# inline pull up/down resistor decrease resistance when encapsulated

I am working with a 500 MΩ resistor, assembled in the fiberglass tube with both ends closed. There is dielectric-silicon filled in between the resistor and fiberglass tube, which gets cured in 24 hours.

When I measure the resistance of the resistor itself, it's showing 500 MΩ. However after assembly into the fiberglass tube, it was showing in 450 MΩ range. Then after cutting the top half of fiberglass tube (horizontally cutting) the resistor was again showing 500 MΩ.

I think the silicon expands during the curing process and it puts pressure on the resistor to make it read low resistance. Can this happen? What would be the impact of pressure on resistor and what is the easy and best way to measure the impact?

Thank you.

• Or the silicone itself is conductive – PlasmaHH Sep 17 '18 at 19:19
• compress the sides of the tube and check for resistance changes – jsotola Sep 17 '18 at 19:25
• horizontally cutting only makes sense if you also mention the orientation of the tube – jsotola Sep 17 '18 at 19:26
• What does the datasheet for the silicone say regarding its electrical characteristics? How long after applying the silicone did you measure the resistance? Going from 500MΩ to 450MΩ implies a parallel resistance 4500MΩ, which is practically a perfect insulator for most applications. – ajb Sep 17 '18 at 19:27
• please draw a diagram of the assembly .... include the cut line – jsotola Sep 17 '18 at 19:29

It's possible the resistor is being affected, but silicone is relatively good that way compared to epoxy and 10% is a big change.

You DEFINITELY need to avoid using acetic acid cured (condensation cure) silicone on electronics. Use an electronic grade of silicone rubber.

Platinum-cure silicones do not have the conductive acid component.

RTV (room temperature vulcanizing) rubber is very soft in Shore Hardness grade compared to the epoxy coating of a resistor, so compression change of 10% is very unlikely.

You must report the RTV specs and method of curing to get more insight from us. There are many types.

The "caulking type" which is acetic acid cured.(may cause rust)
Two part Silicones with Platinum(\$) or tin for curing
One part Silicones which use heat and/or mositure for curing.
GE's N-SIL RTV122, RTV123 and RTV128 adhesive sealants are one- component, ready-to-use, and cure to a tough, resilient silicone rubber on exposure to atmospheric moisture at room temperature.
- Being hydroscopic to accelerate curing, any dust or airborne contaminants can increase the leakage current in the gigaohm range. but being sealed in fiberglass takes longer to cure. ( but that would take a very dirty factory)

## Recommendation

A better process might coat the resistor 1st and rapid cure past the tube end position, then insert and fill the tube with "any" silicone rubber but not past the 1st coating.

3 day full cure can be accelerated in an oven with datasheet advice.

Get GE RTV128 Volume Resistivity, ohm-cm 4 x 10^15

## Anecdotal

(this is what we used in Aerospace in late 70's in my 1st job for HV insulation in space (e.g. plasma probes) and thermal protection of exposed wires on reentry to atmosphere)

The <10^10 Ohms of your results indicate the dielectric added is static dissipative.