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First of all I apologize, I am not an electronics expert, just fairly good at it, but now I am faced with this weird problem that I can't figure out.

I needed to replace a transformer on a power supply. Having no info on the one previously used, I tried to guess using the following schematic:

power supply schematic

The specifications for TP14 and TP15 are +35V and -35V (DC). The transformer seems to have a central tap on the output coil, providing two outputs of opposite phase on J22 and J23. I found a used toroidal transformer, don't know much about this one either, but it's the right input voltage and power rating, it has three output wires (one black and two blue) and on the output it says "36V-0-36V BLU-BLK-BLU". So I powered it with nothing attached to the output, measured 36V (AC) between the black and each blue wire and roughly double that voltage between the blue wires, which makes me think they have opposite phases. I made all the right connections in the circuit and now on TP14 and TP15 I get a reading on +50V and -50V (DC). It's pretty concerning, because 50V is the working voltage of capacitors C67 and C70. Got away without blowing them, but can anyone please explain what am I missing here?

I should mention, the transformer also has another secondary output (14V-0-14V YEL-BLK-YEL) connected to the same circuit board (image below, one yellow and one black on J11 and J8). But this is only used in a simple DC converter to send power to an external board. Could this have any influence?

FX power supply

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2 Answers 2

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There is a disconnect in your information. TP14 and TP15 are at the output of a rectifier/filter capacitor network. Therefore, there should be DC voltages at these points which is implied by the schematic (+40V and -40V). These voltages also imply that the transformer output (on each side) is about 28 VAC (40/1.414). You have applied 35VAC and you got 50VDC which makes sense (35X1.414=50). The result is that the voltage across the dropping resistors (R95 and R96) will increase to 50V-16V-34V. Then the power dissipated in each resistor will increase to 34X34/270=4.3 watts. This is probably too close to their rating of 5 watts. Thus using this transformer stresses the filter capacitors and the dropping resistors. Hence it is not recommended.

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  • \$\begingroup\$ Yes, thank you, I was too tired and wrote AC instead of DC, now it should be correct. I admit I don't have much experience with bridge rectifiers. I just figured the DC outputs should be similar in voltage to the AC outputs, maybe even a bit lower due to voltage drops across the diodes. So this math formula is new to me... Looks like I need to do some research... \$\endgroup\$ Commented Oct 5, 2023 at 22:49
  • \$\begingroup\$ The smoothing capacitors, C67 and C70 will charge to about the peak of the input sinusoidal voltage. However the output of the transformer is specified as the RMS voltage which is 0.707 of the peak. Note 1.414 = square root of 2 and 0.707 is it's reciprocal. If you need to replace the transformer then you need to look for one about 28V-0-28 V RMS which will give you +/-40V DC at TP14 & TP15. \$\endgroup\$ Commented Oct 5, 2023 at 23:17
  • \$\begingroup\$ All right, things are starting to make sense now. Thank you! \$\endgroup\$ Commented Oct 6, 2023 at 1:49
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Having explained where the 50V comes from in the other answer and comments there is another point you might consider when replacing the transformer.
The design is such that the drop across R95 & R96 is excessive IMHO, 60% of your power is being thrown away in them all the time. With no load there is nearly 90mA flowing through each resistor and the 2 Zenner diodes. Assuming you need this 90mA to service the full load condition, a much lower output transformer would suffice. 14V-0-14V would give you about +/-20V at the TPs. The two resistors would have to be reduced to 43 ohms to maintain 90mA ((20-16)V/90mA) but they only dissipate .36W ((20-16)v * 90ma) so use 1W to be on the safe side. This rating does not take into account a short circuit condition on the output, but neither does the original design.

Unfortunately I don't think you would be able to use one side of the same 14-0-14 windings for the 10V supply as one of either J11 or J8 would be connected to J13 on the transformer and this is grounded, thereby shorting out half the D31-D34 bridge rectifier.

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  • \$\begingroup\$ There doesn't seem to be a connection between the black wires on the two outputs, I think they are just center taps on two separate outputs. The +-40v, although not shown in this schematics, are also being used for power amplification and the +-16v are used for powering some op-amps. If I understand the math correctly, the original transformer had 2 outputs, one 56v with center tap (for 28v outs with opposite polarity) and one 7v. So hard to find. Maybe I will try to hack this one, the average between 14v and 36v is 25, which could give me the 35v output, I'll sleep on it. \$\endgroup\$ Commented Oct 6, 2023 at 2:11
  • \$\begingroup\$ @ZenMax Yes, your existing transformer has two separate windings, one 28-0-28 (which appears to be faulty) used for the +/-16V outputs and a completely separate, isolated 14-0-14 winding, one half of which is used to generate the +10V output. As such the two outputs do not interact and only share a common earth (0V) rail. Is there room in the case to add the replacement transformer (whatever output you decide upon) so that it can power the +/-16 and leave the original transformer to just do the +10V. The old, faulty 28-0-28 windings can be disconnected and taped off. \$\endgroup\$ Commented Oct 6, 2023 at 11:32

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