Hi in a previous video which I'll link in down below if you haven't seen it. we played around with a Precision 1 amp current Source uh Ignoring all this sort of stuff, we were basically following uh the application note circuit here in the uh LTC 65 data sheet. this is a Precision 1.25 volt uh voltage reference and this thing oscillated a buggery. it was absolutely awful and uh, and since then I've tried a couple of um I've tried the other recom Ed configuration with the PNP cuz I suspected this thing might oscillate.

that's why I breadboarded the thing up. never trust these application notes just to work. Oh, you got to build the things up and test them. let me tell you.

And since then I've tried uh various configurations I've tried the other uh one here uh with the P&P configuration but that one was a dog too, especially at uh higher current and I tried uh, some fets and other configuration and various various other things to try and stabilize the the loop on this thing cuz internally it's an Op amp and well, you know, um, if you don't, uh, get the loop uh stable which I won't go into um, it can. it can oscillate and that's exactly what we saw, especially at high current load. So I thought I'd have another uh crack uh here using uh another uh circuit you've uh seen before which I'll link in down below as well. If you haven't seen it, this is just my simple uh, constant current, uh dummy load.

but well, hey, a dummy load. It's constant current right? That's exactly what we need. one amp I don't necessarily need the ground referenced uh output that we had on this uh uh circuit. Here, you can see the transistors in the upper half and the output current actually goes down to ground like that.

Well, don't actually need that cuz it's going to be battery isolated anyway. So I figure my load can be on the high side. So I thought I go back to this Uh configuration I've used before which can also be unstable of course. Um, you know, know.

nothing new there. It's still got a loop in here, it's still got the opamp and everything else and you have to stabilize that Loop But I thought I'd give this one a crack and see what we get. So we're going to build it up on the breadboard here. I've got my N Channel Art mosfet I just pulled one out of The Jug bin.

it's an RFP 305 and I've now got which is very nice from V. Thank you very much V for sending me samples of these. Very nice. 1.25 ohm uh four terminal uh 0.02% I think it is precision resistor.

Really quite nice. so this will actually allow us to get some a very nice uh four terminal wire measurement even on a dodgy breadboard which we're going to see. and I'm using the same thing oh I didn't WR in there. but yes, it's still the LTC 65 I'm going to still stick with that.

So 1.25 Vols over 1.25 ohms is going to give us a constant current of 1 amp. but hey, let's build it up. see if it oscillates, it probably will. Um, well, we can almost certainly make it oscillate under certain uh circumstances.

the output capacitance here, and uh, and the and the load current and the input capacitance and all sorts of stuff. So um, yeah, we probably can get it to oscillate, but let's see if we can actually get a Precision 1 amp current Source on our bread B which we intended to last time because these configurations up here just sucked as I said, thank you very much Uh V for sending me samples of this 1.25 Ohm. There's the exact part number there. If you're playing along at home, you can actually, uh, order these on uh, digk.

but I think there's a minimum volume to actually, uh, order them so it's a 1.25 ohm .0 uh, 2% and it's the VPR uh 221 series. I'll actually Link in the data sheet down below. Really awesome. Tempco Fantastic resistor for using in a Precision current.

Source Like this and here's a little uh strip they come in. They sent me a few of them and uh, well, if you're going to make them cuz these are made to order actually they you can order any value you like and they will laser trim it for you. visha. Fantastic! Um and yeah, there it is.

A four terminal uh to 220 package. Fantastic. So the two outer pins on there are the Uh current in and out, and the two inner pins are the two sense pins in here on our circuit. So even on a dodgy breadboard, we can hook this thing up and get really precise four terminal measurements because the two sense pins are tapped right inside the right on the resistor in there.

Hence why I'm going to use a really nice Precision resistor like this. So we've got our like .2% um Precision voltage reference. here. we've got our 0.02% Precision uh for terminal resistor.

worst case, of course, so we should really be able to get you know, like a 0.02% or better uh, one amp current Source reference out of this sucker. But even if we do get this thing stable and build it up, there's going to be an issue with this circuit and I've deliberately left it out. And if you want to try and figure out what it is before we get to it in the video, hopefully I can get the thing stable and we can actually see there's going to be some output error here. there's not.

We're not going to get precisely one amp and try and figure out why so you can stop the video. Now look at the data sheet for the LTC 65. Try and figure out why when not going to get precisely one amp through this resistor H So this is the circuit we're going to build up on the breadboard here. as the data sheet recommends.

of course, the um, uh sense line must have a bypass cap of at least 2.7 microfarads and less than .1 Ohm uh ESR it's got to be a low ESR because the transfer you know function. If you look at the Loop stability and all that sort of stuff, you can't have too high a value. ESR The ESR is an equivalent resistor in there like that. So I've got a 10 microfarad uh, ceramic in there should be good enough.

I've got a series resistor in here I think I've got a Uh 12K I think it is just because the gate capacitance we we just want to slow this thing down. we just want to add, um, some rollof in there. So you want to put a series resistor in there of you know, some nominal value. um, so you know it can change and maybe we might experiment with that.

But anyway, I've just got like a 10 or 12K in there. No problem, just a bypass cap on the input here. they recommend, you know, 100n or something like that. I've got one directly from the last video I got one directly 100n directly solded onto the little Uh surface mount module Plus for good measure I've got another I think 47n outside of that on the breadboards.

Oh, by the way, uh, input Supply Here, we're probably going to need like four or five volts or something like that. Our output voltage is 1.25 volts, but we got to have our gate uh, a sufficient uh Gate Drive voltage as well for our uh n Channel fit over here I I Haven't looked at the data sheet for this 3055 I don't actually remember. but yeah, 405 Vol should be good enough. So I'll set it to five.

Oh, Also, an important thing is that the ground reference for this entire circuit I'm taking off the sense tap for that resistor there. so there is no error because we're trying to generate a very precise 1.25 volts across this point. And so you know, basically the sense output. the force line drives, uh, the voltage for that.

but the sense line here and there and there that's our internal voltage reference is going to produce. Or going to try and produce 1.25 volts precisely across those two points. So you want them directly across your sense line there. You don't want to make mistake of connecting this ground through to here like this.

You want this actually returning if your input is like this and there's a ground there you want actually your return current bypassing all of your um, uh, all of your Precision voltage reference circuit in here and going back to your Supply like that. So you've got two current Loops There one is the huge 1 amp current Loop going around like that and the other is just a separate ground in there for the sense resistor and this, you know Chip's only going to draw a couple of milliamps, but you definitely want that. Very important. That's why we're using a four terminal resistor.

If you don't have a four terminal resistor, then you want to do a proper Kelvin connection. Right on the point and there is our circuit build up there. and uh, this Op Amp isn't used here. That was just from some experience from last time.

We may need that again. We'll see. Um, there's our Precision voltage reference on there as I said bypass cap bypass cap on the input the ground. you'll notice that the ground is ref is separate and actually goes back to the Sense terminal over there of there's my Um 1.25 ohm four terminal Precision resistor.

There's our N Channel mosfet up here. This will be my current Source going from the Uh drain of that thing up to the top there. there's my 10 microfarad output cap there. so let's hook this thing up and plug that in and I'll just measure the output sense line with the scope here and we'll see what we get.

And here's our power supply: I've got it set to Uh 5 Vols here. but of course you want to set the output current limit just so you, uh, protect your circuit. You don't want to blow the ass out of things. I'll set that to 1.1 amps there so it can't go over that.

it'll self limit. by the way. No I don't have any heat sinks on these devices here. The four terminal Precision resistors is only going to dissipate Uh 1, uh, 25 watts maximum and that is within the safe operating range of this thing within.

just into free air like that, what's called free air. So um, without a heat sink, so that'll be fine. The mosfet over here. No, it's going to get pretty hot.

Use arms law, Work out how much power's dissipate in that. but hey, it'll be good enough for just you know. Probe in for you know, 5 seconds and seeing if the thing oscillates it's you know. It's not like we're going to leave it there for you know, an hour or something like that.

It'll just cook the thing to death. So here we go. Let's see what we get. Let's switch this on and my circuit's drawing.

uh 4 milliamps there. That's sort of what you'd expect for the uh, quiescent current of the circuit here. And if I plug in my Scopes hooked up I'm at uh yeah, yeah. 200 MTS per division.

That'll do it. Let's plug in our load. Hello hello. We're getting our 1 Point uh, 25 Vols nominal up there.

But uh yeah, it looks like we're getting some oscillation and if we ac couple that it looks like yeah, that's that's. pretty hard. We're getting about 50 MTS uh Peak to- Peak uh oscillation on this thing? That's that's pretty awful. Let's see if we can, uh, see if we can trigger off that and see what we get in.

Uh yeah, look at that awful similar to what we were getting last time. So hey, but you know we're get in there and I reckon we can stabilize this sucker. We should be able to. So what I'm going to do now is actually hook up the uh hook up my uh Amer here and uh, let's see what value we get because you know it should.

We should get there. We go. We're getting near our amp but of course. oh o here we go.

It's changed. Look at that. Our waveform there has changed because we've got some extra inductance in this lead. We only had little jumper lead before, So there you go.

That's kind of expected. Um, that you wouldn't get precisely the wave the same waveform. And to show you that, let's put that back in and I'll show you that. That's there We go.

Yeah, completely different. There you go when you add the short because you got the inductance of these leads in here. So that is changing the whole uh, oscillation frequency of that thing due to you know the inductance of this the SSR, the output caps and the loop stability and the poles and the whole, you know, the whole, shebang in there. And just to show you that, let's that's what we're getting.

Okay with the uh, huge leads on here. Let's remove the input capacitance. You know how I said I've got the two input caps there. Well, let's physically take out one of them I still got one there directly on the input of the chip.

but we should expect a little change. and we do get it. Look at that. we're getting some more High frequency parasitics happening there.

That's really yeah. that's look at that. So if we whack our cap back in and bingo, we're back back from the future. But if you think you're going to solve this problem by you know, tweaking your input capacitance on this on over here, then you're completely wrong.

You're in the wrong ballpark because the loop stability of this thing is ultimately uh, determined by the phase margin of the Um output Op amp in there for this voltage reference and the As I said the output um capacity Here, the 10 microfarads with the Uh A A it usually specifies in data sheet it does. In this case of, it's got to be less than 0.1 OHS But it's not just the ESR or the output, it's actually the output capacitance as well. So this is where we need to solve our problem. And when you get oscillation on a voltage reference or a regulator like this, usually you're not going to have enough uh, capacitance on the output to ensure Loop stability in your voltage reference.

Here, let's increase the value of that output cap and I think we're probably. well. hopefully we will solve our issue. That's the Uh, that's the plan anyway.

So let's work on a big Ass not a huge value resistor, but a big ass 22 microfarad uh 400v Electro on here and see if that that makes a difference. I'm not going up in value yet, but I'm adding another I'm still leaving in the 10 microfarad uh, ceramic cap, which we got in there, but adding another 22 mic in there in parallel. So let's uh yeah, that's on all right. let's switch this back on.

No, that hasn't solved it, but it's changed it. Look at that. we're on our way. so let's go up in capacitor shall Wei 470 microp Electro 10 Vols So let's give that a B and there we go and I'll plug this in.

Live here. See what we get? Oh hello, hello Oh 1.2 1.27 Look at that. we're AC coupled here on on the Noise by the way. so that's that's 5 Mt per division.

Beautiful! We're getting our Precision current Source near enough to 1 amp. Ah, Beauty What a Bobby desler. So we're down at 5 Ms per division here. and if you're curious about that noise, I've got the thing actually switched off now.

All right, our circuit is Switched Off and we're still picking up this crap. Okay, so oh hello, look just putting my hand, there is enough to, um, pick up the interference from the screen here and near my circuit and couple that in. There's all sorts of AC coupling stuff happening here, so that is not actually the residual noise of this circuit and we won't go into actually measuring the noise of this thing. That's why if I switch it on and because this is actually a pretty low noise reference, right? so the noise isn't actually oscillation of the thing here we go.

let's switch it back on and get our there we go. Noise is exactly the same because we're not using proper low noise measurement uh techniques here. This noise is a fury. It's a red hairy so you know, don't think that is your output oscillating or being nois or anything like that.

No, that's not necessarily so. But anyway, all we're here for today is to stop the oscillation And we have. We have ourselves a stable Precision 1 amp Uh current Source Or do we? Now, as I said before, I gave you a little quiz at the start. this is actually expected the exact value we're getting here.

Please don't let me, uh, scream at me if I'm leaving this on too long cuz that fet's going to heat up anyway if I talk too long which I always do. Um, the output is pretty close to 1 amp there. Look at that and you might think that's bang on. but do the math there.

that's. 27% and you remember this thing should be .02 It should be an order of magnitude better than that, so that should be uh, 1.27 or something like that. and it's not to do with the layout in my circuit cuz I'm pretty confident that is right, cuz we've used the proper four term Kelvin resistance. so we got an extra 2.7 milliamps in there.

Why? Now this of course is where you have to read the data sheet and if you're reading it carefully or if you've seen the Forum before where where we've actually discussed this after the uh, uh previous video, then you'll notice that pin. six V outs right? that sense line coming back in, you might think, well, it's not going to take any current at all, right? It's going to be the input to the Op. but it's not. And look, we got two internal resistors in here.

There is going to be some current flowing into this pin and if you read up here, what does it tell you? Uh, this pin sinks 2 milliamps. The output eror is uh, is the resistance of the PCB tray so that could matter and it talks about this in the data sheet. I recommend you read it times that 2 milliamps regardless of the load current. Aha 2 milliamps Bingo what do oh I I left this on oop should be smoking now it's dissipating 4 five watts in that poor little sucker.

That's where our 2 milliamp erors coming from Gotcha poor little sucker. So what we' got Happening Here is here's our load up here. Here's our am up. We've got our PR Yes, we do have a precise 1.25 volts across our Precision 1 25 Ohm resistor.

Yes! so we are getting precisely 1 amp flowing through here like this, down through the resistor and out the bottom. but we've got a 2 milliamp eror current coming down here and going. oh thank you I need to go in there down to ground so we got a Precision Even though we got that precise voltage across there our resistor we think, oh yeah, we definitely have to generate our 1 Aamp through there. That's Ohms law.

Aha, you forgot about this sneaky little current path going down there little bastard. and that's where our 2 milliamp eror is coming from. In this case, it's only. 2% so it's not much.

But hey, we're trying to design a Precision current Source here at .2% order of magnitude. better. 10 times better than that. So that 2 milliamps is killing us.

So what we need to do is actually put an OP a buffer in here like this. positive negative. So then of course there's no current flowing into that opamp because it's a high impedance input so the error is going to be negligible. Not even half a B's dick and all of the current.

Of course this is powered from plus V up here. so all the current is coming from this terminal up here. That extra 2 milliamps is Source from up here instead of through our load over here. So therefore, if we add in that buffer which may affect the loop stability of this thing of course.

So hey, we could be back to square one with our Loop stability, we may have to compensate. But anyway, that is the way that we're going to fix that. But of course, if you're astute, you would have realized that. Look, we've still got our 10 microfarad, in fact, our 470.

sorry, what did we use like a 470 microfarad in there to make it stable on the output of this opamp. Now, And hey, opamps do not like having uh, large capacitors on the output so that thing could oscillate on its own. So we're going to move that cap back over to the input here. and so I'll put that 470 mic back on the input there and there'll be nothing on the output of the S amp.

It'll just be driving that sense line directly. Let's see what we get. All right. let's switch this sucker on and hook it in.

I Got my 470 mic on there I Hope it's all right. I've got my Opamp in there. It's powered and fingers crossed. Hello, look no wh, No noise and look, we're down to 1LE 06 So we're uh now .6% accurate.

Awesome! So you remember we were getting 1.27 before and the data sheet said, well, it was a nominal uh 2 milliamps into that pin. Well, we've eliminated that 2 milliamps, It's vanished and now we're just getting that 7. We're getting like 7 before. Well, o uh7 before in terms of offset.

If we didn't account for the 2 milliamps, take out the 2 milliamps. There we go Bang! I'm curious to know what happens if we remove our 470 M cap. Now let's see if it goes back to being no. There we go.

it's still stable. We've got a bit more noise, but, uh, not a problem at all. So this thing is now stable without that um, output capacitance there. Because we've Chang entirely changed the uh loop parameters of this uh circuit, the poles, and everything else.

The whole thing has completely changed by adding the Opamp buffer in that feedback loop. Oh hello, I Just put in a 10 microfarad ceramic I Was just going to boast that, uh, putting in this uh, uh. putting in this op has ironically made this thing more stable. But look at that.

that's horrible. That's five. That's 10 ms uh, per division there. So that's that uh, little 10 microfarad ceramic.

It doesn't like that at all. But if I put in that uh, well. it. it works without it.

um, or with the 470, but looks like that that, 10 microfarad is in The Sweet Spot which makes a little bit of oscillation within the loop we've got here. So that's why. uh, this kind of circuit you can make stable for one precise load you've got. and well, and that's what I'm going to use this for is a Precision 1 Aamp current Jetter and that's fine.

but if you're using this to power all sorts of different loads at different currents, then well, you could come a gutsa. So just be careful that uh, getting Loop stability in all sorts of circuits like this is not easy. And really, unless you've got the exact uh simulation model of that reference chip I better turn that off, it's getting a bit hot. Um, then you, uh, you know you really have to build this thing up and you know, try it on your actual load to make sure it works.

And that's what we get if we put in 100 Nanofarads. look at that 100 narad. By the way, I'm putting these caps directly across the uh sense line of the input of the Op amp there. So there you go.

That's 50 Ms per division. That's that 100n. Pretty horrible. So looks like we still do need that high Val value cap across here to make this thing stable.

Not surprising. and you know how I said this? uh gate series resistance in here is going to matter at the moment. I've got a 12K resistor in there just to, uh, slow the thing down because we're going to get some gate capacitors here and we can cause the thing to oscillate that way. So what I've got now? Okay, let's so let's actually short that out.

There's our output. Okay, it's stable. I've actually got no capacitance here at the moment, but you can see with the 12K resistor in there it's nice and stable. Now what I'll do is I'll short out that.

There we go. I shorted out that 12K resistor. Ha. look at that.

So we got some pretty darn awful 500 MTS per division noise there. So let's with that shorted out resistor, let's see if we can on our even our 470 mic cap across our sense resistor there. can't uh, you know, can't uh, settle that oscillation down. we really have to remove that.

uh, we really have to in include that 10K in there. So the other thing to note about this is that this thing really doesn't change much. even though these devices are heating up. I mean they're you know, way too hot to touch.

The fet's probably up at 100, 80, or 100 or something now and that you know, uh, the I don't know the uh four terminal uh Precision shunt resistor you know might be at 60 or something I don't know I haven't put a thermometer on there, but yeah, getting pretty warm. So um, but look, I mean it's pretty Ultra stable. That's because this: if you go, have a look at the data sheet for this Precision resistor. it's Tempco is bugger.

All these things practically have no drift at all like 2 PPM per degree seat. Oh and if you're wondering what Op Amp I used in there, yes, it does actually matter I've used a really Precision uh trimmed Op amp the OPA 376 V offset voltage of 5 microv volts typical which translates to our with our 1.25 volts here ofle 4% error. So it's adding bugger all error into our circuit here and that's what you want. and I just tried another uh, four terminal uh resistor in here because we are still getting, you know .6% Which is higher than we'd like.

so higher than what we really expect. So I tried a different one and there you go. It hasn't changed by a huge amount as you'd expect because these are 0.02% Precision resistors or better and I'm sure they're actually better. That's sort of worst case.

These things you know never hit their worst case. So I can only presume that extra error that we're getting in there because we expect better than that. um is due to the contact resistance on my breadboard in here, so this is not ideal. Either that or my voltage reference is out of tolerance.

So what I've done is I've solded up another chip. This is the old one with the dodgy uh pins that I had to uh Bend inwards to make it uh fit. Anyway, I've sold it on a uh, brand new LTC 65 and let's have a look at what we get Bingo that is uh, half I think roughly half of what we're getting before. So we're now 0.03% and technically that's with inside our error budget because our Um because the voltage reference is up 0.025% I believe uh, nominal in itself.

And then we've got the Precision current shun of 0.02% So yeah, there you go within our era budget. So maybe I don't know. Maybe this little puppy has had a bit too much abuse from all the uh experimenting that we've got there. But I'm I'm pretty happy with that.

It's more than good enough for my Um application here. So there you have it. There's our Precision 1 Aamp current source and we got it stable and we got it to work on a breadboard. Brilliant! That's what we want, even within our error budget of our two main parts here, which is our voltage reference and our Precision resistor here.

So I think I might uh do a custom little board for this I'm not exactly the final uh form factor. It's going to be built in little box. It's going to be powered from batteries. going to be powered from like four uh diesel batteries I didn't uh test the input voltage range there.

The voltage range can perhaps uh go uh down a bit. I'm not sure. probably add like a little low battery indicator or something like that, but this is going to work a treat. And of course that fet there needs a heat sink.

Um, technically this resistor here. uh, Precision resistor doesn't need a heat sink, but I would put a small heat sink on that as a matter of course. and uh, this one. yeah, got to test it over.

uh, you know, over continuous load and all that sort of jazz. But jeez, Pretty. Dar Happy. And if you're curious to know what the uh switch on performance of this uh current source is with the 470 microfarad cap across there.

So let's uh, give it go. Little dodgy alligator clip here. So oh yeah, there's our contact bounce There we go. I that's that's pretty dodg.

Let me try that again. So yeah, there a switch off and let's see if I can get a clean switch on. There we go. that's pretty clean.

That little bit of noise in there is just from my contact bounce there, but that turns on pretty nicely, but you're only going to get that nice. Uh, turn on with a large value cap in there if you got. you know, a smaller value or no value at all as we found it was still stable with no value cap in there. Um, then, well, your transient performance isn't going to be smooth like that.

It's probably going to overshoot. In fact, can probably give that a go. Here we go. So I've taken the cap out and whoa, there we go.

Now let me try that again. it's hard to capture I Need a proper switch? There we go. Got it There we go. Got a massive amount of overshoot there so our 470 mik helps with that.

So it's a fairly clean switch on with that 470 mic value. So there you go I hope you enjoyed that video of this Precision 1 amp current source. And yeah, it's good enough for my application we were getting. You know, 0.03% here on the breadboard.

Not a problem. it's switch on. performance seems quite stable, so I'm pretty happy with that. More than good enough for my application.

So I'm going to build this sucker up and, uh, use it. and if you want to have a play around with it by all means, do yourself. So thank you very much. Uh U Val at Vishe for getting me that Precision 1.25 ohm current shunt resistor Awesome! Going to link in the data sheet for that thing down below.

Highly recommend you take a look. Vishe make some awesome resistors and they actually sent me some other goodness as well which we might take a look at. Uh, one day. Very very nice.

Top of the range Precision Resistors M Resistor porn. Love it. Catch you next time.