Repurposed Old Multimeter = DIY Capacitor Discharger With Readout - Project

[t1d]

My previous DIY Capacitor Discharger was difficult to handle. I had built it in a box. The type that is built on a wooden stick appealed to me. So, I got on YouTube and looked for inspiration.

There were all of the typical stick and wire models. However, Haseeb (YouTube channel) had made a jig that allows him to watch the discharging voltage drop across the bleeder resistor.

I got the idea to take Haseeb’s idea one step further. Why not re-purpose an old multimeter into a dedicated Capacitor Discharger? So, I did just that…

I used a low-price multimeter (precise readings are not needed) that has been a drawer for years. All that was necessary was to attach the bleeder resistor across the probe terminals inside the case.

This type of inexpensive meter only has tubes, for its banana plug sockets. They are difficult to solder to, directly, so I found common attachment points on the PCB and solder the resistors there. See pictures. Be careful to use traces that do not include the wire current sense resistor.

I used four 1K resistors, two each in series and those two pairs in parallel. This gave me 1K Ohms at 2 watts.

I used such a high resistor value to be able to handle higher voltages. I am encountering around 400 volts, now that I am learning about switch mode power supplies and inverters. But, I think the high value may be too slow, as there is a trade-off between too fast of out-rush current stressing the cap and too slow cooking the bleeder resistor.

I may rebuild it for a couple of reasons…
- As said, the slow discharge time.
- I stacked the resistors and the height of the bundle is interfering with the lid closing, just a tiny bit.

Safety points…
- I was concerned that the increased size of the solder joints might make it easier for current to jump the gap. So, I doped the solder joints with insulation. This is not shown in the pictures.
- I will add a big label to the front of the unit stating that is has been modified and that it is to only be used to discharge capacitors.
- The multimeter's labeling claims that it can handle high voltages and current. I am very aware that it is under-engineered for the task, particularly the probes. I will use it with great caution.

That's it... A neat package, with digital readout, that I expect to be very easy to use.

[BrokenYugo]

Definitely upgrade the leads, even cheap ebay test leads are better than the crap they include with those.

[t1d]

Great suggestion! I have some of those from Ebay in a drawer, but the idea to use them had escaped me. Thanks!

[BrokenYugo]

Note that you may have to trim back the shrouding around the banana plug to match, these don't use the "standard" shrouded banana design.

[t1d]

Yep, exactly. Coincidentally, I had originally bought my Ebay probes for an old Tenma 72-1005 Universal Test Center, which includes a multimeter with the same style of probe sockets. So, I have already removed the shrouds, as you suggest.

[trobbins]

If you had a small pushbutton with 250VAC rating, it may fit on the front panel where the DCA label is - it could add an extra parallel load resistor if your probe setup gives you an extra 'hand' (ie. if you use a clip probe for ground).