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PSU safety tips for DIY Eurorack?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author PSU safety tips for DIY Eurorack?
ub1k
Hi there, this is my first post on here!

I recently got into modular and after building my first kit and protoboarding another schematic, I realized that real fun was in breadboarding and playing with tuning existing designs. I had already some basic knowledge of electronics (mostly digital) but through experimentation, these las weeks, I have managed to learn quite a few things about signal processing and opamps. My journey was being nothing short of glorious hyper until this morning, when I realized my PSU had stopped working. It's one of these Frequency Central things (http://www.frequencycentral.co.uk/?page_id=1911) that require a 12VAC wall wart (which was not an easy thing to find, BTW). Turns out that the transformer is dead. Dead Banana

I have not yet understood what I did wrong (maybe some short circuit?) but I was wondering whether someone else has experienced this in the past and, if you're working on your own DIY modules, how you can prevent things of the sort from happening. I guess that this PSU being DIY, it probably lacks some protection circuit?
All tips will be very much appreciated! help
sduck
Protip: before plugging in anything you've built, check for (lack of) continuity between the power rails. It's one thing to have to troubleshoot a newly built module, but if you have to replace the PSU to do so, it's twice as much bother.

By transformer, do you mean that the wallwart is dead? Have you tested it with a DMM? If you need to replace it, get a cincon - TRH70A120-12E01 - they're fairly bulletproof. i have a bunch of them. They power my TTSH, DDRM and numerous other toys.
ub1k
Thanks for your answer. I read that continuity tip before but if I'm prototyping something on the breadboard it's quite easy to mess things up. If I'm soldering something on a PCB I will obviously test before connecting.

By "transformer" I mean the one inside the wall wart. I tested the resistance between the primary terminals, it's an open circuit. I suspect that the thermal fuse inside it is gone. The adapter you suggest looks nice but it's DC and thus PSU requires an AC-AC adapter, with 12Vrms output.
Thanks once again!
KSS
You probably did nothing really wrong to have this result.

This type of WW fed supply is sensitive to uneven draw from the positive and negative rails. Try to keep them as equal as you can. Including using a load resistor on the less loaded, usually negative rail, to increase its draw to more closely match the positive rail.
Avoid using the positive rail to support a five volt rail.
WW transformer excessive heating and failure is not uncommon when these steps are not followed.

sducks advice to use a cincom is worth pursuing.
ub1k
Thanks KSS, it's good to know that I probably didn't do anything too wrong.
As for the Cincom, have you seen my reply to sduck? I need 12VAC, not DC.
degeneratedsines
KSS wrote:
This type of WW fed supply is sensitive to uneven draw from the positive and negative rails. Try to keep them as equal as you can.

Do you have any other source or reference on this? I have never seen this stated anywhere and that sounds crazy to me (but I am not very knowledgeable either).
infinitemachinery
degeneratedsines wrote:
KSS wrote:
This type of WW fed supply is sensitive to uneven draw from the positive and negative rails. Try to keep them as equal as you can.

Do you have any other source or reference on this? I have never seen this stated anywhere and that sounds crazy to me (but I am not very knowledgeable either).


I curious about this too. It seems to me as long as you stay with the current limits of the power supply there should be no issues using one of the rails for a 5 volt suppy.
snercle
i always send the power rails through 10 ohm resistors when prototyping so that if i short +/- it only kills the components on the board Dead Banana
cygmu
degeneratedsines wrote:
KSS wrote:
This type of WW fed supply is sensitive to uneven draw from the positive and negative rails. Try to keep them as equal as you can.

Do you have any other source or reference on this? I have never seen this stated anywhere and that sounds crazy to me (but I am not very knowledgeable either).


If you look at the schematic of a typical wall-wart based PSU such as the MFOS one:
http://musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/wallwart _added_load_rs_schem.pdf
and consider what happens when the +12V and -12V rails carry unequal currents, you will see that the difference in the two currents has to flow in the transformer windings.

My understanding is that this DC in the windings is bad for the transformer, and can cause it to heat up, or fail in other ways. But I've got that from reading posts in this forum (such as this one: https://www.muffwiggler.com/forum/viewtopic.php?p=2223054&highlight=#2 223054 ) rather than from anything more definitive.

I think the reason KSS warns against powering the 5V from the 12V is just because that will tend to generate exactly this kind of imbalance.
Graham Hinton
cygmu wrote:
If you look at the schematic of a typical wall-wart based PSU such as the MFOS one:
http://musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/wallwart _added_load_rs_schem.pdf
and consider what happens when the +12V and -12V rails carry unequal currents, you will see that the difference in the two currents has to flow in the transformer windings.


Eh?
The WW voltage is AC and current only flows through CR1 or CR2 when the voltage exceeds the reservoir voltage plus a diode drop. That will be less than half the half cycle that each half wave rectifier has available. So it is most likely that the WW was underrated and overheated. The WW AC current rating should be more than 2 x root 2 x maximum rail current and then some. I bet it wasn't.
cygmu
Graham Hinton wrote:
cygmu wrote:
If you look at the schematic of a typical wall-wart based PSU such as the MFOS one:
http://musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/wallwart _added_load_rs_schem.pdf
and consider what happens when the +12V and -12V rails carry unequal currents, you will see that the difference in the two currents has to flow in the transformer windings.


Eh?
The WW voltage is AC


Maybe I have mis-thought this. But let me try again. Suppose we draw 100mA from the +12V line and nothing on the -12V. I believe that this means that an average of 100mA must be flowing in the transformer windings. It will be in bursts because it only flows when the caps ready to receive charge through the diode, but the average over time must be that 100mA, or perhaps it is better expressed as 1 coulomb every 10 seconds.

That is the part of this that I think I understand properly. If I am wrong then I would like to know what I've misunderstood. The follow-up claim which matters here is that this net charge flow in the transformer windings is bad for the transformer. For this all I have are posts on this forum such as the above by KSS and the one I linked by ElectroSteam and several others by that user. It would be great to get to the bottom of this in a convincing way.
Mungo
cygmu wrote:
Maybe I have mis-thought this. But let me try again. Suppose we draw 100mA from the +12V line and nothing on the -12V. I believe that this means that an average of 100mA must be flowing in the transformer windings. It will be in bursts because it only flows when the caps ready to receive charge through the diode, but the average over time must be that 100mA, or perhaps it is better expressed as 1 coulomb every 10 seconds.

That is the part of this that I think I understand properly. If I am wrong then I would like to know what I've misunderstood. The follow-up claim which matters here is that this net charge flow in the transformer windings is bad for the transformer. For this all I have are posts on this forum such as the above by KSS and the one I linked by ElectroSteam and several others by that user. It would be great to get to the bottom of this in a convincing way.
How about a thesis on the matter:
https://eprints.usq.edu.au/75/1/AshleyKarlZEIMER_-_2004.pdf
Even if you can't access the references it covers the details of why DC bias is bad in a transformer. Their example measurements of a 1200VA transformer are very interesting.
ub1k
Coming back to my original question, if I just added a resettable fuse to one of the secondary's terminals would that at least make it less likely that the transformer is damaged in case of a short circuit?
Graham Hinton
ub1k wrote:
Coming back to my original question, if I just added a resettable fuse to one of the secondary's terminals would that at least make it less likely that the transformer is damaged in case of a short circuit?


A real fuse would be better. Those polyfuses have a weird characteristic and they trip by current x time. The manufacturers only show part of the curve, but it extends beyond that shown. The outcome is that you may find them tripping after conducting the current you want for an hour or so.

Transformers need protecting against rectifiers and reservoir caps going short circuit and when that happens you need to repair the pcb, not just fit another fuse.

What was the current rating of the WW you blew and what current were you drawing from the PSU?

cygmu wrote:
Suppose we draw 100mA from the +12V line and nothing on the -12V. I believe that this means that an average of 100mA must be flowing in the transformer windings. It will be in bursts because it only flows when the caps ready to receive charge through the diode, but the average over time must be that 100mA, or perhaps it is better expressed as 1 coulomb every 10 seconds.


The WW is rated as AC volts rms and AC amps rms which give a continuous power, but it is not being loaded continuously. Multiply the AC volts by 1.414 (root 2) to give the peak voltage that the caps will be charged to, less a diode drop. Divide the AC amps by 1.414 to give the maximum continuous DC current available, but you won't get all of that because the diodes are not conducting all the cycle.
The charge put into the reservoir caps per cycle has to equal that taken out on average. Transient demands will change the voltage on the caps which is a distorted sawtooth where the lowest voltage has to remain above what the regulator requires.

We don't have to find obscure effects to blame when the basic design is questionable.
ub1k
Quote:
What was the current rating of the WW you blew and what current were you drawing from the PSU?


The WW is 1A (https://www.poweradaptorsuk.co.uk/1000ma_12v_AC_AC_Power_Adaptor_p/ta 100012ac.htm).
I don't know how much current I was drawing exactly. I was trying to make a DIY lofi echo effect, so I had a PT2399 connected to 5V and then an opamp (one of these russian things used in the Polivoks filter) to the +-12V rails. The plan was just to amplify the signal a bit so that I could hear it through the headphones.
Graham Hinton
ub1k wrote:

The WW is 1A (https://www.poweradaptorsuk.co.uk/1000ma_12v_AC_AC_Power_Adaptor_p/ta 100012ac.htm).


That is rated at 12VA. That means the volt amp product and is given that way instead of watts because there may be a phase difference. You only get a maximum of 12W when the voltage and current are in phase.

Allowing for the typical 50% efficiency of linear regulators and the half cycle that leaves less than 1.5W for the positive and negative rails, but it should have been enough for what you were doing provided that the circuit had no fault.

Quote:

I don't know how much current I was drawing exactly.


There's the rub. You would be better off using a bench power supply for development to get the overcurrent protection and current monitoring readout and only use a cheap PSU when you know it is working and how much power it needs. Having a bare PSU on your bench while cutting wires is not a good idea.
ub1k
Quote:
Having a bare PSU on your bench while cutting wires is not a good idea.


Then I guess I should buy a dual bench PSU if I want to do any prototyping at all. If you have any suggestions that won't mean selling any of my organs, I'll be very thankful Mr. Green
Sin_Phi
Others may know better, but I expected to find a budget bench supply everyone used like how Rigol or Siglent are for their scopes. There doesn't seem to be one really. I haven't gotten it yet, but my intent was to get one of the 3 supply ones from here https://www.newark.com/c/test-measurement/bench-power-supplies-sources -loads/bench-top-power-supplies?brand=tenma for about $200 USD. There seem to be a lot of clones and re-badges on amazon and ebay that all have mixed reviews in the budget market with concerning behavior like coming on at full voltage.

Was looking at this https://www.newark.com/tenma/72-13610/dc-power-supply-adj-fixed-335w/d p/56AC4684 or https://www.newark.com/tenma/72-8695a/power-supply-3ch-32v-3a-adj-fixe d/dp/47X1650 which I have seen similarly priced clones of.

I used to DIY power supplies. Haven't had any truly bad experience, but it gave me enough anxiety to just trust that someone who has their product certified knows better than I. Feel better with a bit of assurance that I will just be blowing up ICs and not my hands.
Graham Hinton
ub1k wrote:
If you have any suggestions that won't mean selling any of my organs, I'll be very thankful Mr. Green


There are always old Farnell ones on eBay at reasonable prices. Old Hewlett Packard models are better and fetch a little more, but still reasonable.
fuzzbass
If just building and testing modules, there are some inexpensive alternatives. You don't need high accuracy or power, but internal protections are necessary. Both of these have tolerated dead shorts between ground and a power rail, with no damage.

1. Used HP 6236B. Triple supply but the +/- 0-20V outputs only provide 500ma each. No current limit function, but if you go over 500ma, output clamps down to near 0V. No frills and incandescent meter lamps are usually dead. Current readout on mine is not accurate. Built like a small brick house, nonetheless. Takes a lickin' and keeps on tickin'.

2. Mutable Instrument Module Tester. For Euro - only +/-12V on 2x8 .1 pitch header, no current indication, but probably good for ~400ma. Provides audio and cv signals to inject for testing basic functions. This one uses the same 12VAC wall wart as mentioned in OP.
ub1k
I also read about the Mean Well RT-65B (https://www.mouser.ch/ProductDetail/MEAN-WELL/RT-65B), which seems to be protected against overload and short circuit. Still, it would be nice to have some kind of indicator that either of those is happening.
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