You may get Deja Vu here because I Recently looked at another digital display. well, this one this one here and uh, it was all right. The circuitry was very basic, but uh, here is another one that looks pretty much identical, but the circuitry is different. It looks possibly better quality.

and the first thing I'm noticing here oh let me Zoom down a bit is that the Uh display looks as though it's multiplexed in a standard format where each digit is WR is H lit individually in the go: 1, 2, 3, 1, 2, 3 and interestingly, when you turn it off and on again, it initially displays o12 I wonder if that was the Uh? They're basically either a piece of a software code or they're just displaying this is digit zero. this is digit one. This is digit 2 just as part of their Diagnostics when they first made it. Now the accur accuracy of this one seems okay.

Well, let's get the fluke in. Hold on. I shall get the FL meter which has Croc clipss attached at the moment and I'll set it to AC Volts need new leads for this? uh and then I should just stuff these crocodile clips into this connector here. and the flick meter says 2 46.

That display down there says Al 2 46 as well. They seem to be matching up fairly accurately. Well, that's quite good, right? Let's explore. So I'll just uh get IM out the way.

I shall turn the power off. turning the switch off turns the power off to everything in here I reward it to make sure everything was perfect. Uh, and we'll open this unit. Uh, this display here I me probably mentioned it's not accurate because I I've not calibrated this I should calibrate.

it shouldn't I I Think it does have adjustments available now. which of these is which? Uh, this is the new one. Yes, it is. okay.

So to get the circuit board out and and this one, it's the same as usual. you basically hinge this little thing back. Hold on. let me just grab hold of this.

oh tell you what, I might be able to just hinge it up like that. Is it going to come out? This is when I say oh, it comes out easily. No, it doesn't. There's this little diffuser plate, slight tint.

Oh, this is a a red display again because uh. I prefer red. The red is the most reliable of those colors. The green and blue phosphor H green and blue and the white phosphor are less reliable.

I'm just going to peel the tape off this. this is used during manufacturing. It's not needed. It provides a more a better contrast h less reflection.

not that that's such an issue with that. So right initially I'm seeing that it is capacitive dropper based. but the difference is that I'm seeing uh, different circuitry in the actual the voltage divider and the extra capacitors. The other one was Miss missing right? Okay, I shall take a picture of this and then we can explore the circuitry one moment.

please. and let's explore. So there are some differences to the other unit. Uh, there's quite a few differences to the other unit.

Let me get down close to this. I We can take a look at the components on the circuit board. Uh, is this does this have Andy Tracking bastard, right? Okay, it doesn't have a discharge resistor across the capacitor right? so that holds a of a charge and gives you a Zing H Does it have anti- discharge? Uh. tracking slots? I'm not sure it does okay I don't think it does have an tracking slot.

So uh, things worth mentioning: This does not have a a discharge resistor. I Should know that because I've already drawn the schematic out for it. Tingle tastic, right? Okay, so uh, there is a connector here and uh, one leg of the connector goes to this resistor and then to to the bridge rectifier, the other goes via this capacitor to the bridge rectifier and that provides a power supply with this smoothing capacitor, a Xeno diode, and a little decoupling capacitor. the other one didn't have that.

This is good. The chip is driving. Uh, a standard Multiplex s segment display with three positives for the digits and 1, 2, 3, 4, 5, 6, 7 eight Uh segments including the decimal point. Uh, there is a diode feeding the Uh divider circuitry for the Uh measuring the voltage.

Now it's worth mentioning I Tried this potentiometer across its scale and it's very small. It's only about 7 volts across the scale and at the moment for the correct accurate reading, it's actually slammed to one side. Um, let's take a look at the schematic. here.

Here is the schematic: Uh, the incoming Supply comes here. It goes via that tingly capacitor there with no discharge resistor across it. This isn't really a huge issue in Uh, a machine because there'll be other loads. See, for instance, this is in a control circuit.

Those other loads would potentially discharge this. but when you've done what I've just done and you've had it running and then you've taken off and then stick your fingers across it, that does risk a slight tingle. So there's the capacitor. Uh, it is rated 630 volt to give this a a wide voltage range.

not sure its voltage range this particular model is the other is 70 to 500 volts. The 75 Oh is right on the input here. The sort of inrush limiting which limits the peak current as thator initially charges um, but also um, it's If there's any transients, it will help, but you know I still would have gone for higher value. Um.

So on every half cycle, the capacitor allows a small portion of current through that gets rectified. the bridge rectifier. Smooth By this capacitor, there's a decoupling capacitor which is better at absorbing transients and it can. Basically, it's got a lower impedance.

it can react faster than in this. And then there is a Xenor Diode. The Xenor Diode is called Um a W7 which is let's just write that W7 which is a 4.7 Vol iner diode and that's exactly the voltage are measured across it. The incoming Supply also goes via this diode and then two high value resistors 470k the 22k potentiometer and then the other side of that bridge is a 4.7k resistor.

So by tuning that potentiometer, you basically get a division of the main Supply voltage that can then go into this chip. I should just put a question mark in the chip. It it might be an application specific chip, it might just be a microcontroller. I'm going to guess microcontroller, but well suited to applications like this and that then drives the display as the current limiting init.

It's got the three lines out to the digits and common anode and then it's got the Uh. The negatives are switched the eight negatives to actually light the Uh segments. So initially it will turn on one of these and it will display, say the digit two and then it will uh, turn that off and turn the second one on and it'll display the digit four and so on. Um, that's it.

I Have to say this was literally just it. might as well have been out of circuit. H It was basically just completely um, unadjustable in the sense that you know it. was slammed against the end on 246 Vols I measured the range is 246 to 253.

Vols That is literally just 7 volts across the full range of that potentiometer. Um, but these things can be adjusted. You know, by nudging resistor values, you can actually tune things to give yourself a wider range. H But it's nice in the sense that because they gone for a small range, it means that uh, it's more accurate to adjust it.

The other one had a much wider range. But there we have it. This one is kind of better. It's got It's got pros and cons.

it doesn't have that discharge resistor tingly H I Don't think it's got the anti tracking slot, which means these connectors at one10 of an inch part 2.54 mm are a bit close for sticking it over a 415 Volt or higher Supply Um, but it it seems the other ones seemed accurate as well. It seems quite a nicely made, functional little unit. Um, and it does have those little things. the little features like the covering capacitor and this little uh, smoothing capacitor, but the other one didn't see to really struggle without that.

But there it is. Uh, the other display. Uh. two identical looking displays.

Very different circuitry. Quite interesting to actually compare the two of them.