150d bench power supply ammeter not working

[Dannyx]

Re: 150d bench power supply ammeter not working

If you do not need the full output of the transformer then pull the tape around the secondary winding and separate the center tap winding in two then you have two separate outputs and retape the secondary winding and you be ready to rock and roll

I was thinking of that too at one point, especially since I'm now feeding around 23v into the board which gives me a max output of around 15v, so even if I just fed 12v (one winding), I should still get at least a 12v output, especially since the output of the transformer measured after the bridge rectifier is going to be higher than 12v, so there is some headroom...might be the best suggestion. Not sure how easy it is to do that without wrecking the xformer though....

[sam_sam_sam]

Re: 150d bench power supply ammeter not working

Most of the time it is very easy to do

The hardest part is if it has a metal cover over the winding then it is not as easy to do

The biggest problem is if you get in a hurry when doing this project you have to take your time and work carefully and it should turn out alright

I have done this a few times before and it has worked great for me

The one thing to be careful doing is untaping —->

( you only need about two turn of tape then cut the rest of the tape on one side of the wire close to the metal core of the transformer and peal it back to get to wires on the secondary winding )

<——— the transformer use a very sharp exacto knife with a very small blade to help you untape it that is the easiest way to do this

Here is what exacto knife looks like https://www.amazon.com/dp/B01FOQK1VW...a-301672080909

[Dannyx]

Re: 150d bench power supply ammeter not working

I know what an exacto is LOL and HAVE replaced fuses in transformers before, so I know what to expect It most definitely has a metal surround, but that comes off easily with some encouraging and a flat head screwdriver to bend out the metal tabs holding the bottom piece to the frame. Might start working on it today....HOPEFULLY

[sam_sam_sam]

Re: 150d bench power supply ammeter not working

Let us know how this works out for you

[Dannyx]

Re: 150d bench power supply ammeter not working

Here are a few shots of that control board...they might not be great, but will hopefully give you an idea. What's weird is that this thing, again, appears to be grounded: there's a GND wire going from this display board to the power board, so even if there are two separate windings, they'd still have a common ground...after the rectifier that is...this is so confusing, must admit

The red and black wires botch-soldered on the back of the board are for the fan I added...basically it's in parallel with that 12v winding obtained from the center tap and the GND leg of the bridge rectifier.

My buddy wasn't exactly keen on the idea of me trying to gut his transformer, so I haven't tried it yet - might have to do it one day when he's not around

[Dannyx]

Re: 150d bench power supply ammeter not working

Another thing I tried with this occasion is powering the "instruments" with a 12v adapter and it worked: I got the correct reading for the current. I set the output to 1v, got a 1ohm resistor and placed it on the output. Ohm's law tells us that 1v divided by 1ohm equals 1A, which is what my displayed showed (of course, not precisely 1A, but 998-something mA is perfectly acceptable) so that part is working. When I switched back to my hack, the display only showed 23mA, so there IS a grounding/loop/whatever issue.....

I also did a rough schematic of how the thing is powered: on the left hand of that very dirty piece of paper we have the original setup: two separate windings. The right hand side is the current setup: a center tapped transformer. Can't really wrap my head around what's going on here. The dotted line indicated the instrument panel. The diode and 7805 regulator are on this panel. GND is common...yeah I know I used the earth symbol there, but you get the idea

[stj]

Re: 150d bench power supply ammeter not working

hey, you know the 4pin chokes on the mains input of stuff like dvd players?
the windings are equal - see where this is heading?

[Dannyx]

Re: 150d bench power supply ammeter not working

hey, you know the 4pin chokes on the mains input of stuff like dvd players?
the windings are equal - see where this is heading?

Yes, I already suggested that somewhere along the way, but someone who's obviously more tech-savvy than I am said it wouldn't work. I also believe the coil would act as a short-circuit, since it's just like a piece of wire...might try it tho

THIS ?

[sam_sam_sam]

Re: 150d bench power supply ammeter not working

Have you looked at the data sheet for the meter chip to see if it need to be a separate power supply or not

[Dannyx]

Re: 150d bench power supply ammeter not working

Not yet - didn't have the energy

[budm]

Re: 150d bench power supply ammeter not working

Looking at the meter display board, it uses simple half-wave rectifier, one of the transformer wire is connected to circuit GND which is connected to the main power supply GND, no isolation.
MK1841D3: http://poormanelectronics.blogspot.c...-858d-smd.html

[eccerr0r]

Re: 150d bench power supply ammeter not working

MK1841D3 is an OTP MCU, can't make assumptions about how it works. However, we don't need to, we just need to know what the reference point of the display board is.

I also did a rough schematic of how the thing is powered: on the left hand of that very dirty piece of paper we have the original setup: two separate windings. The right hand side is the current setup: a center tapped transformer. Can't really wrap my head around what's going on here. The dotted line indicated the instrument panel. The diode and 7805 regulator are on this panel. GND is common...yeah I know I used the earth symbol there, but you get the idea

So where are that red and black wire soldered on the back of the board connected to? Did that used to connect to that second transformer?

What I don't understand is why (originally) the 10V winding has one leg grounded as shown on the left side circuit. If it really was grounded then the right side circuit tap *should* work though a bit wonky; but my guess is that drawn connection is in error - please check. It should just be connected to the 7805 GND pin, but isolated from the main supply bridge.

In any case, that noise filter coil will not work. Yes I had to try it myself many years ago, but failed. It saturates real fast.

I can't think of an easy way to isolate the two without a separate winding unfortunately. May be SOL without redesigning the PSU board - which is equally as painful as adding another supply.

Find another wall wart to sacrifice?

[Dannyx]

Re: 150d bench power supply ammeter not working

Just realized that last night I wrote my reply just as the forum was undergoing maintenance and it scrapped what I THOUGHT I posted, so:

So where are that red and black wire soldered on the back of the board connected to? Did that used to connect to that second transformer?

Let's talk about those wires: the red/black wires are just a PC fan I added later on, so they're "leaving" the board so to say and don't really concern us. On the other side of the board there used to be a 2-pin plug which I removed and soldered in the yellow/blue wires for convenience. Yellow is negative and goes to common GND (negative leg of bridge rectifier), blue is positive and goes to the center tap in this case. In the original setup, yes, those two would go to that 10v winding.

To the right of that there's that 4-pin connector connecting the two boards together which is like a "data" connector which carries the V and I sense (ok, data is an overstatement but you get the point). You can't see it in the pictures I took, but just underneath the plastic connector is the pinout of that connector, as follows: V, I, GND and 5V (not sure about the order though), so yes, there IS a common GND going between the two and I'm also quite positive the schematic I drew is correct in this regard.

However this opens up yet another bit of mystery which is WHY there's another 5v wire there :| There's a 7805 regulator on the power board itself, so I think those 5v are coming into the instrument board for some odd reason and not the other way around. The reason I say this is because when I tested with individual supplies, another thing I tried was to power just the control board, without powering the display at all and it works just on its own: putting my meter's probes on the output and turning the pots adjusted the output up and down just fine, even though the display wasn't illuminated, so the main board is not reliant on any "external" power to work, so those 5v likely go into the display and not the other way around - it's for some other purpose... Cheers.

[stj]

Re: 150d bench power supply ammeter not working

hey, go to a railway/canal or abandoned building - wherever people just dump junk.
i'm sure you can find an old radio-cassette player or something.
any old 6-9v transformer will do.

[Dannyx]

Re: 150d bench power supply ammeter not working

hey, go to a railway/canal or abandoned building - wherever people just dump junk.
i'm sure you can find an old radio-cassette player or something.
any old 6-9v transformer will do.

I already have one of those. I got a box full of adapters and stuff, so I'm positive there's a transformer in there somewhere as well Trouble is there isn't much space inside the enclosure so I'm not sure it will fit....plus it's messier that way, though, of course, it may be unavoidable and be the only solution eventually, as I doubt there are other solutions at this point. The manufacturers wouldn't have adopted this dual rail supply setup for no reason if there HAD been a better solution. Ok, our hack-happy community out there really enjoys hacking stuff and might be able to come up with something

When it comes to junk, unfortunately we don't have any place where you can "dumpster-dive" around here, however we have something that comes fairly close, it's like a flea-market or swap-meet: think I mentioned this before somewhere before but the idea's there's a whole bunch of people who gather in this purposely-organized place and they bring any junk/stolen stuff they no longer need and/or want to get rid of. There's no rules and no regulations: sometimes you get a guy who barely speaks the language who gives away practically new stuff for free, while other times you get a chap who's selling junk for outrageous prices. Negotiating is highly encouraged obviously and sometimes you're able to score bargains, especially since most of these people often have no idea what they're selling and what that stuff normally goes for, like tablets, phones and so on....

[stj]

Re: 150d bench power supply ammeter not working

yes, car-boot sales.
we have those every sunday morning!

[eccerr0r]

Re: 150d bench power supply ammeter not working

To the right of that there's that 4-pin connector connecting the two boards together which is like a "data" connector which carries the V and I sense (ok, data is an overstatement but you get the point). You can't see it in the pictures I took, but just underneath the plastic connector is the pinout of that connector, as follows: V, I, GND and 5V (not sure about the order though), so yes, there IS a common GND going between the two and I'm also quite positive the schematic I drew is correct in this regard.

There can't be a common ground at this part of the schematic - this breaks isolation. If there was a link, then your hack using the center tap should have worked. Not only that, it would have been cheaper to simply put a larger heatsink on the 7805 (or bolt it on the case) and draw power off of the main winding if isolation wasn't needed.

[Dannyx]

Re: 150d bench power supply ammeter not working

There can't be a common ground at this part of the schematic - this breaks isolation. If there was a link, then your hack using the center tap should have worked. Not only that, it would have been cheaper to simply put a larger heatsink on the 7805 (or bolt it on the case) and draw power off of the main winding if isolation wasn't needed.

I however disagree and insist one of those wires is GND, further backed up by the silk screen I mentioned and a continuity reading. I shall open it up once again and snap some pictures - my mate uses it as it is at the moment....this is getting very strange

[budm]

Re: 150d bench power supply ammeter not working

Meter Bottom side board layout.

[Dannyx]

Re: 150d bench power supply ammeter not working

Yes, upon first glance that appears to be correct.