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Panel meters arrive!

The panel meters for my DIY bench top power supply have arrived from china. One is a 20v voltmeter and the other is a 5A ammeter. I ordered them from ebay and at a cost of around £6 each, they are far cheaper than anything available locally.

On unpacking the meters i found this rather interesting note:

(excuse the crappy photo)

It seems that the meters require fully isolated power supplies. It may be possible to power them from battery but then i have to mess about with relays switching them on and off and changing the batteries. Instead i hunted around and ordered some of these DC-DC modules. They are XP-Power IA1209S modules but any dc-dc module with between 9v and 12v isolated output will do. I chose these because they have a 12v input, 9v output and are cheap. They have dual outputs but this just means that they provide a -9v output as well as the 9v output. Obviously the -9v output isn’t needed.

Panel Meter Power Circuit

Parts List:

  • 2 XP-Power IA1209S dc-dc modules
  • Small piece of strip board (also called vero board)
  • Solder
  • 6 lengths of multi-core wire (7/0.2mm core)

Tools Required:

Only basic tools are required for this circuit. These should form part of any electronic engineers arsenal, as discussed here. I just used the breadboard as an extra step

When they arrived I tested them on breadboard first:

I’ve drawn up a nice schematic to show how simple the wiring is:

Remember to cut the tracks on the board between the two supply’s outputs if you’re using strip board.

This time I powered up the circuit using the jump starting trick i mentioned in my previous post and tested the output using a multimeter. The output read 10.5v but there was no load on them so this isn’t unusual. After wiring up one of the panel meters, the output voltage dropped to a more sensible 9.3v

Notice that i have a power resistor (large white block) to load the supply and ensure i get a nice consistent dc source. This is sitting between 5v and ground and is 10Ω. This gets quite hot so until i can give it a heatsink and some airflow, i’m only running the supply for short periods.

Next i soldered the dc-dc modules to some strip board in the layout shown in the schematic above. Remember to cut the tracks between all the outputs of the dc-dc modules, you want those outputs to be completely isolated.

The result is some nice blue:

You can see the circuit in the top right, just below the ATX cable connector. I used a salvaged piece of stripboard from my earlier mishap, hence the additional yellow wire to bridge previous cuts. The breadboard is just to help distribute power and to hold the power resistor.

My next step is making the aluminium box that all this will go into. After that it won’t be long until this project is complete

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  1. September 16, 2010 at 9:49 am

    I just had a thought. You can probably use a dual output 5v isolated supply as this will give you -5v, 0v and 5v. Connecting between -5v and 5v should then give you 10v, which is perfect for driving these meters.

    5v dual output regulators are much easier to come by and you should be able to get them from most electronics shops such as maplins and rapid online.

    If you choose this route, let me know how you get on.

  2. Christian
    November 17, 2010 at 2:12 pm

    i just bought a pair of LCD panels for this project and i found as well those isolated power modules but they are way too much expensive, > 10 uss for each one, its more expensive than the LCD panels them-self, so im gonna try with batteries i guess 😦

    • November 17, 2010 at 3:16 pm

      I’m in UK not USA but a quick search on digikey.com brings up a product for $5.15 each. Digikey part number is 945-1104-ND. This would need to be powered from 5v not 12v, but other than that it seems identical

      I hope this helps

  3. December 11, 2010 at 12:57 am

    I was looking into the panel meters, and found some that look identical on eBay. Although, most of them have a measuring range of about 7.5 to 20v. Does this REALLY mean I won’t be able to measure anything below that minimum? What are yours?

    • December 11, 2010 at 11:36 am

      You need to shop around a little more. The ones that measure 7.5 to 20v are designed to measure the supply that they are running from. They are generally used in boats and trucks to measure the voltage from the battery/alternator.

      I bought mine from asia-engineer

  4. March 6, 2011 at 9:00 am

    @Mike I found exactly what you are looking for:
    http://cgi.ebay.com/Mini-Blue-LED-DC-100V-Digital-Volt-Voltage-Meter-/350261423350?pt=LH_DefaultDomain_0&hash=item518d352cf6#ht_1193wt_1139

    that will measure from 0v-99.9v

    I am getting that and also the one to measure amps from that seller.

  5. heddy
    March 29, 2011 at 11:06 pm

    Can you hook the DC-to-DC converter directly to the DPM, or do you need filter cap/choke across the output?

    Thanks.

    • March 30, 2011 at 8:34 am

      You can hook them up directly

  6. Ryan
    April 13, 2011 at 12:11 pm

    if the DC-DC converters are connected to the power supply, isn’t that coming from the same source as panel meters?

    • April 20, 2011 at 11:27 am

      the DC-DC converters here are used as voltage isolators. They output 9v relative to their common pin which can be any voltage relative to the power supply ground (up to 1000v difference). This means that you could hook the common pin to a 200v source (relative to ground) somewhere and the 9v pin on the DC-DC converters would give you 209v relative to ground.

      Its important to think of voltage as a potential difference, you measure the potential difference between two places. We usually measure potential difference relative to ground but we don’t have to, for example on a battery the -ve terminal is usually used. A battery would have a similar effect as these DC-DC converters. If you get a 9v battery and hook the negative terminal to 12v dc (relative to ground), you will then be able to measure 21v on the +ve terminal relative to ground, but you would still only get 9v relative to the negative terminal. Similarly, if you hooked up the +ve terminal to 12v then you would measure 3v on the -ve terminal (relative to ground).

      I wasn’t sure if you already understood that but thought it was relevant anyway.

      • April 20, 2011 at 11:31 am

        In fact you can actually use a 9v battery in place of these DC-DC converters, but i chose not to since i don’t like the idea of disposable batteries (environmentally wasteful), especially when they’re not needed.

  7. Anon
    January 20, 2013 at 2:58 am

    Why not connect both meters to one DC-DC convertor? These things hardly draw any power.

    • January 21, 2013 at 8:55 am

      Each meter needs to be electrically isolated from the other as they each measure current/voltage from different points.

  1. November 26, 2010 at 2:04 pm

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