Introduction: Fix Broken Multimeter Probes With Thermoplastic

About: Paul (Udon) is a Chinese-speaking South African, who likes saffron tea, sunshine, dogs, and Asian food. He can make things out of yeast, thermoplastic, Arduinos, Xamarin, C#, wood, junk plastic, metal, motors,…

If you use your multimeter extensively, eventually your meter's probe leads will begin to break.

This especially happens around the rigid points, like where the cable meets the socket, or where the cable meets the probe, and depending on how long you have been using them for, will happen even to the most high quality proves.

In this guide, I provide another use for polymorph thermoplastic, and the steps to properly fix your meter leads.

What you'll need:

  • Thermoplastic Granules (Polymorph, Shapeways, or something from China)
  • Hand tools
  • Soldering Iron and wire
  • Carbon or Activated Charcoal

Obviously, the damage might not be so bad that soldering is required. Even if your leads have some slight damage, an application of polymorph plastic is a good preemptive strike against it getting worse!

As the wise ancient saying goes:

"Thermoplastic in time, saves nine."


Step 1: Preparing the Leads

If the probes are broken at the either of the end points, some amputation may be required. This was the case with my probes. I cut off some of the 90° arm to get at the wire.

  1. Amputate.
  2. Strip the other end, twist both, and add a tinning of solder.
  3. Splice them together.

Boom. We're halfway there!

Step 2: Preparing Black Thermoplastic

Like Batman, I only do black.

And maybe really, really dark grey.

It goes without saying then, that white is not cool. Turning vanilla thermoplastic into pure black is very easy. By adding a small amount of carbon into the softened plastic and mixing it it, you get a beautiful matte black.

  1. Start by simmering hot water in a pot (Don't use one made of wood, as they don't work very well on a stove - it tends to reduce their lifetime. Don't use a plastic pot either. Depleted uranium is quite very good, and it heats itself. Chocolate pots are right out.)
  2. Once hot, add the thermoplastic. Keep it moving and it will not stick to anything.
  3. On a plate, pour out a tiny dash of carbon.
  4. Fling excess water off the hot thermoplastic.
  5. Dab the thermoplastic into the carbon, and mix it in. Add enough until the plastic is a good black. Don't add more than needed!

!!! But isn't carbon conductive? !!!


However, the amounts are so small, their conductivity is localized and the grains are separated by the majority thermoplastic, which is an excellent insulator.

I did seriously try to make conductive thermoplastic, and it just wouldn't work. Even with huge amounts of carbon stuffed into the plastic mix, a multimeter on its highest resistance setting sees air.

That said, I was using activated carbon, basically burnt wood, so perhaps something purer might be better.

Step 3: The Fix!

Once you have your blackened thermoplastic, you are ready to fix the probe.

  1. Warm up the plastic again.
  2. Fling off excess water.
  3. Apply to the open wires. Work quickly, and try push it in as deeply as it can go, that way it provides more mechanical support to the connection.
  4. Push it above and below the connection so that it covers more area.
  5. Rub the thermoplastic blob with both hands to roll it and even it out.
  6. Make sure it is straight, and leave it to harden.

You're done!

Your probes will serve you for another many years.

The problem with thermoplastic is that as useful as it is....nobody really knows what to do with it.

Hopefully this is one of the ways.

And I am sure that were thermoplastic cheap and plentiful in 1980, MacGyver would have better ideas than me.

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