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I would like to replace the voltage and current analog meters on my Topward 2300 bench power supply with a newer digital one (DSN-VC288)Datasheet. Unfortunately, I'm very confused about how to wire it. None of the wiring diagrams address my particular scenario. For clarification, I've added two diagrams. The first describes the power supply using the existing analog meters, while the second is my preliminary wiring diagram.

Simply replacing the meters and keeping the same wiring won't work. Analog meters require no additional power to function while digital ones do. I'd like to power the meter from an existing LED indicator rail reducing the need for an external power supply (annotated as 9V meter rail).

Unfortunately, it's unclear whether it should be wired in an isolated or shared configuration. While they do share ground, it's difficult to determine if they have the same voltage sources, since they appear to be different rails. Moreover, the relative voltages add another set of complexities as to what is negative vs positive.

Topward Benchtop Power Supply

Combo Digital Ammeter Voltmeter

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Would you please post a link to the data sheet for the digital meter? It's difficult to tell what's happening from a block diagram. \$\endgroup\$ Commented Nov 28, 2022 at 0:42
  • \$\begingroup\$ Can you tell us the voltage and current ratings of that supply, and what rating shunt the digital meter has, if any? From the datasheet it looks like it would have a 10A shunt with an external one needed for higher current. \$\endgroup\$
    – GodJihyo
    Commented Nov 28, 2022 at 1:27
  • \$\begingroup\$ I would have thought you could find the answer to this just by googling, but no. There is one Youtube video in a language I don't understand and a number of web pages which are either very poor quality (IMHO) or are for automotive applications. \$\endgroup\$ Commented Nov 28, 2022 at 1:37
  • \$\begingroup\$ The rectifier bridge in the upper left corner appears to be drawn incorrectly. One of the diodes is drawn shorted, and the bridge is upside down. \$\endgroup\$
    – RussellH
    Commented Nov 28, 2022 at 2:00
  • \$\begingroup\$ The +9, -12 volt supplies are riding on the output positive rail. That means the 9V relative to the output negative is 9V+Vout. What is called 0V on the schematic diagram is actually Vout. Be careful where you connect the thin red, black wires. \$\endgroup\$
    – RussellH
    Commented Nov 28, 2022 at 2:46

2 Answers 2

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You've got other problems besides powering the digital meters.

The original ammeter is set up as a voltmeter across a shunt resistance made up of R23, R31, and R65 and R66 if those two are present in the model you have. This means you can't directly replace it with the digital meter as that will have it's own shunt resistor. You would need to remove the existing shunts and put the digital current meter leads where they were, but then the shunt resistance would be different and the current limiting circuit wouldn't work right.

And all that brings up another problem, these digital meters tend to use low side current sensing, the shunt that senses the current goes in the supply's negative lead whereas the existing one is in the positive.

If the meter you have works that way you will need to connect it the way the instructions show, with the thick black to the supply minus and the thick red to the negative side of the load.

Here's an example of a typical connection. See if that helps you out. image from linked site

image from linked site

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  • \$\begingroup\$ I don't think you can just remove the existing shunts as they are also connected to the base of Q03 via R22. I think this is some sort of current limiting feedback. When the current goes above a certain level, the voltage across R23, R31 is sufficient to turn on Q03 which, in turn pulls the base of Q02 down reducing the output voltage. A back of the envelope calculation suggests this happens at around 9-10 A. \$\endgroup\$ Commented Nov 28, 2022 at 1:31
  • \$\begingroup\$ @PeterJennings Yeah, you would be replacing them with the digital meter shunt but that would likely change the voltage at that point and cause the circuit to not function properly. I'll edit that part. \$\endgroup\$
    – GodJihyo
    Commented Nov 28, 2022 at 1:35
  • \$\begingroup\$ My power supply doesn't have R65 and R66. Is it possible R23 and R31 only turns on Q03 whereas R65 and R66 are shunt resistors only for the 23010 model? What's interesting here is that the R23-R31 are the same across all of the power supply models with different current and voltage outputs. Could it be the exisitng ammeter has a built in shunt? It would definitely simplify the manufacturing, allowing them to use standard gauges. It also should be noted that my ammeter/voltmeter has a trimmer for calibrating the ammeter. \$\endgroup\$
    – user148298
    Commented Nov 28, 2022 at 3:55
  • \$\begingroup\$ I forgot to mention that the 23010 model outputs up to 10 amps. Many analog gauges require a shunt for such high currents. Is it possible the ammeter's built in shunt is sufficient? \$\endgroup\$
    – user148298
    Commented Nov 28, 2022 at 4:40
  • \$\begingroup\$ @user148298 R23/31 form a 0.075 ohm shunt, in the 10A model they add R65/66 to make it a 0.0375 ohm shunt. The voltage across the shunt will be the current times that resistance, so for 3A and 0.075 ohms it's 0.225V. The existing ammeter is wired as a voltmeter (uA or mA meter with series resistance) to read that voltage. In any case you can't connect the digital meter there anyway as it is low side sensing, you would need to leave the existing meter circuit and connect the digital one in the negative output path as the datasheet shows, \$\endgroup\$
    – GodJihyo
    Commented Nov 28, 2022 at 5:23
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If I had to guess, I'd say that the thick red and black wires are supposed to be wired in series with the load. I suggest between the positive side of C10 and the + output terminal, with or without an additional shunt as required. The yellow "current sense" should be connected to the - rail. Without knowing the exact model I could be wrong. It looks as if you should be able to power it as you propose, but check with the data sheet

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  • \$\begingroup\$ Yes, if they hooked it up the way they are proposing it would likely short the output of the supply and possible burn up the meter shunt. \$\endgroup\$
    – GodJihyo
    Commented Nov 28, 2022 at 1:07
  • \$\begingroup\$ @GodJihyo I spotted that a well, but didn't comment on it. \$\endgroup\$ Commented Nov 28, 2022 at 1:30

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