Resistors and capacitor going bang.

[SirKit Breaker]

Been doing some electronics experiments with the students. Fairly easy stuff, but i have to admit the last time i did this stuff was when i was where they are now, and that is more years ago then i will admit. Anyway, we have soldered some resistors in series and parallel, added capacitors and diodes, no problems. Connected up a 24v AC power supply rated at 2a. Measured voltage and resistance, then disconnected to connect ammeter and we had no problems, the maximum current we are pulling is 0.018a. When we connect the oscilloscope to watch the wave sometimes the resistors burn out when i change the screen to get a slower picture of the wave, and when we add a capacitor they think it is great watching the capacitor shoot up in the air when it pops. What am i doing wrong with the scope, oh and i have no instructions for the scope, and make is unknown.

Cheers.........Howard

 

[MarkieSparkie]

I'm assuming you’re using AC rated bipolar capacitors (e.g. motor start or speaker cross-over types) and not unipolar aluminium or tantalum electrolytic capacitors, as unipolar types will get very hot and explode on AC due to being reverse polarised every other half-cycle.
If you’re trying to use unipolar electrolytics you will need to either:- 1) Rectify the 24Vac supply to produce a DC supply with a full-wave bridge rectifier before your circuit or 2) Use DC bench power supply to feed your circuit. In either case the scope probe ground issue below should not cause a problem providing you measure on DC side only.
If your using bipolar capacitors and 24Vac supply is not electrically separate from the mains supply earth (i.e. one side of the ELV supply is connected to mains earth), then the scope probe ground connection which is at mains earth potential is acting as a shorting link. The ground connection to your scope probe is connected to the scope chassis and thus to mains earth (Class 1). You need either to:- 1) Connect the scope mains plug via a safety isolating transformer or 2) Supply the circuit from a 24V SELV supply or 3) Electrically separate the existing 24V supply with a safety isolating transformer or 4) Use a differential scope probe if you have one (expensive option), thereby electrically separating the circuit under test from mains supply.

 

[DPG]

If it's an electrolytic cap (tubular type, usually blue or black and with a stripe down one side) then they are polarised and will not like 24VAC across them, since it is reversing polarity every half-cycle. Reversed caps go bang! Daz

 

[GLENNSPARK]

Well i have just added that one to "favorites" on my pc mark as that sounds like a damn good bit of info. Now I can digest that post at my lesure.
Thank you.
Glenn.

 

[GLENNSPARK]

If it's an electrolytic cap (tubular type, usually blue or black and with a stripe down one side) then they are polarised and will not like 24VAC across them, since it is reversing polarity every half-cycle. Reversed caps go bang! Daz

Told us about this one last week at college mate.....