74. right so that's it in its carrying case and if you've got even more money to spend you could actually buy better than a million meter a micro ohm meter but we're not going to concern ourselves with very expensive test equipment because this one you can build yourself for next to nothing right so i've got the screws undone on the case first let's take the lid off and see how remarkably simple it is as you can see there's next to nothing in it now all this side this is the power supply so the actual million meter only consists of four semiconductors so for everybody who's wondering why we've got four test sockets and only two clips and if you look these are these wires are actually in parallel and well these are called kelvin clips and this is the most important part of a million meter the reason we've got two wires is to cancel out the resistance so we get an absolute zero so i'll just give you a quick demonstration we'll just move to this flute multimeter and um what i'll do is i'll put it on ohms and then i'll short when you get it out the light i'll show the test connectors together and if you look there we've got 0.3 or 300 milliohms now what we're reading there is the resistance of these test clips the resistance of the wire but with the kelvin clips because we've got two in parallel on this side and two in parallel on that side it actually cancels out the resistance of the lead and now the most important thing with these see a quick example we've got two connectors there but you've got to take a measurement from the middle of them two connectors so if we draw a point there and another point there this is the place where we're taking the reading from and that will give you that center point of these two connectors that gives you an absolute zero so whereas on the flute we're measuring the resistance of the leaves if we turn the millivolt meter on with a connection shorted it should read zero and that's it that's reading absolute zero and that's why it's most important to have these kelvin clips now of course you could make a pair yourself um with a multimeter probes but what you'd have to do is you'd have to solder them together right at the very very end and if you don't do that if you was to put to the lead say here then you get a false reading because you'd actually be measuring um you'd also have from there to there that will measure a certain resistance in this case it's probably micro ohms but to make an accurate measurement both them tips have got to be absolutely joined at the very edges right so here's the schematic of the uh million meter as you can see there's next to nothing to it and now here we've got a digital multimeter well i didn't want a digital multimeter because that ties up your meter as you can see in this design it was designed to plug into a digital meter well i didn't want that i wanted an independent meter so i'm not tying up another meter and so what i did is i bought one of these little panel meters it's not this one it's a different one to this but the most important thing is uh you've got a resolution of zero to 200 millivolts um now i think with these panel meters most of them come with that resolution anyway 200 millivolts and you just put shunts across them to suit your needs and so that's the first thing i deviated on here now also you'll notice it's got on this design it's fed from aa batteries well this thing actually consumes alone 100 milliamps which is quite a lot of current to be drawn from batteries especially if you leave this on for a long period of time and so what i wanted is a mains powered one um now there's two slight problems because you're using a digital panel meter you can't connect the ground of that to the ground of this so if you're going to be using a little panel meter as i've done you need two separate power supplies you need one to feed the millivolt meter circuit and one to feed the um digital panel meter so as you can see here i've got a little mains transformer with two independent six volt secondaries um feeding a bridge rectifier and two linear regulators um now one of these regulators is feeding the panel meter on the front and the other regulator is feeding the million meter part so that's what all i've deviated from the magazine is i've actually mains powered it um because also if your batteries go down you could get false readings so by parallel mains you've got a cut i think you can run all the time you don't need to worry about batteries you don't need to worry about faults readings because it's always on power let's have a look that's the digital panel meter at the back um as i said this is just a little thing i bought in probably cost five or six pound i can't remember now um and the four test sockets on the front so as you can see really really simple to make i mean you could use it with a multimeter you don't need um a separate display on the front but i just found it a lot better to be not tying up a multimeter and to have a big red led display um so now let me show you where the schematic is um please don't ask me to post you the schematic on i just don't have time to scan it and send it to everybody but i'll show you how to find it i'll show you where it's come from and you can look it up yourself nice and simple right so here's the magazine um just so we don't lose the page let's just put something into there um so as you can see um it's television and consumer electronics is july 2005. and there it is on the front page turn it over and it's page 534 and i'll show you where you can get a copy of this magazine from and you can actually you can get this on the internet and just download the you can just print these out yourself right so that's the magazine go on to the internet and type in world radio history television magazine which has come up there click on that uh click on the one that says practical television and then if we move down here we want july 2005 which is that one there and there we go that's the one so now you can look that up yourself um you can read it on the internet or you can just go to the pages you want and just print it off right so to buy a set of these kelvin clip leads which um it's a lot easier and a lot better to just buy a pair than try and make your own uh go on to aliexpress on the internet type in kelvin clips and that is the set of kelvin clips um so um just less than 10 pound for a set right well the only other thing to mention now because we're getting towards the end of the video is the components so you can see here we've got a chip tle2425 and an op amp tl071 i would strongly recommend that you do not buy these two chips on aliexpress because you're likely to get fakes and you can get both of these from rs components and leads i think that one cost about two or three pounds they're not expensive but if you buy on ebay or if you buy on aliexpress you're likely to get fakes and you might not get this project up and running at all so all right then guys guys and girls many thanks for watching my video and like i say i'm very sorry for everybody who kept asking for this schematic but it's taken me a long long time to find this magazine so we're all right guys and girls many thanks for watching my video and i'll catch in the next one goodbye for now

Channel: Michael Dranfield

Views: 50,702

Keywords: millivolt meter, lo ohms meter

Id: hdVU9VYUCRE

Length: 12min 33sec (753 seconds)

Published: Mon Jun 06 2022