Will that test show a capacitor that's gone high impedance though?ljcox said:The way to check the filter caps is to set your meter to the Ohms range, connect the black probe to the - wire and the red to the + wire of the cap.
The reading will be low at first, but it will start to increase as the cap charges - if it is healthy. If you are measuring it in-circuit, there may be other components connected across the cap that may affect the reading.
I keep switching the range of my meter up as the cap charges, ie. start with the low Ohms range and switch it progressively higher as the "resistance" goes off scale. It should eventually go off scale on even the highest Ohms range - provided that there is no other path in parallel, eg. a resistor to ground. If so, you will eventually read the resistance of the resistor.
Then I switch the Meter to the DC Voltage range and measure the voltage across the cap. It should read the meter's battery voltage initially (or a bit less) and slowly decrease as the cap discharges through the meter.
Hero999 said:Will that test show a capacitor that's gone high impedance though?
uh, a shortcut to the grave.Or you can just disconnect your scope's earth connection but that's just retarded.
Eric,dbtoutfit said:Thanks Len will do
How about the Diodes?
They look like these.
Thanks,
Eric
Hero999 said:Or you can just disconnect your scope's earth connection but that's just retarded.
Nigel Goodwin said:OK, I'll mention it YET again - disconnecting the earth on a scope is an EXTREMELY common practice in the service industry.
For many purposes it's FAR safer than an earthed scope, personally I wouldn't ever use an earthed scope, I don't consider them safe enough.
Hum Bucker said:"Extremely common" it may be - probably due to a lack of suitable isolation transformers - but just because it is "extremely common" does NOT make it a safe practice! The "Service Industry" is notorious for its cost-cutting measures - usually to the possible detriment of the staff. Plus, you'll still get the display problem that I referred to earlier.
Using an isolation transformer and an earthed scope is the ONLY safe and sensible option in the scenario under discussion. An unearthed 'scope is a safety hazard under ANY circumstances.
Hum Bucker said:You simply don't understand the whole concept of "earthing", do you?
By placing the electrical circuitry inside a metal box, which is connected to earth, and with one side on the a.c. supply joined to earth (at the sub-station), an electrical failure that results in the metal case becoming 'live' blows the supply fuse - thus saving the user from receiving an electrical shock. Therefore, to ALL test equipment should be earthed at all times in the simple interest of user safety.
You don't seem to understand the basics of SMPSUs, either.
If I wish to examine the waveform across the filter capacitors in a switch-mode PSU, where these caps. are fed from a bridge rectifier DIRECT from the a.c. mains, I MUST, repeat MUST connect the earth lead of my scope to the -ve of the caps.- otherwise, there is no reference for the 'scope to work to. To do this, a mains isolation transformer is ESSENTIAL between the a.c. supply and the PSU - otherwise there will be a dead short across the bridge.
There is no debate to be had on ANY of these aspects: they are simple, provable facts. Only the ignorant and mis-informed would disagree.
I'm only 25 and I've heard about it and to me it's common sense why the earthed, mainly because if it isn't earthed it can float at whatever voltage it wants to.Nigel_Goodwin said:As you're blindly rattling off domestic earthing standards you're obviously far too young to remember the huge controversy over earthed and earth free environments?.
Not if you're using an isolation transformer as there's no return path to earth.Hand on earthed scope, other hand on live wire - oops! - full mains straight across the heart.
No, it isn't safe if you don't have an isolation transformer. The -Ve pulses up an down at mains potential with respect to earth so as soon as you touch that scope you're a goner. You're better off blowing a fuse or rectifier then learning from it and buying an isolation transformer.Yes, you connect the chassis of the scope to the -ve of the caps, that's blatantly obvious - and I do so on at least a weekly basis, and have done since SMPSU's first appeared in the Thorn 3000 series TV's. That's why you don't have an earthed scope in service departments - it's then perfectly safe if you use an isolation transformer or not.
When was this?I might also mention I've been on training courses at huge international electronics companies, and in their training rooms there are no isolation transformers, and all scopes have their earth leads removed. I can't remember the last time I ever used an earthed scope?, at technical college all scopes were unearthed - again, for greater safety.
Mind you, at technical college they also had live bare brass 440V three phase terminals on the walls! - that was in the heavy machines lab, which we had a number of lessons in (by complete mistake as it happened).
Eric,dbtoutfit said:It described that it's highly typical for the diodes next to the filter cap/s to dead short for no reason. This seems to be true in my case but I want to make sure other components down the line after the diodes were not affected. You will have to measure them to know. Some components can be measured in-circuit if there are no parallel paths.
It was described that it is possible that the filter cap/s could go bad.
I am sure that this is typical but I thought I would just mention that there is a capacitor that crosses beside with the fuse. After installing a few fuses as I was trouble shooting I notice this capacitor would finally "pop" so to speak. Blow a little ceramic pc off it's self. I don't understand this, what does pc mean? I would assume this wouldn’t help as it is probably a result in the dead short.
Eric
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