Designing on the run, sourcing parts, 2

A project log for DIY 6-digit multimeter

Open source multimeter that measures more for less

jaromirsukubajaromir.sukuba 07/29/2020 at 19:420 Comments

Now when front panel was prepared to go, I needed to start with most basic part of every circuit - power supply.

Since I didn't have too much of good FR4 boards, I opted for the cheaper PCB phenolic based perfboard for this. I started by looking at what transformers I have in my junkbox

Since I knew I'll need symmetric 15V, symmetric 21V and two separate 5V sources (for digital part and digital portion of analog board), I started drawing schematics for the complete package:

It looked somehow too complicated, so I took some liberty to decrease components count while keeping functionality the same, so the upper portion would look like this

Aparat from that, a few 78xx and 79xx regulators, capacitors and miscellany stuff was needed for analog board. For digital board, 7805 regulator would be probably dropping too much power (displays alone consume around 0,25A), so I searched for some switching regulator. I was unable to find anything other than this isolated TRACO power block.

Isolation isn't needed here at all, but for years (notice the ordering date from 2010) I had no better use for it, so choice was made.

Oh, and for rectifier diodes I chose those old ones in metallic case, made by TESLA probably 40 years ago

After a bit of planning, components were all placed

Row of diodes

And point-to-point wired, as well as I dremeled out isolation barrier, so that live parts aren't close to "earthed" parts, as well as analog parts are isolated and separated from everything else

Some thermal compound and mica isolators

And power supply block is ready to go

How it fits in the case? Quite nicely

Notice the power switch is located directly on PCB, with its push rod quite out of the way. I have a plan how to operate it from front panel. Also, the two PCBs are meant to be analog and digital boards. Those are the next on the list.

Before I could proceed, I needed to look around. I definitely needed some precision parts, namely resistors and opamps. I took a look into e-waste box

Oh, some OP-07 opamps.

ADM707 reset circuit, analog switch, that could be handy

And look at this old Zebra printer control board. That's actaully pretty schmick resistor, quite good as current shunt for 1A range of my multimeter

Friend of mine brought me this old PLC, made in Czechoslovakia right after iron curtain fall, in early 90's, one of the first designed and manufactured in here. Unfortunately, programming interface is somehow limited and old, many of the devices are broken or replaced, so I got a few in my hands

I'm pretty sure you would be hard pressed to find ANY infromation about those. Unfortunately, apart from setting museum there is not much I can do with it, so under knife it goes.

After a bit of soldering iron action (spot the differences)

I recovered a lot of relays, PCF8574 expanders and more miscellany stuff.. Those boards are really exotic, being part of large gas laser control circuitry, made in 80's in East Germany.

One of boards, a bit later

So, I have power supply and a quite bit of parts to start with. Let's build some more, in next log.