Electronics > Repair
Question about Dual supply power supplies and shorting
<< < (3/3)
shockpoint:

--- Quote from: Kleinstein on September 25, 2024, 02:31:46 pm ---Some "leakage" from the negative side to the positive side is not a big surprise. It is kind of expected to a low level, e.g. for power a few OP-amps, of a resistive divider.

Depending on how important and complicated the ciruit is, it may be worth  doing at least a crude reverse engeniering.

Reverse diodes at the supplies (input and / or output) would not be for regulation, but for protection, e.g. when one half drops out or just during turn on.
The weaker half would be the one with the lower current limit, intentional or just from the tolerances. If both outputs are current limited and the +12 V are shorted to the -12 V one of the regulator will go into current limit and the other voltage could cause the wrong polarity for that half. Diodes could limit at least the voltage in the wrong direction and this way limiting / preventing additional damage from reverse voltage.

--- End quote ---

Kleinstein

Yes the digital piano's modular power board has those elements that you have mentioned
the current limiting is 400milliamps - I suppose that's why the fuses (M205 slow blows) blew on both the -12 and +12 rails
there are reverse diodes at the outputs of the voltage converters tied to GND for each of the regulators for -12 and +12
I have disconnected that module and am assessing the op amps where the short seems to be

There was only one op amp that was lighting up on the thermal camera

What doesn't quite make sense at this stage is 24V across the effective resistance of the short (128ohms) would only actually produce a current of 0.1875amps - considering my fuses were rated for 400milliamps I am not sure why they would blow
maybe I am understanding this wrong
I confirmed the draw the op amp - with injection of power at 1V and limiting at 1A, the resistance of 128ohm was causing a current draw of 0.007A
Kleinstein:
The current drawn from a semiconductor circuit may well be nonlinear. So one can not extrapolate from the 1 V measurement to 12 V. The 128 ohms could be closer to a diode and than maybe 40 ohms of resistance.

There is also a chance a chance the the "short" that blew the fuses was temporary, like a latch up event that can clear once the power is removed.
shockpoint:

--- Quote from: Kleinstein on September 26, 2024, 07:01:35 am ---The current drawn from a semiconductor circuit may well be nonlinear. So one can not extrapolate from the 1 V measurement to 12 V. The 128 ohms could be closer to a diode and than maybe 40 ohms of resistance.

There is also a chance a chance the the "short" that blew the fuses was temporary, like a latch up event that can clear once the power is removed.

--- End quote ---

Thank you - this makes sense.

An update - I removed the faulty op amp and the 125 ohm reading is gone. I think the op amp was letting current through between Vcc and -Vcc

Waiting for a new LM833 op amp now and I will report back.

I took some time to learn how to use a hot air gun without blowing away everything else on the board
Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod