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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 30th Jun 2020, 10:33 pm   #1
Aubade65
Triode
 
Join Date: May 2012
Location: Oxfordshire, UK.
Posts: 35
Default AVO 8 Universal Shunt Rating

I have an AVO 8 MK2 with the 300R section in the universal shunt O/C

I intend to replace it with a 1% metal film resistor - can anyone suggest a suitable power rating for the replacement resistor?

Regards
hh
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Old 1st Jul 2020, 10:51 am   #2
pmmunro
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Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,288
Default Re: AVO 8 Universal Shunt Rating

hh,

This will be the 300 ohm resistor labell 'S' on the circuit diagram in the service manual?

This is part of the universal shunt for the 250 microampere range, along with a 1200 ohm section, 'T'. It is also used for the 'ohms' range. It is the resistance range which is more critical concerning power rating as resistor 'S' will carry about 1.2 mA, less the 37.5 microamps for the moving coil, in that mode.

Even then however, it is not called on to dissipate much power; I calculate it to be around 4.3 mW. It will in fact have to be rated for considerably more power so that the heating effect, even with a low temperature coefficient of resistance, is negligeable. A quarter watt metal oxide resistor should be quite adequate, but a half watt component would give an even better margin.

To confirm this, in the Model 8 Mark V, uses component resistors and R13 (360R) is used for the 1 mA range; the rating is given as 1/4 watt.

Up at the high current end of the universal shunt, things get a bit more demanding. Section 'M' for the 10A range, 0.05 ohms, which needs to dissipate nearly 5W.

PMM
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Old 1st Jul 2020, 12:04 pm   #3
Aubade65
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Join Date: May 2012
Location: Oxfordshire, UK.
Posts: 35
Default Re: AVO 8 Universal Shunt Rating

pmmunro

Thanks for the reply

I did not think it would need to dissipate much power and I was considering a a half watt resistor to give a decent margin of safety - its good to have some verification

Regards
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