[volt-nuts] Suggestions for 10 milli Ohm 0.04 % 100 W resistor

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Tue May 17 04:58:59 EDT 2016

I have an HP 6674A power supply with option J06,  which means that the PSU
is 70 V @ 30 A rather than the standard 60 V @ 35 A .

I've replaced a couple of bits in this and will need to replace some more,
so it would be prudent to get this calibrated.

The service manual calls for a 8.5 digit 3458A, which seems a bit over the
top given the displays for voltage and current are only 4 digits. I think
my 6.5 digit 3457A will be good enough. This is a 2.1 kW PSU, not a
precision measuring instrument. But I don't have any ammeter that can read
30 A, so I can not just use an external ammeter if I want to know the

The service manual also calls for a 0.010 Ohm 0.04 % 100 W resistor, with a
recommended resistor of a Guildline 9230/100. I am wondering how practical
it is to make such a resistor and verify its performance on the 3457A.
Some versions of this PSU have a lower output voltage (56 V) but higher
current (42 A). With the 30 A PSU I have,  the maximum power that could be
dissipated in a 0.01 Ohm resistor is obviously 9 W.

Any suggestions about what I can use that will not cost a fortune?

Looking on eBay, Guildline resistors are several hundred USD each. I can't
justify that given the cost of a Keysight calibration of the PSU is $199 in
the USA.

I much admit that I have never really much attention to calibration of a
PSU. It seems a bit of a waste of money when you can use a decent
multimeter if you really want to know the voltage or current. But with a
PSU of this size, I don't have an ammeter good enough.

The other tricky bit about calibration of this PSU is the need for loading
it to full power (2.1 kW) then dropping the load to 50% and measuring the
recovery time. It should recover to within 100 mV in 900 us. Likewise it
should recover the same if switched from 50%  of load to 100 %. The manual
calls for an electronic load,  but I suspect a FET switch and some big
resistors in water will do. I don't see any need for such resistors needing
to be very high precision, but obviously something decent is needed to
calibrate the ammeter.


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