Isolated power supply woes - Page 1
Forum section: electronics / projects

author: stigwurx
author_rank: Contributor
timestamp: July 27, 2019, 02:01:58 am
content: So I have a few old oscilloscopes I was hoping to rejuvenate and a tube amp that I've been putting of repairing for way too long. So I built an isolated power supply to reduce my chances of frying myself among the high voltage supplies. 560 VA with 120 , 230 and variable 0-260V as my selectable output choices. Just switching the taps of the isolating trafo for the fixed voltages then feeding the 230V into a variac for the variable.It's working but the variable part is being a bit of a pain. With no load attached to the output the variac seems to be drawing a chunk of current from the isolating trafo, haven't metered it yet but my ears don't lie and I can hear the hum of the iso traf pick up a notch or two when the variac is connect in series.I haven't worked with variacs before so I'm wondering if this is just par for the course ( seems silly ) or if there is something that needs to be tweaked. The problematic circuit is literally just an isolating transformer with a variac in series- the metering is done after an isolating switch on the output so no load at all .Could impedance mismatch between output and input of the respective transformers be an issue ? If so , is there a fix . Or is this just variacs all over, what're your thoughts.Any suggestions gratefully received. Lacking a diagram of any informative value I attach a pic of the offending machine.Regards.

author: H713
author_rank: Regular Contributor
timestamp: July 27, 2019, 06:14:42 am
content: First figure out how much current we're talking about- it's possible it's negligible.Two possibilities come to my mind.1) Is the variac in question designed to be used at 240V? It's possible you are saturating the variac core, in which case it will pull heavy current and buzz.2) If this is not the case, it may just be a coupling issue that can be solved by putting some capacitance across the input "primary" of the variac. If this is the case, I wouldn't worry too much.I built a very similar power supply for the same purpose, and I put the isolation transformer after the variac. Yes, this means you don't have the fixed output voltages whilst adjusting your variac, but in practice it is rarely necessary.I would figure out what sort of current we're talking about here. If it's a lot, then you are probably saturating the variac core.

author: stigwurx
author_rank: Contributor
timestamp: July 28, 2019, 12:22:25 am
content: 2 was indeed the answer to my problem. I hadn't checked the current (got too hung up on having loaned out my clampmeter), but having one of the scopes that're in the maintenance queue to hand I checked the output and it was a nice and smooth and sinusoidal which suggested to me that the core wasn't being saturated , should expect distortion? Nonetheless (as it was second hand ebay and who knows it's history), I switched the primary tap on the variac to the top of the coil , now just cut available on the output voltage - no boost, to see if it improved matters which it did only slightly.Then I tried your capacitor suggestion , started at 100nF across the variac primary and incremented the value until I eventually found a sweet spot around 470 and this was using the boost tap. Now when I switch to variable output there's the additional hum of the variac but a hum that's no longer drowned out by the drone of the isolation transformer (it too is used-ebay so I forgive it's laminates for some infirmities).Many thanks for your suggestion , it worked a treat. May you have a very good day.