Panel Pooper

Institutionalized Dumpster Dive

Over the weekend, I had occasion (in other words I made an occasion) to visit A1 parts and surplus in Etobicoke. I have mentioned it before as a candyland for the junk enthusiast and this is essentially correct. Despite its remote location, it is well worth the visit, just be prepared to dig and expect no help from the rather cantankerous guy at the counter. Total comic book guy for the electronics store which I find endearing rather than offensive. Anyway, if you ever end up there, be prepared to wade through tons of unsorted junk, find a few gems, and score a few surplus deals.

For my part, I scored a few critical components for my dummy load, which is so close to construction I can taste it. I found a 4PDT switch in a box marked simply “switches” and got it for probably half the price it’s worth if I was to ask the dude at the counter for it. I also finally got my panel meter.


Panel meter pains

I was excited, thinking the last hold up with my dummy load project was behind me. I had the Velleman PMLDCDL for $10 and it would be good enough for my purposes. Almost.

Today when I got home, I tested it with a 9V battery to confirm function, all was well. I then whacked on an 78L09 onto the dummy load breadboard to power it and hooked it up as it would be in the final product only to be greeted with “-1″. Poop.

I knew pretty much straight away what the problem was. “-1″ means “over-limit” and I know for a fact I was generating 100mV for it to measure from the 10kΩ trimmer (Iset). The one problem I knew might happen and did: this meter cannot be powered from the same power supply it’s trying to measure.

This is essentially because it’s a cheap shit meter that doesn’t have a bit of extra circuitry included in it to allow it for use in common-ground applications. I had run across this problem before once, when I was building my own voltmeter using the ICL7107 DVM IC and was able to fix it with a jumper, because the chip had provision for that. This one, unfortunately does not.

Stupid thing is, the guy at A1 never heard of anything to do with common mode problems in panel meters even though it’s quite a familiar reality. Also, the package and instructions for the meter itself said nothing about it needing to be powered from a separate supply.

Options and a solution

There is a solution for everything in electronics. Usually one is presented with a number of them. Some can be ruled out for complexity, scarcity of critical components, cost or whether or not one gives a shit. It’s part of it’s charm.

The following options came to mind or came to my mind after a quick google:

  • Power it separately using a 9V battery
  • Power it separately using a 2nd 9V wall wort
  • Add an inverter and rectifier to isolate a supply for the meter
  • Buy another panel meter online that can handle a common ground
  • Internalize the power supply and provide two, one for circuit and one for meter

One can see pretty quickly that these options are quite annoying, especially considering that I thought I was almost done for chrissake! The first option, using a 9V battery is just … stupid. Not only stupid, but lame and annoying. I definitely do not want to deal with batteries dying, replacing batteries and the whole mess of batteries in general. Screw it.

Option two is likewise lame and annoying. Why do I want to plug one box in using two wall worts. No way man.

Option three involves using a sort of switchmode inverter to convert it to AC at a fairly high frequency, then rectify it back to DC and regulate it to 9V. Talk about complex! Sure, I could get the frequency going with a 555 timer easy enough, use a mosfet for switching, a transformer to… I don’t have an appropriate transformer. Screw that then. It’s just too complicated and unnecessary for just powering a tiny panel meter.

The fourth option is just plain… annoying. I just went to the ends of the earth (which is Etobicoke if you don’t drive a car and rely on public transit as I do) and now I either have to go back there, or order online and wait… nope.

The fifth and last option sprung forth an idea – I can do this, and not only that, I can do this with stuff I have.

Frankenstein’s power supply

I was currently powering my circuit using a Panasonic 12V 200mA linear wall wort. It actually put out about 16V which as I mentioned before actually works out better for me. I had previously used a 9V 210mA linear wall wort which, as I had noted, proved insufficient voltage wise to give my opamps their swing.

I thought, why not put them both inside the case and have one wall plug externally to the mains. It’s simple to wire them in parallel and then I have my two power supplies, one plug. Brilliant, simple and FREE.

I thought of many mounting solutions, the easiest of which is to strap them to the inside of the case, and use crimp female mains sockets to get wiring to the mains wiring and power switch etc. Eh, kinda dicky. Reeking of lameness, I can do better.


With some swearing, sawing, hammering and dremeling, I was able to open the wall worts up and free their boards. Those things are a pain to get open by the way, I believe deliberately. Bit of advice – chisel on the seam, a good whack or two of the hammer, repeat for all sides, and they split right open. Some anyway, the Panasonic one was plastic-welded shut, that needed a-dremeling.

I mounted them using cable ties onto a piece of plastic perfboard and added standoffs and tied all the wires down. DONE.


As you can see from the photo, it’s nice, neat, fits in the case which had lots of room anyway and no messy wall worts to deal with. Given that I quite literally have a milk crate full of surplus wall worts, I doubtless will use this solution again and again.

Questions and next steps

I did manage to eliminate the mains pins by means of my linesmans pliers (essential bit of kit that!) as they were pokey-upy-shocky bits that I didn’t want exposed inside the case. The solder lugs on the transformers are adequate and I can hook up the appropriate wiring tomorrow. I will wire them in parallel and have a pair of wires heading off to the mains connector I plan to install on the case. Will use a standard IEC one so I can also use up my huge collection of IEC power cables.

I do wonder if the transformers, mounted rather close together, will have an effect on one another from their magnetic fields. I will have to run some tests to make sure I’m still getting what I need from them and if not, look at spacing them out some. Something tells me I will not have issue with this.

After that, I will run some more tests (yay for tests)! I want to test the unit using the new panel meter to make sure everything is lovely. I will also hook up the fan and run everything all at once to get an idea of my total power consumption. It should be well well within the limits of the two power supplies I’m using and am anticipating no problems.

The panel meter will also need a range setting. This, I will accomplish by adding a switch and a couple of divider resistors. Since the reading across the shunt (and Iset) will be 0-320mV, I will use the 200mV and 2V scales and omit the decimal point for the low setting, and add it on the high (reading in mA and A, respectively). I will be able to get mA resolution if I keep the load under 1999mA and 10mA resolution from 2.00A-3.20A. I believe this is why Martin Lorton spec’d his device to a maximum of 1999mA at least partially, he didn’t want to bother adding a range switch for the display and losing display resolution to the mA. I can’t say I blame him. I most definitely need a load that can pull 3A so this is a necessary evil.

Also coming up, I will wire up the 4P switch as detailed in the last post and test it out to make sure I’ve got it right. Beyond that, it’s wiring up everything nice so it’s ready to go into the case with proper wiring and connectors and etc. Soldering together the PCB will also happen at that stage. Then it’s case time.

Ah, I’m excited! If you aren’t sleeping at this moment, that means you are too and I thank you.

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