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timhoward

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Would a small UPS, running on batteries, be a suitably stable 230v 50hz supply while I do some tinkering with the loop calibration settings on an old Robin 1610?
Any better idea's that don't cost a lot?
This is entirely a hobby project and not worth investing in lots of equipment for,.
My copy of the service manual (which looks as though it was typed onto toilet roll and then faxed from Australia via a piece of wet string) seems to suggest that as it's an old-school tester I can do most of it (not RCD stuff) with a stable power source and a decade box., making adjustments to 5 variable resistors.
Knowing the supply loop impedance is usually required, though I'd guess the N-E link on the secondary side will make that almost negligible.
 
No. It is likely to be inferior to the mains in every important respect. Importantly, the output of typical desktop units is non-sinusoidal, which might or might not throw the measurement process of the MFT out of whack. The 'modified sine wave' or whatever they like to call it is actually DC of one polarity, then off, then DC of the other polarity, then off again. The duty cycle is chosen to make both the RMS and peak voltages correct, and the ratio between the two is the only parameter in which the output represents a sine wave. If the MFT is injecting a test current during the voltage zero it might not make any difference, but how would we know whether it did?

Then, even with an inverter that produces a genuine sine wave, the effective loop impedance is likely to be unstable. The loop impedance of the mains is low and constant because it's an infinite bus. Nothing the tester can do will measurably influence the national grid, therefore the impedance it sees is mainly due to the local copper circuit which is linear and has a long time constant. In contrast, the effective source impedance of the UPS output is dependent on active voltage regulation, influenced by everything from the battery charge level to where in the program the microcontroller happens to be when it detects a load variation and tries to correct the voltage. Again, it might not matter at the voltage zero if the closed-loop response is neat and tidy but there are so many variables that I wouldn't trust it.

Is the mains really too unstable to carry out the adjustments?
 
Thanks Lucien. Makes sense
I was trying to achieve 3 things.
1) The very first step is to connect to a stable 230v / 50hz supply and adjust a VR until the display reads 230. Thinking about it, another calibrated tester could measure the voltage and I could match it
2) A further step involves knowing the external loop impedance and my thinking was the N-E link in the UPS would make this nearly negligible. But again, it could be measured by other calibrated testers.
3) As the design of this particular tester needs it to be be open with mains connected to adjust things, and isn't designed very safely in terms of live parts and their locations, and further requires a non-RCD protected circuit as 25A is leaked to earth, I was thinking that being on the secondary side of a transformer would be nice. I'll just have to be careful! (They changed the design for the next model, the 1620 which is entirely calibrated digitally without opening the box)
 
the N-E link in the UPS would make this nearly negligible

The loop impedance is that of the energy source plus the wiring from source to tester. In the case of a DNO substation, the transformer has low impedance so the wiring tends to dominate the measured value. In the case of the UPS, the impedance of the energy source, in this case the UPS transformer and all its miscellaneous electronics, will dominate over the wiring. The link is neither here nor there, and indeed might not be a link at all.

What about a shaver socket

Even if it will withstand the test current, the effective impedance of the transformer will be very high, possibly out of range for calibration. The test current might saturate it and what the tester would then read is the DC resistance of the secondary winding, not the mains at all, and perhaps abort the test.
 

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