On Thursday, July 30, 2009 11:12:25 AM UTC+10, Avionics Tech wrote:
We have a Fluke 5220A Transconductance Amplifier with a problem
that's been getting progressively worse over the years. It's supposed
to put out at least 20 amps, ac or dc, with light inductive loads.
The overcompliance circuit is tripping the unit out at around 19A DC,
and much less for AC, around 13A. These are both with loads connected
with minimal voltage drop across them. The loads, a shunt and coil of
wire, have been used for years with no problems.All supplies are clean
and stable. After troubleshooting for hours and getting nowhere, we
disabled the overcompliance circuit to see if it could indeed put out
full current. R114 and R115 on A7 burnt up (page 129,
http://assets.fluke.com/manuals/5220a___imeng0300.pdf ). We replaced
all the transistors on A7 although none were blown, re-enabled the
protection, and we're back where we were.. I'm curious why those
resistors burned. I'm not familiar with this type of amplifier
configuration. We checked, and the amplifier output is stable and not
oscillating. At this point, we'll take any input. Next up I'm
planning on replacing all the active parts on the driver board, I'm
tired of troubleshooting and wasting time, it's shotgun time.

Thanks,
Steve

Hi Steve,

this is a massive long shot - I found this old post on google groups regarding a problem you had with a Fluke 5220A Transconductance Amp.
I've got a similar problem with one here - it can drive resistive loads at 20 A DC and AC without an issue, but is very hit and miss with the 50-turn coil (used to calibrate current clamp meters up to 1000 A). Sometimes it will drive 1 A or 2 A through the coil, other times it will just go straight into an overcompliance fault.

Sometimes I can slowly ramp up the input voltage and get up to 5 A or 6 A through the coil but it depends on the day. Ramping up in ~100 mV steps is usually okay but very, very tedious, and sometimes even this causes it to cut out.

I did some reading and it seems that there is meant to be a feature in the 5220A where the current will slowly ramp up if the input voltage is applied first, then the 5220A set to 'Operate' mode, rather than having the 5220A in ' Operate' mode and changing the input voltage. I suspect this is what is causing the problem in both your case and in my case, but don't even know where to start in terms of fault-finding.

If you did ever end up identifying a faulty component or an alternative solution for this problem I would be eternally grateful for some assistance!

Thanks,
- Alex