Dog Bone resistors

[Crackle]

Do you think dog bone type resistors will continue to increase in resistance value after 50 years?

Mike

[Terry]

"Continue to increase in value"?

From this I assume that they are already much higher than their marked value and no longer fit for purpose. I see no reason why this process shouldn't continue, Mike. How long it might take for any significant change is another question!

What is the reason for your question? Are you thinking of repainting them with their new values and reusing them - or what?

[Crackle]

The simple answer Terry is yes.
I needed a 60k resistor and a 50k one I have measures just right at 61k. So I am now waiting for the base coat of blue to dry.

It would have made matters so much easier if they had devised the colour code as tip spot body as the multiplier is far less likely to have changed over time.

Mike

[Crackle]

And here is the new 60k resistor.
   

Mike

[Pat Pending]

Ingenious! As long as they're not noisy, they should last.

[boater sam]

If reusing carbon dogs I cook them first to make sure they are dry and stabilize them.
To up the value to what you need I file notches in them. If a big notch or several is needed I cover the body with the correct colour of heatshrink, then paint the bare end & tip and spot and test, so far they are OK.

[Crackle]

Hi Sam
I wonder if cooking and drying them will simply result in them absorbing more moisture over time, or if they absorb a certain amount and then stabilise.

Mike

[Mark Hennessy]

Every time I've read of people cooking carbon-comp resistors, the effects are always temporary.

Some more information in this thread: http://golbornevintageradio.co.uk/forum/...p?tid=4384&pid=46480#pid46480

Good job the values are rarely critical in practice.

Nice paint job though

[boater sam]

I've not checked if they go high again, would damp make them go higher or lower?
I paint with enamel paint or oil paint, perhaps that helps.
When cooking them I have not noticed any significant change in value once they have cooled, they invariably are high in value.

[Craig]

The increase in carbon comp resistors seems to be related to the value. Lower resistance values, say less than 1k seem to be fairly stable, whereas higher values, say more than 10k tend to increase by more. I've seen 47k resistors be close to 56k, and 1M resistors are a bit of a lottery where they end up. In my experience they never go lower in value.

But they have excellent HF behaviour, since they are just a rod of carbon particles in a binder. That is why tektronix used tiny Allen Bradley 1/10W carbon composition resistors in their high speed 7000-series plugins, like the 7T11. And they can take surge overloads with ease whereas a metal film resistor will burn out.

Craig