Measuring 2KVDC. How would you do it safely?

Thread Starter

Tonyr1084

Joined Sep 24, 2015
5,856
I have a microwave oven that is acting up a bit. Seems to not heat as fast as it should. I'm thinking it may have a bad transformer, but how do you measure something that is supposed to be 2KVDC?

The circuit consists of a step-up transformer, a high voltage diode and a high voltage capacitor. I think (I said "I THINK") the diode and capacitor work to double the voltage as well as rectify it. With a positive going sine wave, the capacitor is charged through the diode. When the sine wave goes negative the capacitor adds 1KVDC to the transformer output and makes for a 2KVDC voltage source. (or something like that)

I want to measure it but only have standard measurement tools. And yes, I know enough to be careful around the high voltage AND the capacitor. How would YOU measure the voltage? My standard DVM only measures as high as 600AC volts.
 

#12

Joined Nov 30, 2010
18,217
Buy the right high voltage probe.
I've never measured the KV. With about 3 parts in the circuit, you can measure the other two parts and figure out who's dead.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
5,856
So far I haven't done ANYTHING. I want to be sure to be safe.

I assume if the HV diode is bad then I shouldn't get any heating. And if the HV capacitor is bad, could it be open? Shorted? If shorted wouldn't it have gone POOF!?! If open then maybe that might account for a low output. Not sure here. So I'm taking this very slow, and I may just opt for buying a new oven over the chances of having to buy a casket for my golden little toasted body. Safety is my #1 concern. #2 is - I'm cheap. I'd rather fix it than replace it. Too easy to replace it. In the end I probably will replace it.

So I come here to ask of those who know more than I know.

Thanks y'all.
 

AlbertHall

Joined Jun 4, 2014
11,212
There is another part that may be causing your problem - the magnetron, and that's a lot more difficult to test.
The diode could be shorted or breaking down at a low reverse voltage. That would reduce the available voltage to the magnetron.
The capacitor could have lost capacitance. Does your meter measure capacitance?
The transformer could have shorted turns which would reduce the voltage but then it would also get much hotter than normal. Does it?
High voltage probes don't come cheap but you could, for a one-off, use a string of resistors suitably heat-shrinked, to reduce the voltage to something your meter can measure. You would need to know the resistance of your meter.

[Edit] This suggests your low heat is very bad news :(
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
5,856
I thought about building a high resistance voltage divider, something that will turn 2KV into 200V.

I've also considered taking a 12 volt transformer and using the 12 volts AC as a source for the primary. Since we operate on 120 VAC, 12 VAC input into a normally 1KV secondary should result in 100 VAC. I think. And I don't want to light up my bench OR my fingers, arms, heart muscle.

Like I said, I haven't done anything yet. I want to KNOW where I'm going before I start messing with things that could be potentially dangerous. 2KV capacitors ARE dangerous. Being careful is the buzz word here. I may dip the cap leads into a salt water bath to discharge any potential before I start messing with probes and stuff.

Yes, I have a capacitance meter but I have very little faith in it. I've tested high value (farad) caps before and found that it takes a very long time to get a reading even close to what the cap's rating is supposed to be. The HV diode? I assume it's like any regular diode except it can handle much higher voltages than regular diodes can. Testing it should be straight forward enough. But to be sure I may take a 12 volt source and drive an LED through the HVD(iode). If the LED lights up with one polarity and not with the other then I'll assume the HVD is good. Provided an HVD can pass 12 VDC.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
5,856
A pound of water increases by 1 degree F per btu and a watt second is 3.413 BTUs.
Water, thermometer, math.
Didn't know that little factoid. Thanks 12. Since I haven't done anything yet, and I know I have a 1100 watt oven I should be able to come up with some real numbers. Just one question: How much water is one pound? I know a gallon of water weighs around 8.2 pounds (or something close to that - I can google it). Come to think of it I can probably google a pound of water.

My altitude is 4550 feet. I know water boils at 203.4 degrees F. But I don't think I want to boil water I just want to measure the time to heat it one degree as you suggest. Thanks.
 

paulktreg

Joined Jun 2, 2008
799
Use the correct test equipment or just replace the microwave?

You're going to need a high voltage probe of some description whether it's for a DMM or an oscilloscope but if it is the transformer then I'll bet you can't buy parts.
 

ScottWang

Joined Aug 23, 2012
7,004
The high voltage probe is a good way, but it is more expensive, if you don't use it very often then buy it just waste the money, you could buy the high voltage resistor 90MΩ and 10MΩ to make a voltage divider, it can be expanding 10 times of the measuring range to 6KV.

You can also buy the black Heat Shrinkable Tube Shrinking Tubing Sleeve in your local store, and choosing the diameter a little bigger than the high voltage resistor, and using the heat gun to make the Heat Shrinkable Tube smaller and match all of the high voltage resistors, just left the pin out, you could put double Heat Shrinkable Tube for the resistors.
 

#12

Joined Nov 30, 2010
18,217
I used to work on these as my day job, and I never measured the voltage. It was never necessary because they don't break, "a little bit". Door switches, timer, power relay, diode, capacitor, overheat safety, fuse, turntable motor, fan...that's about the whole truck stock for a uwave repairman. Therefore, I discourage trying to attack the most dangerous diagnostic possible. It probly ain't broke.
 

#12

Joined Nov 30, 2010
18,217
Isn't a BTU about ~1000 watt-sec?
If you don't know and you're too lazy to google it, don't guess. It isn't helpful.
Why not use a voltage divider to measure the voltage?
Because nobody has established that it's broken and there's no point in trying to perform the most dangerous and least helpful measurement possible if it's not broken.
 

dendad

Joined Feb 20, 2016
3,810
I would encourage you to buy another if you are in doubt. It sounds like your electronic knowledge is a bit limited and so I think trying to work on about the most dangerous device you have in the home may not be a good idea.
Not only microwave ovens have lethal voltages but there is the microwave radiation to worry about.
That being said, there are plenty of Google results for high voltage probe designs. For example...
http://rimstar.org/equip/hv150kvprobe/hv150kvprobe.htm

You need to make the high value resistor out of many lower value resistors in series to spread the voltage across them to prevent breakdown.
Then calibrate the probe against a known voltage. But, do you know the required voltage anyway?

Testing the capacitor is a good idea as these do fail, going low capacitance. A lot of multimeters have a capacitor tester builtin. Also remember the capacitor can hold dangerous voltage for a long time, so make sure it is well discharged before doing anything.

"If in doubt, DONT"!
Your life is worth a lot more than a suspect microwave oven.
 

BR-549

Joined Sep 22, 2013
4,938
It's true it's been awhile since I looked it up. I found that 1 BTU/sec is 1055 watts. It's hard for me to believe that one watt/sec can heat a pound of water 3 degrees F.

A voltage divider will easily allow the monitoring of the voltage without any special equipment. Once you get the voltage down...you can scope it. If it's fuzzy or too much rf......you can use a detector (diode,cap,resistor) on scope probe......and check out DC component.

This will confirm/not confirm your suspicions.

The turn table is working, right? That happen to me once.
 

GopherT

Joined Nov 23, 2012
8,012
If you don't know and you're too lazy to google it, don't guess. It isn't helpful.

Because nobody has established that it's broken and there's no point in trying to perform the most dangerous and least helpful measurement possible if it's not broken.

I think out OP is looking for a reason to buy the high voltage probe that he saw at the hardware store. Why on earth use basic phish a when he has a perfectly good reason to buy that big-ass HV probe. He can grab onto it with both hand and...
 

#12

Joined Nov 30, 2010
18,217
1100 W/hr x hr/3600 seconds = 0.30555 w/second
0.30555W x 3.413 BTU/w = 1.03 BTU/sec
1 lb x 1.03 BTU = 1.o3 degree F
100F/1.03 degrees = 95.89 seconds / 100 degreesF

Now, if you really want that big, shiny probe, buy it and find out how your meter reads when it's trying to measure half a sine wave with a 2KV peak every second half cycle. There is no smoothing capacitor so all you will get is half a sine wave and the voltage might or might not go to 2KV. What should the current be if power is only being delivered for less than 1/2 cycle of the power line frequency? A magnetron designed for 2KV surely won't go into oscillation at 10 volts. Maybe not at 100 volts. Else, why hit it with 2,000 volts?

If you find 1800 volts, how are you going to measure the current, and how much should it be? Are you going to plug your DMM into the 2KV line and hope you insulate it well enough that it doesn't arc to case and turn into a ball of flame? If you do get both measurements, how are you going to know what the duty cycle of the magnetron tube is?

Either that or heat a pound of water and use a $10 thermometer.:rolleyes:
 

recklessrog

Joined May 23, 2013
985
Please, Please, take this warning seriously. The power available from the transformer and associated rectifiers and capacitor can blow you into the next world in a split second!! Without meaning to insult you in anyway, you do not seem to have the required knowledge to work on such a dangerous device, so please desist for your own safety.
Even though the capacitor usually has a built in discharge resistor, this can take several second to discharge after the oven is turned off.
Most likely the fault is a low output Magnetron, and a replacement is usually more expensive than a complete new oven from a supermarket.

Is risking your life worth less than $100??? ..............Just leave it alone before you injure yourself or worse, bite the bullet and buy a new oven!!
 

GopherT

Joined Nov 23, 2012
8,012
Please, Please, take this warning seriously. The power available from the transformer and associated rectifiers and capacitor can blow you into the next world in a split second!! Without meaning to insult you in anyway, you do not seem to have the required knowledge to work on such a dangerous device, so please desist for your own safety.
Even though the capacitor usually has a built in discharge resistor, this can take several second to discharge after the oven is turned off.
Most likely the fault is a low output Magnetron, and a replacement is usually more expensive than a complete new oven from a supermarket.

Is risking your life worth less than $100??? ..............Just leave it alone before you injure yourself or worse, bite the bullet and buy a new oven!!
Come on!, $200 for an HV probe, a budget for replacement parts, no clue how to find the problem, a couple of hours experimenting, a grand kid screams or doorbell rings, slight movement of the HV probe from the noise, kids try to figure out how to sell the house and split the money - they have to buy a new microwave before closing the deal on the house. Tombstone has loving tribute that includes the word "zapped".
 
Top