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I opened up the power supply to a HP printer and I see that the output has three pins. I measured voltage on one pin to ground and it shows 8.3V.

The adapter says: Output 24V @ 1A. Is this a dual rail power supply?

Why don't I don't see 24V on the output?

The power supply is fine as it powers the printer.

Here is the photo:

enter image description here

JRE
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Grepsoft
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  • Most probably. Any markings? Two distinct diodes and capacitors after the transformer? Add some load on both rails and measure again. – winny Jun 24 '20 at 17:23
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    You should be able to trace out the circuit fairly easily as there doesn't seem to be much on the low-voltage side of the transformer. – Transistor Jun 24 '20 at 17:31
  • no specific markings but just component markings. What's the second transformer for? – Grepsoft Jun 24 '20 at 17:38
  • What is the original model number for the power supply? Or printer? What text is hidden behind the red square you drew? It might reveal something. One of the pins might be for sensing voltage, or setting voltage, or simply turning the thing on. There is also only one transformer in the picture, the other thing that looks like a transformer is common mode choke for filtering mains. – Justme Jun 24 '20 at 19:08
  • after Adam Lawrence answer i took a closer look at the connector and the middle pin says 'mode conn' or something, The right pin says GND and left one VH. I am assuming the mid pin probably selects between different voltages. – Grepsoft Jun 24 '20 at 21:10

1 Answers1

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I found a thread on BadCaps.net Forums discussing a different printer with this specific power supply. The schematic was user-generated so YMMV.

Schematic from BadCaps.net forums

Pin 1: +24V

Pin 2: On/Off Control (marked as ON/OFF 3V)

Pin 3: GND

The on/off pin is coupled to an N-channel MOSFET which gets tied into the main feedback 431/optoisolator circuit. If the MOSFET is off, the voltage looks like it would be set to around 8.5V. If on it adds parallel resistance to the divider and the output would be around 24.3V, so this makes sense.

It most definitely is not a dual-rail output. And there is only one transformer (the coil near the AC connector is a common-mode EMI inductor).

Adam Lawrence
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  • Is there a specific reason for doing this? – Grepsoft Jun 24 '20 at 18:55
  • Specific reason for what? The on/off control? Most likely the 8.5V setting is a low-voltage standby mode to allow the printer to be awoken remotely on-demand, or something like that. The higher voltage is probably only used by the printing and motor elements, the logic would be lower voltage. – Adam Lawrence Jun 25 '20 at 12:42