DIY Modular Synth Problem - Voltage between power supply grounds

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the_fig
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DIY Modular Synth Problem - Voltage between power supply grounds

Post by the_fig » Sat Jan 09, 2021 5:52 pm

Hello, I'm fairly new here so please let me know if I'm violating any protocols.
I have a technical question regarding a modular synthesizer I'm building. I'm using Sam Battle's Kosmo format for my modules, and I'm using two Frequency Central Power Supplies for +12/-12/Gnd. The two power supplies are the same model built with the same parts, both driven by the same external power converter (120VAC to 12VAC).

The problem I have is that when I have modules plugged into both power supplies, there's a short circuit that fries one of the ground wires. From my own investigation I found that there is a potential difference of around +/-8mV (it fluctuates, but who really knows with my crappy multimeter) between the two power supplies' grounds (note this is the same whether or not modules are plugged into the power supplies). So because each module's face plate is grounded and the face plates of all the modules touch, the two grounds are connected and it creates the short circuit. My question is this: is there a way to make the ground from both power supplies equal, and if not, is there a way to make sure that things still work properly with this voltage across the grounds? (Also, when the modules' faceplates are not touching and I tap them together, there's a noticeable spark, which my intuition tells me means there's more than 8mV, but I don't really know)

My first idea was to place a very small resistor between the modules of one of the power supplies and the ground connection to the power supply, which would mean it doesn't create a short circuit because there's a load in between, and I think a 10R 1/4W resistor would work (P=V^2/R with V=0.008V and R = 10Ω means P = 0.0000064W, much less than the 0.25W that the resistor is rated for), but I think that would mess up the modules it's connected to (discharging capacitors would take longer and stuff like that). But then again, if the two grounds are connected, couldn't those capacitors discharge through the other ground?

Basically I'm out of my depth and would love some advice. Thanks!

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col
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Re: DIY Modular Synth Problem - Voltage between power supply grounds

Post by col » Sat Jan 09, 2021 6:27 pm

Some photos would be a good start. Have you tried it with a SMPS (power converter (120VAC to 12VAC) powering each PSU separately? Perhaps the PSUs are not designed to be run in parallel off of the same source?

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Re: DIY Modular Synth Problem - Voltage between power supply grounds

Post by guest » Sat Jan 09, 2021 6:43 pm

if the grounds are at different potentials, then there is a problem with the powersupply. when you say they are powered from "the same external power converter", does that mean the same type, or the same exact one? im assuming the former. when you say "fries the ground", does a wire actually burn up? what is the failure mode?

one thing you can do is unplug the external powersupplies, let them discharge for a bit, and then measure the resistance from the input pins to the output pins of the external powersupply. this should be infinite across all combos of pins (between inputs and outputs). a small leakage in the external powersupply could cause a ground issue. the other thing to check for, is whether the cases of the two synths are connected to anything else. unplug all cables except for the powersupply and measure the voltage difference. then jumper between the case grounds with a 1k resistor, and measure the voltage drop across that resistor.
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Don T
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Re: DIY Modular Synth Problem - Voltage between power supply grounds

Post by Don T » Sat Jan 09, 2021 8:09 pm

You haven't inadvertently connected the two power supplies in series, have you?

the_fig
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Re: DIY Modular Synth Problem - Voltage between power supply grounds

Post by the_fig » Sat Jan 09, 2021 8:16 pm

Thanks for the feedback. I was messing around with things and noticed something. At the time I thought it would be inconsequential, but I found the problem. I had flipped the wires into one of the power supplies, so it was getting an inverse sine wave. My thinking at the time was that this wouldn't matter because they're both outputting DC voltage, but I guess not. Anyway, I flipped the wires and now everything works perfect! So problem solved.

jamos
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Re: DIY Modular Synth Problem - Voltage between power supply grounds

Post by jamos » Sat Jan 09, 2021 8:24 pm

No offense, but this is really screwed up.

Can you provide a schematic of the system? If you cannot, again no offense, but you should NOT be working with power supplies at this level. AC power is DANGEROUS and if your first attempt to build this resulted in sparks and burned wires, you have crossed the line. Buy a commercial power system and stop risking harm.
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jingo
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Re: DIY Modular Synth Problem - Voltage between power supply grounds

Post by jingo » Sun Jan 10, 2021 2:46 pm

Probably aiming for a darwin award...

the_fig
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Re: DIY Modular Synth Problem - Voltage between power supply grounds

Post by the_fig » Sun Jan 10, 2021 7:01 pm

hi jamos, here's the schematic for the power supply I'm working with Image
I realize that this stuff can be dangerous, but I never work with the full 120V, using a commercial transformer to step down to 12V.

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Re: DIY Modular Synth Problem - Voltage between power supply grounds

Post by Elahrairah » Wed Jan 13, 2021 8:00 pm

So you said there are two of those, and one had the AC reversed?

And that led to a ground potential difference? That's not what I would expect. Makes me want to build same and test it...

nigel
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Re: DIY Modular Synth Problem - Voltage between power supply grounds

Post by nigel » Wed Jan 13, 2021 10:01 pm

the_fig wrote:
Sun Jan 10, 2021 7:01 pm
hi jamos, here's the schematic for the power supply I'm working with [...]
See this thread. That schematic is incorrect, and does NOT match the circuit on the PCB. On the PCB, one of the AC inputs is connected to 0V, the other to both diodes. If you are trying to drive two of those from a single AC supply, then you will need to ensure that the same input is connected to 0V on both supplies (as you discovered).

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