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  • Author Author: jw0752
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Shop Tips -  Intrusive Meter Probe

jw0752

I frequently work on circuit boards from Dental Equipment. Since these circuit boards are often used in wet or humid environments the manufacturers like to coat the boards in a thick layer of epoxy or other hard sealant. Probing these boards has always been a challenge as my meter probes would not penetrate the epoxy and I would get false readings on the components under test. The usual method for working with boards like this is to use probe tips that are very sharp and to push hard until a reading is registering on the meter. The sharper the tip the faster they dull and break and it is a nuisance to have to be constantly sharpening. In addition once you have sharpened and the nice nickel coating is gone the underlying brass is way to soft to hold a point. The final frustration with these very sharp tips is that it is easy to get stuck by them.

 

Tonight I assembled some parts and made some tips that solve the problem of penetrating the epoxy, are not dangerously sharp, are very durable, and are easily replaceable. To do this I am going to adapt another piece of Dental technology, the drill or bur that is used to cut tooth material. One particular bur numbered 557 is a good fit for my needs:

 

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The 557 bur is about 1.5 mm in diameter and a little less than 2 cm long. The tip of the bur that usually cuts the tooth material is made of tungsten carbide and has flutes along its sides and has a sharp star pattern on the tip itself. My plan is to make a chuck for the dental burs and mount the chuck on a meter probe handle. I will have wires coming from the chucks back to pin jacks which will accept the tips of my normal meter probes. Now when I need to probe a board covered in epoxy I will attach the new probes to the meter probes and use the dental burs as the tips. Here is how the fabrication came together. Please excuse my poor machining which is 50% having very cheap tools and 50% having a poor machinist. Here is a picture of the assembled probe tip:

 

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I have taken a piece of solid brass rod and threaded it so that it will screw into my probe handle. I next drilled a 1.5 mm hole down the center to accommodate the bur. Finally I cross drilled and tapped a hole for a set screw to retain the bur. Here are all the parts for the probe extensions before and after assembly;

 

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Placing the bur on the epoxy above the pad or component that I want to probe and with light pressure and a slight twist the sharp points of the bur cut down to the metal and a dependable reading is obtained on the meter. These burs are very common and probably can be purchased from any dentist. You will find that even used burs will work well as the dentist uses the flutes on the sides to cut and we are mainly interested in the star flutes on the tip. The bur shaft is made of steel and the tips are tungsten carbide. This material has a higher resistance than the usual metal used for probe tips. My probe extensions and tips added about 0.1 ohm of series resistance which I can mentally add into my readings. It of course doesn't impact the voltage readings.

 

John

Parents

  • Hi John,

     

    I just had a moment where your tips made making an accurate low resistance measurement possible where using standard tips the job was impossible.

     

    I guess you saw on my E36313A road test that I was initially having odd DMM readings when I was using 4-Wire mode and it initially made me think that something wasn't right in the supply, then I started to wonder if it was actually something to do with my DMM. But it actually turns out that a lot of the resistive losses in the setup were in the banana plugs going into the DC electronic load, more so than I had expected. The voltage drop across the banana plug where it went into the binding posts on the load and the point on the binding posts where the sense wires were clamped was enough to make my measurements look totally bizarre.

     

    Anyway, to cut a long story short, I wanted to measure what the resistance was for the banana plugs being plugged together so I took my DMM and my best pointy standard probes and nulled out the impedance of the cables and probes. I then tried really hard to measure the banana plug connection but I just couldn't get the pointy probes to stay put on the round profile of the plug. Much frustration ensured and I decided it wasn't going to work this way. So, I added the set of your probe tips and touched these on the plug right next to each other and nulled them out and then took one to the other side onto another plug into the back end of it and both tips gripped perfectly and I was able to make a really nice stable reading without any effort. So it seems there is about 20 mOhm of resistance when I insert my standard hook up cables into a binding post which with 10A running through it explains the results I was seeing.

     

    So thanks again for the set of probe tips, I'm sure they are going to come in really handy again and again! image

     

    Best Regards,

     

    Rachael

  • in reply to rachaelp

    (dodging 's comments - he says I'm plugging these probes image ):

     

    Great probes for pcb work. Especially for surface mount boards - and especially for ham fists me.

     

     

     

    That's it. Happy 2018 John image

  • in reply to Jan Cumps

      wrote:

     

    (dodging 's comments - he says I'm plugging these probes ):

    image

     

    Well I am quite happy to plug these, they are great! image

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