Schematic / Transformer Help - Page 1
Forum section: electronics / beginners

author: djmwj
author_rank: Contributor
timestamp: March 21, 2012, 12:38:29 am
content: I am trying to figure out why my microwave is not working. I found the schematics for the unit and i can't figure out how the design does not show a short at the transformer. As far as i read it there should be 120v supplied to both ends of the coil.... which should short it.... I attached the schematic and would appreciate any input. The switches on there will show closed during operation. Thanks for your help getting me to understand. The Schematic is here:http://www.mediafire.com/download.php?66ittq0kbeklpox

author: IanB
author_rank: Super Contributor
timestamp: March 21, 2012, 01:23:59 am
content: Here's the picture to make it easier to see:

author: IanB
author_rank: Super Contributor
timestamp: March 21, 2012, 01:29:13 am
content: It's a transformer. A transformer is not a short circuit, it is a transformer. It only draws the amount of current it is designed to draw (unless it is faulty).(I should add that it's really dangerous to open up a microwave and try to fix it unless you are an expert in such matters. You could easily die.)

author: sonicj
author_rank: Frequent Contributor
timestamp: March 21, 2012, 01:45:03 am
content: heres the schematic.i'm with ian... microwave ovens are extremely dangerous to service if you don't know exactly what you are doing. aside from checking continuity on the interlocks, thermal cutoffs & resistance values, this project might be best left for a professional.-sj

author: Psi
author_rank: Super Contributor
timestamp: March 21, 2012, 03:09:28 am
content: yeah, be very careful, 1-2A at 2000V is crazy dangerous.It's nothing like 110/230V.2000V at that current will kill you dead instantly.And it's not just the 2000V transformer output you have to worry about.There is a large 2000V capacitor which will store a charge that's equally as lethal, even after you switch it off.Especially if there's a fault preventing the capacitor from discharging normally.

author: djmwj
author_rank: Contributor
timestamp: March 21, 2012, 05:00:12 am
content: Thanks for the cautionary comments. I understand the risks. As for the Schematic, I meant to upload the other side of that.... as you did sonicj, Thanks. I am impressed that you have it! Its a rather old GE microwave. Anyways, I am trying to understand whats going on on the high side of the main transformer. it outputs to the cap which is hooked to the magetron. The magnetron is merely looped back through to the transformer, at a different winding. How is that a complete circuit, and Why do it this way?Also for whatever reason, i don't understand how the primary side of the transformer is powered. If you look at the wiring diagram, it appears that 120VAC is applied to both ends of the primary coil.... which makes little sense to me.Thanks for your insights.

author: IanB
author_rank: Super Contributor
timestamp: March 21, 2012, 05:27:36 am
content: 120 V AC is applied between terminals P0 and P120 on the primary side of the transformer to power it. A high voltage is developed on the secondary side which is rectified and stored in the H.V. capacitor. The negative DC voltage is then applied between the filament and ground of the magnetron. The magnetron is behaving a bit like a vacuum tube in the way it is connected.

author: sonicj
author_rank: Frequent Contributor
timestamp: March 21, 2012, 06:31:25 am
content: Quote from: djmwj on March 21, 2012, 05:00:12 amactually, the internet had it... credit goes to google and some other guy on some other forum.-sj

author: SeanB
author_rank: Super Contributor
timestamp: March 21, 2012, 01:12:16 pm
content: The secondary side is basically a voltage doubler. One end of the transformer is connected to the chassis of the microwave, and the other is applied to the capacitor of the voltage doubler. What is confusing is that there is only one diode shown, the other diode is the magnetron, which is funny enough also the load. On each cycle the capacitor is charged to 2000V via the discrete diode, and on the other half it applied 4000V to the magnetron to get it to run. The thick 4 turn or so secondary that is very well insulated is there to heat the cathode of the magnetron. generally a 4v heater running at 10A current. Do not try to measure anything in the secondary side, even if the power is off. Disconnect the unit from the mains, wait for 10 minutes for the capacitor to discharge through the internal resistor, then short it out with a very well insulated wire jumper across it's terminals. If there is a spark replace the capacitor as the resistor inside is faulty. The diode is hard to test as the forward drop is around 40V, if in doubt replace.Most faults I come across are due to door switches out of alignment ( remember there are 3 switches, 2 apply power, and one shorts the transformer if the door is open) or faulty, either shorted or open. Timers are also a problem, especially electronic ones where the keypads go intermittent or open ( pricey parts there) and where the transformers or relays fail. Most faults blow the mains fuse. To check if it is primary side or high voltage side disconnect the terminals from the primary winding ( direct on the transformer, not elsewhere) and run the unit, opening and closing the door and setting various power levels as well. If it runs fine time to look at the secondary. Disconnect the single secondary lead ( not the heater wiring) and reconnect the primary and test it again. No fuse blowing and then you know it is only 3 components to change. First replace the diode and capacitor as a pair, and note the value used, as it is critical. If it still trips then replace the magnetron.In most cases an aging magnetron will be low power anyway, and a new one will give a big boost.

author: djmwj
author_rank: Contributor
timestamp: March 23, 2012, 02:57:12 am
content: Thanks alot for the replies. SeanB, In my exploring i discovered that the primary coil inputs both look to be driven to 120v individually which makes little sense to me. By this i mean i measured at B2 and y12 (first wiring diagram) of the transformer, while the lines were not attached to the transformer and discovered both had 120V. I thought that at least one of those two would have to be neutral/ground in order to induce current around the primary. If both sides are at 120v there would be no current induced.Am i not correct that B2 and W5 (first wiring diagram) correspond to P120 and p0 (second diagram)? if so how is that possible? Both are at the same potential.

author: djmwj
author_rank: Contributor
timestamp: March 23, 2012, 03:05:09 am
content: so i just answered my own question.... the switch to short the transformer is busted. with the lines shorted they of course both read the same! Thanks! Any ideas where to get a new switch?Another question just to verify my understanding, the ouput of the capacitor to the magnetron is in DC, rather than AC as the diode only allows current through on half the cycle, and the capacitor levels the gaps between cycles?

author: SeanB
author_rank: Super Contributor
timestamp: March 23, 2012, 03:08:19 pm
content: The switch is a standard V3 microswitch, available at any place that sells either washing machine spares or most electronic retailers.The magnetron runs off pulsed DC, half a cycle goes via the diode, and the other half via the magnetron. In all cases the magnetron can be regarded as a thermionic diode, it only starts to act like a magnetron when the voltage gets high enough in the forward direction to start it, but it still is a diode valve, just with a 2.4 GHz ripple on the output current.

author: djmwj
author_rank: Contributor
timestamp: March 27, 2012, 10:43:29 am
content: Thanks for the help. I am going to have to learn more about magnetrons. I got the switch and the microwave works good as new thanks alot.

author: Randall W. Lott
author_rank: Regular Contributor
timestamp: March 31, 2012, 04:37:26 am
content: Microwaves can be extremely dangerous. If you don't know exactly what you're doing, these are one of those things best left to the experts.Be cautious; I hope you can get it to work as well as stay safe.

author: SeanB
author_rank: Super Contributor
timestamp: March 31, 2012, 10:23:20 am
content: Fixed one at work this week. Forklift upgrade, as the capacitor was shorted, and let a little of the magic smoke out of the transformer. Kept the mains cord, wiring, switches, timer, fan, turntable and diode, as well as magnetron for the next one. Rest is in 2 piles - steel and copper, for the scrapyard.