Microwave will not heat

[kwired]

Whenever working on capacitors with bleed resistors, I still check for voltage or short across the capacitor, etc. You can not always visually tell if the resistor is functioning.

 

[Electric-Light]

Whenever working on capacitors with bleed resistors, I still check for voltage or short across the capacitor, etc. You can not always visually tell if the resistor is functioning.

That's a good way to destroy your DMM. Dump the energy into a 40W light bulb. The inertia of the filament is more than enough for the residual energy. The worst that can happen is that the filament will blow.

 

[kwired]

That's a good way to destroy your DMM. Dump the energy into a 40W light bulb. The inertia of the filament is more than enough for the residual energy. The worst that can happen is that the filament will blow.

How is measuring the voltage going to destroy my DMM? That is what it is designed to do is measure voltage. One needs to know what kind of voltage to potentially be present and use a meter that measures the proper range and is at the correct setting, but that is necessary for measuring any voltage.

I realize that this thread is talking about microwaves, and the capacitor in those may be storing a pretty high voltage. When I run into capacitors they are typically for 600 volt or less motors, lighting, or power factor capacitors, and are typically small enough capacity that just shorting them with a screwdriver is what I typically do. That is better for me than shorting them with my fingers unexpectedly later on. I will do this even if there is a bleed resistor, better than finding out with my fingers that the resistor is open for some reason.

 

[G._S._Ohm]

just shorting them with a screwdriver

Shorting caps may shorten their life.

 

[mivey]

Shorting caps may shorten their life.

And won't necessarily remove the charge. After shorting, a cap can still "recover" a little bit over time and have to be discharged again. At the moment, the offical terminology escapes me.

 

[Electric-Light]

How is measuring the voltage going to destroy my DMM?

By measuring a voltage source far beyond the rating of the meter that can provide a significant current.
The high voltage capacitor inside a microwave is rated 2100v AC, so depending on where the peak was, it can hold about 3,000v DC (2100 * sqrt 2).

If the bleeder resistor is dead and your DMM sees 3kV, it goes poof.

The safety factor isn't the same proportion across all voltage. A 5v rated IC may survive 20v fine. That doesn't mean that 1,000v device will withstand 4,000v.

 

[Electric-Light]

And won't necessarily remove the charge. After shorting, a cap can still "recover" a little bit over time and have to be discharged again. At the moment, the offical terminology escapes me.

A very low leakage capacitor acts like a bucket as well and can pickup a charge. A high voltage capacitor can build up a significant charge when exposed to say something like negative ion generator type air cleaner much like leaving a bucket outside on a rainy day.

After a while, the capacitor has the ability to dump the contents.

 

[K8MHZ]

And won't necessarily remove the charge. After shorting, a cap can still "recover" a little bit over time and have to be discharged again. At the moment, the offical terminology escapes me.

Yep. We found that out the hard way testing large DC conductors.

We followed the same procedure we did for the AC conductors. Hi-pot 25kV, ground out and short to discharge for a couple minutes, label and move on.

So, we checked a big run of shielded DC conductors (for a power house), bled them off, tore down and went to break. 15 minutes later the terminating crew went in to put terminals on the conductors and were getting shocked by the cable.

 

[mivey]

A very low leakage capacitor acts like a bucket as well and can pickup a charge. A high voltage capacitor can build up a significant charge when exposed to say something like negative ion generator type air cleaner much like leaving a bucket outside on a rainy day.

After a while, the capacitor has the ability to dump the contents.

Not quite what I had in mind.

Dielectric absorption was the official term I was looking for.

 

[kwired]

Shorting caps may shorten their life.

Better the cap life than my life. I don't disagree that placing a load across them is probably a better practice, a meter or some shorting device is still the only way to ensure they are not storing any charge.

 

[kwired]

By measuring a voltage source far beyond the rating of the meter that can provide a significant current.
The high voltage capacitor inside a microwave is rated 2100v AC, so depending on where the peak was, it can hold about 3,000v DC (2100 * sqrt 2).

If the bleeder resistor is dead and your DMM sees 3kV, it goes poof.

The safety factor isn't the same proportion across all voltage. A 5v rated IC may survive 20v fine. That doesn't mean that 1,000v device will withstand 4,000v.

Like I said, I mostly run into capacitors operating below 600 volts. I have seldom measured for voltage, and usually just short low capacitance motor capacitors I commonly run into. Only occasionally do I get much of a noticeable arc when doing so. So I am guessing the amount of energy stored in these is generally not that much energy, or they often have little or no charge at all.

If I were to mess with a capacitor from a microwave I would much rather short that capacitor vs. trust a resistor, than to find out just how hard that thing will bite me if I come in contact with the terminals. I may place a load across it before shorting it, but shorting it at some point should result in a voltage of zero.

 

[ActionDave]

And won't necessarily remove the charge. After shorting, a cap can still "recover" a little bit over time and have to be discharged again. At the moment, the offical terminology escapes me.

Dielectric absorption was the official term I was looking for.

This can happen with large transformers too, from what I have been told. Any truth to it?

 

[G._S._Ohm]

This can happen with large transformers too, from what I have been told. Any truth to it?

If the interwinding capacitance is significant. Small capacitance will reduce the time duration of the shock you get and voltages above 600v will puncture the skin and so increase the current into your body.

 

[mivey]

This can happen with large transformers too, from what I have been told. Any truth to it?

I'm not sure but a transformer certainly has some capacitance and some polarization takes place so I would not be surprised.

I know they can store voltages and SOP is to ground tanks, bushings, everything. I'm sure I have something around here listing safety concerns but I would think static would be the bigger concern (after de-energization of course).

 

[ggunn]

I sometimes work on my guitar (tube) amplifiers, and the power supply filter caps likewise can store enough charge to be dangerous. I made a probe from a wirewound resistor and a clip wire that I use to ground all the high voltage points in the circuit before I start working.

 

[John120/240]

Working on a microwave that the display lights up and accepts inputs but when you press start the microwave does not heat, I have done the resistance checks on the magnatron and it checks good. would appreciate any help. thanks

Chattahoochee what did you end up doing to fix the problem ? Replace a part or replace the

whole microwave ?