That's definitely not "a little bit". By very far, not!
Muscling around a scope chassis (not the probe tip, but the probe ground and the big scope chassis connected to it on the other end of the cable) from zero to some 800 V in several dozen microseconds is no small feat, much less doing that 20000 times a second repetitively.
Not many power supplies will do that on an internal node without running into major stability issues (unless you have a very small battery-operated "pocket" scope, sitting on a wooden table far away from any earthed metal, thereby being a "light" load).
The probe tip is not the issue, but the scope itself, hanging from the probe ground, that is.
It is NOT higher rated. A 100 watt incandescent in that spot will limit the current even lower and there is less chance of blowing anything else.
I did not mean to suggest that you change your plan of troubleshooting, jus t that next time when it comes to a hot test, use the bulb. If something is still shorted you have a hell of alot more time to figure out what, rather than having overheat in seconds. When the light dims, you probably found t he problem. the light goes down in resistance as all the filters charge and get almost to full voltage, once it does that it works without a net. Regu lar fuse and all that.
I consider a dim bulb tester a must for this type of work.
That's not a problem. It only ever sees 320 V from the mains, plus any little remains of the mains surges that may come its way past C1802+3.
Even with surges and such, 450 V is likely the highest thing it will ever see, so a 630 V rating is a good and conservative one.
It won't ever see the 800 V. But the transistor V1806 (collector) will.
Basically, the input caps will "see" only normal rectified mains (320 V). The resonant caps will also see some 300 to 320 V, but because the sinewave resonance signal is bipolar, and one end is tied to the positive end of the input caps, there will be times (each half cycle) where the voltages will add and the result (referenced to the emitter) will reach some 600 - 620 V. At these same times during the cycle, L1806 will also be reset via V1811, and the reset voltage (some 200 V, also being in series) will also add to this, plus any little remains (50 V or less) from the L1804 circuit. So the collector of V1806 will "see" quite a lot of voltage when V1806 is in the "off" phase. But this high voltage only applies to the V1806 collector, not to the other parts / signals.
P.S. Since you indicated that you have some background with radio...
Consider the collector of V1806 as a signal source. As a signal source that can basically drive a 800 V peak-to-peak square wave.
Consider the whole power supply board (including any cables and the isolation transformer or variac that you are using to feed it) as one half of a dipole antenna.
Consider the scope (the whole metal chassis) and the probe cable as the other half of the same dipole antenna.
Consider the two halves connected in the middle by the probe ground clip, at that overheating power resistor in the supply.
What you get, is a center-fed dipole, sitting on your table, and being driven with a 800 V peak to peak fast square wave. Not a light load.
An isolated high voltage differential probe would "separate the halves", so that the big (parasitic) dipole would no longer exist.
OK, Dimitrij, a *huge* quantity of info to digest here and in your other new postings on the subject and more detail than I can assimilate in a single sitting! Many thanks as ever; that will certainly be all I need to know for the time being. I'll get to work on the checks when I return to base after midweek. Laterz...
There is a simple though unwritten rule about power supply testing:
"Never connect the ground (common, chassis etc.) of any test equipment to the switching node (power transistor collector, drain or power IC output pin and its associated signals) of a switching power supply!"
It is valid for all types, no matter if flyback, forward or resonant.
The reason for this rule is that a "switching node" usually drives a square wave with high voltages (some 500 to 600 V in a flyback, may happen to be as much as 800 or 1000 V in a resonant one), and that a significant amperage is readily "available" at that node too, due to the output transistor's low impedance. Neither is the supply designed to safely drive that into "RF ground" nor is the test equipment made for being "muscled around" at that sort of voltages and dV/dt rise times.
Grounding the test equipment would mean that the whole power supply (plus any safety isolation transformer) is being swung around and letting the test equipment "float" would mean to also swing around the test equipment. Apart from the obvious safety hazard, this can also damage the test equipment and even compromise the test equipment's electrical safety by frying the "Y" capacitors between mains and secondary or stressing the isolation barrier in the test equipment's power supply and / or mains transformer, possibly beyond the level of stress that it was rated for.
So, whenever you troubleshoot some switcher, take heed of this rule.
It's simple to remember, and it can save lives, test equipment, and some power supplies under test too :)
I'm grateful for that expansion, to be honest. I was kind of struggling to get my head around what you were getting at in your earlier postings; didn't make much sense to me on the first read through and although after a second read I was beginning to sense your meaning, it still wasn't 100% clear. At least now I think I can finally see where you're coming from. Naturally I read up on safety precautions when dealing with switchers from books I have and all sorts of diverse sources on the net, but I can honestly say that what you have outlined above has NOT been covered by anything I've seen or read up until now. This would seem to be a glaring omission on the part of those who we rely on to prime us up on the hidden dangers and pitfalls of troubleshooting such equipment. Another good reason for me to avoid dealing with switchers in future if at all possible!!
Oh, I just noticed you did in fact actually state it's an "unwritten rule" - well my personal experience of searching on the subject can certainly confirm that!
At least myself, I have not seen it being being explicitly explained or written anywhere yet, but it's sort of "common knowledge" in a way...
Among engineers who design power supplies, this seems to be taken for granted - too self-evident to warrant explanation apparently. Others, among them the many who design low voltage circuits and prefer to buy their power supplies off the shelf, rarely get to see switching nodes driven with significant fractions of a kV with fast rise times. That leaves their awareness of the "tricks of the trade" rather limited.
Power supply design is both a science and an art, and the power supply "artists"' rites of initiation can sometimes involve strange things :)
Anyway, never fear, but always exercise logical thinking, conservative judgement and be aware of side effects - that would be my advice here.
Well now that you wrote it, it is no longer unwritten. :-)
But I know what you mean. It is pretty much RF and if the caps don't short it out it can burn you some. If the voltage is high enough you don't even h ave to touch it. I got burned by the cathode of a damper tube in a color TV set once. That is half of about a 70 KHz sine wave clocked at 15.734 KHz. Arced to my finger, burnt and cauterized all the way to the bone. On blood, but it sure did smart. And then it felt funny for like six months after.
Can you imagine that on the chassis and probes of your scope ?
Not really. It's too weird. Maybe I could simulate it in spice to get a better idea of what's going on here. Anyone got a model suggestion for a 'hanging scope'?
It's very simple really. Just remember to keep the scope grounded to the DUT ground AND the probe clipped to the power node (X1 setting on the probe) at ALL times and bob's your uncle, you can't go wrong. That way you are only placing about 15pf || 1M loading on the DUT. HTH.
Fair point! But I use an externally selectable decade attenuator for anything over 400V so X1 is good for me. But yes, in the absence of that, X10 would be the way to go.
As an aside, I'm just a bit mystified as to why anyone would want to do this anyway?
Now, apologies for the delay, but I had the usual accumulation of pressing things to deal with on my return so have only now got around to carrying out the checks last suggested here.
OK, I measured the resonant frequency of the primary circuit (with the chopper NOT disconnected, see notes below) by sweeping a frequency range across the main tranformer's primary input terminals. It's not particularly peaky, so there's a Khz or so on either side of Fo before we get to the -3db shoulders. Fo, with no load connected came out as
17.35kHz.
Under power, with frequency counter connected between T1 and T2 with V1812 removed from circuit shows the PWM chip pulsing at 22.55kHz.
Unfortunately I have no idea what the factory figures should be and whilst it seems like there's a big difference between the PWM chip's output and the primary circuit's resonance, AIUI, they're not supposed to be in sync at any time anyway. But are they supposed to be this far apart?
Notes:
I know somewhere it was stated that the chopper transistor should be removed for the resonance test, but I couldn't see the harm in leaving it in. If it invalidates the test, of course, then I'll whip it out and re- do it. If you think it's relevant let me know.
I pulled V1812 as someone suggested because the noise coming back down its collector from L1803 might have interfered with the frequency counter's ability to read the clock pulses.
The quote function musta got screwed up. I always use 10X unless I really need the gain, which is rare. I also recommend others use the 10X at all times as well. Not only does it reduce circuit loading, it also protects the scope to some extent.
Not the first time Usenet quoting got screwed up. I expect to see >> on a quote of a quote and > on a direct quote but it seems not to work that way all the time.
I haven't tried this yet as I can guess sod's law making it the case that I'd have to pull the plug just at the point where the CRT has warmed up sufficiently. The other slight problem is to test this requires the board to be completely inserted with every connection made plus a temp probe to the power resistor which is all rather fiddlesome and not to be done repeatedly if it can be avoided. I can see a situation arising (sod's law again) where someone here will post saying - "oh, whilst you still have the board out, just check this..." Nevertheless, if nothing is said in the next 18 hours, I will test it all reconnected and post the outcome here.
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