1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Please help me recognize clamp-meter's cap ratings for replacement

Discussion in 'Repairing Electronics' started by BGAmodz, Sep 18, 2013.

  1. BGAmodz

    BGAmodz Member

    Joined:
    Sep 15, 2013
    Messages:
    395
    Likes:
    9
    Thanks nigel ,
     
  2. BGAmodz

    BGAmodz Member

    Joined:
    Sep 15, 2013
    Messages:
    395
    Likes:
    9
    Why does magnetron needs such high voltage (2100 VAC) witch i guess is delivered by a step up transformer inside the microwave ?
     
  3. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

    Joined:
    Oct 30, 2010
    Messages:
    9,915
    Likes:
    1,096
    No, a magnetron needs a source of electrons which happens to be a low voltage AC supply similar to a CRT filament. It also nned a HIGH voltage DC voltage to accelerate those electrons. See here: http://www.thermex-thermatron.com/mw-theory.html

    The very thick wires on the transformer is for the filament. There is also a half wave High Voltage Rectifier and capacitor filter. The High voltage shares a filament connection.

    You basically tried to measure 2100 VDC or whatever it died down to. There is usually a bleeder resistor across the capacitor because of the dangers.

    A small CRT oscilloscope like the Tek 211 or 212, I think uses about 3000 VDC.

    One instrument I worked on with the 15 kV power supply had a 1 Megohm 200 Watt resistor for the bleeder resistor.
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. BGAmodz

    BGAmodz Member

    Joined:
    Sep 15, 2013
    Messages:
    395
    Likes:
    9

    yes i read that they come with a built in 10 Mohm resistor for self discharge .
     
  6. unclejed613

    unclejed613 Well-Known Member

    Joined:
    Apr 3, 2009
    Messages:
    1,828
    Likes:
    99
    it's not uncommon for the internal bleeder resistor to go bad. when it does, it doesn't affect the operation of the microwave, but it does mean the cap will remain charged for a very long time after the oven is shut off (or even unplugged). so i never rely on the internal bleeder, but i always discharge the cap with two crossed screwdrivers (with plastic handles). after the cap has been discharged, i disconnect the diode, transformer and magnetron.
     
  7. BGAmodz

    BGAmodz Member

    Joined:
    Sep 15, 2013
    Messages:
    395
    Likes:
    9
    The internal transformer , is it a step up transformer or step down ??? i don't understand how it works , and i don't see any connection with mains (phase and neutral).
     
  8. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

    Joined:
    Oct 30, 2010
    Messages:
    9,915
    Likes:
    1,096
    The transformer is actually both. A step up and a step down transformer.

    It's similar to any electron tube power supply, whether a photomultiplier, x-ray set, e-beam evaporator, TV picture tube, valve amplifier etc. It's just medium power like 1 KW. an e-beam evaporator is about 10 KW, a CRT probably (40 kV*20 mA) W

    There should be an SCR/TRIAC on the mains that turns on the power to the magnetron. Power is somewhat non-linear, so 10% power is not on for 10% of the time.

    So it's no more than a high voltage power supply (about 2100 V) riding on a low voltage AC supply (< 6 VAC) with a power of around 1 KW. There are no electrical grids, screens or deflection mechanisms in a magnetron. I believe there is some magnetic focusing.

    There are inverter based microwave designs,
     
  9. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,214
    Likes:
    640
    Location:
    Derbyshire, UK
    Nothing like that, usually 25Kv at a maximum (and limited to) 1mA - some really large sets might be 30Kv..

    Mostly it's switched by a realy, TRIAC's are fairly uncommon.

    The only reason for it's non-linearity is the time taken for the magnetron to actually fire up, so you have the warm-up time before it starts producing RF.
     
  10. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

    Joined:
    Oct 30, 2010
    Messages:
    9,915
    Likes:
    1,096
    Nigel is completly right. I didn't verify a source for the TV anode voltages and currents. True, both relays and triacs are found in the primaries of the HV transformer in non-inverter microwave ovens.

    My statement about non-linear power behavior failed to mention the reason which Nigel nailed.

    Thanks.
     
  11. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

    Joined:
    Oct 30, 2010
    Messages:
    9,915
    Likes:
    1,096
    Tracing the primary to mains:

    What may be making it difficult is the complexity of the interlocks. There is usually a mains fuse and a NC interlock switch across the mains after the fuse. Then there are two NO interlock switches in series.
     
  12. unclejed613

    unclejed613 Well-Known Member

    Joined:
    Apr 3, 2009
    Messages:
    1,828
    Likes:
    99
    you will usually find at least one interlock switch on the door latch, and one or more thermal switches in series with the transformer primary. the door interlock can get out of position and not close the contacts when the door is closed, the thermal switches can fail open circuit. if the door interlock acts indirectly by energizing a relay, the interlock relay can fail.
    KISS wrote "There is usually a mains fuse and a NC interlock switch across the mains after the fuse." (??????)
    such an interlock would blow the fuse every time you open the door..... most common is a microswitch that drives an interlock relay on the control board, since microswitches that would actually be in the primary circuit wouldn't last long, or they would be expensive because they would need to be rated at 10A.

    the most common failures in the primary circuit that keep the oven from heating, are:
    thermal switch open no power to the transformer, but seems to be operating normally
    interlock switch failed, or out of position, depending on which switch, may not get primary power to transformer, or controls act as if the door is open
    control board failure, usually bad relay contacts if everything seems to be working normally except the transformer never gets primary power. you need to listen carefully to the relays on the board.

    secondary circuit failures:
    magnetron, usually filament goes open circuit transformer hums, and cap is charged, but no heating.
    cap shorted, or shorted to ground. usually blows main fuse.
    shorted diode, again, usually blows the fuse
    open diode, cap never charges
    shorted turn in transformer, blows the fuse
    open primary in transformer. power gets applied to primary, but transformer doesn't hum
     
    • Like Like x 2
  13. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

    Joined:
    Oct 30, 2010
    Messages:
    9,915
    Likes:
    1,096
    On page 10 of the service manual I posted in #22, shows the "monitor switch". This switch can short the primary of the transformer if the door isn't quite right and the magnetron tries to turn on. That's the switch I was trying to describe. My head is really hurting today and I probably shouldn't even be posting.

    NC normally closed. Or closed when the door is open.

    So, if power is inadvertently supplied to the magnetron with the door open, the fuse will blow. Usually the other interlocks will prevent power from being applied.

    There were a few other interlock switches too and one that was monitored by the microprocessor.
     
  14. BGAmodz

    BGAmodz Member

    Joined:
    Sep 15, 2013
    Messages:
    395
    Likes:
    9
    Hey enclejed , yoo seem to have a clear idea on how microwave ovens work .
    I have fixed a lot of microwaves in the past , and there is mostly these situations :
    1) Oven does not turn on :
    Usually the electronic control card is faulty , but the power on the card is delivered by the big transformer or directly from mains ???
    2) Poor heating /oven turns on but no heating :
    In the first case the magnetron needs replacement , but in the 2nd case the magnetron usually works fine , but its hard to know if its the capacitor /half wave diode/transformer that is bad , any ideas ??
     
    Last edited: Sep 24, 2013
  15. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,214
    Likes:
    640
    Location:
    Derbyshire, UK
    Although it's not very well written, it's actually true - there's usually two switches which turn the mains OFF, and a third (the monitor switch) which shorts the mains out and blows the fuse if the other two fail in any way.
     
  16. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,214
    Likes:
    640
    Location:
    Derbyshire, UK
    No disrespect, but your knowledge is so limited you shouldn't be going any near a microwave oven, you're not safe.

    You're also obliged (in the UK) to power and leakage test every oven you repair, with a calibrated (yearly) professional leakage tester.
     
  17. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

    Joined:
    Oct 30, 2010
    Messages:
    9,915
    Likes:
    1,096
    1. Physical inspection

    2. Wait some time, like 5 minutes and discharge the cap. Wait a few minutes and measure the voltage across it. Only ten, can you try an ohmmeter or capacitance tester.

    3. The diode usually shorts. The diode test won't work unless it's shorted. You can test the diode by applying a larger, current limited voltage and measuring the voltage drop. e.g A 9V battery in series with a 9 V/10e-3 A resistor and measuring the voltage across it.

    4. The transformer: Look for insulation breakdown and measure the continuity of the windings. The actual ohms can be confirmed in a service manual.
     
  18. BGAmodz

    BGAmodz Member

    Joined:
    Sep 15, 2013
    Messages:
    395
    Likes:
    9
    Hi guys , i got some news , i have received a bunch of high current clamp meters similar to the one i had gone bad but bigger in size , some of them turn on but missing some capabilities like AC voltage/current readings , and some of them just don't boot at all .
    Will see if i can manage to fix them .

    I will post pictures of them later .
     
  19. BGAmodz

    BGAmodz Member

    Joined:
    Sep 15, 2013
    Messages:
    395
    Likes:
    9
    I managed to check the transformer , the primary side got two connections , the secondary got 3 wires , the middle one is connected to the magnetron .
    So when i power up the transformer with 220 VAC , i measure the voltage between the middle one with each one of the two secondary wires , i get about 140 VAC .
     
  20. debe

    debe Active Member

    Joined:
    Mar 26, 2011
    Messages:
    538
    Likes:
    30
    Location:
    South Australia
  21. debe

    debe Active Member

    Joined:
    Mar 26, 2011
    Messages:
    538
    Likes:
    30
    Location:
    South Australia
    Please read these notes VERY carefully. They are from a Toshiba Wksp manual & note it is VERY dangerous to measure the secondary voltage of the transformer & un necessary. ( Ive been repairing Microwaves for 35yrs & survived by being well informed & Carefull around High Voltage) Hate to see some one Killed due to not knowing what is involved & not knowing. Ask questions & make sure you understand what is being said. Stay SAFE & alive.
     

Share This Page