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G8MNY > TECH 27.06.06 18:12l 153 Lines 7628 Bytes #999 (0) @ WW
BID : 36236_GB7CIP
Read: OE5RCO DL1LCA DB1YAK GUEST
Subj: Microwave Ovens
Path: DB0FHN<DB0THA<DB0ERF<DB0FBB<DB0IUZ<DB0GOS<DB0RES<ON0BEL<LA2MV<GB7CIP
Sent: 060627/0919Z @:GB7CIP.#32.GBR.EU #:36236 [Caterham] $:36236_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Apr 06)
(8 Bit ASCII Graphics use code page 437 or 850)
Here is the circuit that is used in all Microwave Ovens. It is deceptively
simple & masks the features of the circuit.
Mains Main Timer Power _________________ (-7kV peak)
L ÄÄÄÄÂÄÄo-oÄÄÄo\ÄÄo\ÄÄÂÄÄÄo\ÄÄÄÄÄÄÂÄÄo\ÄÄÄ¿ º(_______________ ³Heaters
o Fuse SW1 SW2 ³ Switch ³ % on ³ º __ 2.5kV ³ ³_³
Small | Door ³ or Relay ³ Relay )º( |<fuse ³ / V \ 2400MHz
Fuse o Switches o ÚÄÄÄÄÄÄ´ )º( ³ Cap ³ ³ ___C³Ä RF
³ SW3 \ ÚÄÄÁÄÄ¿ ÚÄÁÄ¿ )º( ÃÄÄ´ÃÄÄ´ÿ \_³_/
ÚÄÄÄÄÄÁÄÄÄÄ¿ ³ ³TURN-³ ³FAN³ )º( ÀÄ10MÄÄ´ ³ Magnetron
³controller³ ³ ³TABLE³ ÀÄÂÄÙ ³ º( _³_ ³
ÀÄÄÄÄÄÂÄÄÄÄÙ ³ ÀÄÄÂÄÄÙ ³ ³ º( 900W \_/ ³
N ÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÙ ³ Trans- ³Multi ³
³ former ³Diode ³
E ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÙ
FEATURES
SAFETY. To stop misuse 3 sequenced door switches are used to stop the power
SW1 & SW2, & to blow the fuse SW3!
CONTROLS. If there is a controller & clock etc, it may have separate mains
transformer & fuse etc. & operate the on & & on relays. Otherwise
clockwork time switches may be used.
POWER LEVEL. As the power can't be varied & the valve magnetron has to warm
up for a few seconds to reach full power, a simple interrupted on/off
ratio timer over several seconds is used to control the mean cooking
power.
RF COOKING. The turntable motor or moving RF stirrer is needed to ensure food
(containing water/fats) gets evenly cooked otherwise the 13cm standing
waves would produce very strange results, & potentially dangerous to
eat cooked/raw banded food! Try grated cheese on a still sloping piece
of toast to see the standing waves.
MAINS VOLTS. Mains Voltage variation is cunningly catered for by using a
under saturated transformer in near resonance with the small Cap. As
the voltage reduces the transformer L increases & partly resonates
with the Cap to produce increased voltage to the magnetron exactly
compensating for the mains reduction.
N.B. Generator users:- this effect only works at the rated mains
frequency!
3.5_kV _
2.5_³ /'~`\ 0v ³~`\ /' RF ³ _
³ | | ³ | Cathode | Frequency³ /' `\
0vÄ´³- - - -³- - - -³ ³ ³ Volts ³ & ³ | |
³ | | kV³ | | Power³ | |
³ \._./ -7_³ \._./ ³___³_______³____
QRM. Energy dispersal on the 13cms band is ensured due to the raw power
supply 0v to -7kV peak 50 times a second on the cathode, causing the
RF oscillation to be FM & AM over a broadband. Usually 2 RF peaks
cross a spot frequency per cycle eg. @ 100Hz.
MAGNETRON
This electrically is a just valve diode, but it has resonant cavities in the
anode & a powerful magnet (hence name) with pole pieces above & below the
cathode, which makes the electrons curve passed the cavity capacity gaps. Each
cavity works like a whistle & oscillates. Link wires may be used to RF couple
all the cavities to the output cavity.
³
/~~~~~~~~~~~~~~~~~\ ³³ O V E N
/'/'~`\ /'~`\`\ ÚÄÄÄÄÄÄÄÄÄÄÙ³
/' ³ ³ ³ ³ `\ ³ :
/' \._ \.---./ _./ `\ ³ :
/' `> gaps<' `\³ :
³ /' `\ ³ :Plastic
Finned ³ /'~`\__³ CATHODE / ~ \ ³__/'~³\ ³ Output :Cover
Heatsink³ ³cavities| (0) | \=³ ³========= :
³ \._,/~~³ \ _ /Curved ³~~\._,/ ³ 1/4 wave :
³ \ Electron / ³ :
\ `> Paths <' ANODE /³ :
`\ /'~' /`---'\ `~"\ /' ³ :
`\ ³ ³ ³ ³ /' ³ :
`\ \._,/ \._,/ /' ÀÄÄÄÄÄÄÄÄÄÄ¿³
`\_________________/' ³³
³
The outer anode case is at earth potential & has finned area for the fan to
cool, this waste heat is ducted through the cooking chamber.
FAULTING (dangerous DEADLY voltages, definitely 1 hand behind your back!)
Always leave tumbler of water in the oven during test! NEVER DISABLE THE
SAFETY SWITCHES! R.F. B U R N S.
1/Does the mains transformer hum? If not check fuses, door switches, & relay.
2/Using a high voltage probe (10x 10M in series in old pen case to a scope)
a) see if 2.5kV AC is present from transformer. If not DC check pri & sec.
b) see if 0-7kV is present on diode/cathode. If not disconnect cathode &
retest. If still no voltage, disconnect diode, if AC appears suspect the
multi section diode. Do a cap test otherwise ( ÷ 1uF 3kV ).
c) if all OK except when cathode connected replace magnetron/scrap.
d) if all OK but no RF replace. Test heater circuit, no heaters replace
magnetron/scrap. Heater OK, test secondary circuit & repair connectors or
broken wire etc.
3/If arcing noises or thumping are noticed, try checking in the dark, if there
is light from any connections/insulation take remedial action. If from
magnetron replace it. If radiator arc (RF O/P of magnetron), check the 1/4
wave cover is making good connection to valve pin if it is a separately
connected type.
4/RF leakage. The RF is radiated into a fully welded box with 1/4 wave overlaps
door choke coupling. It should not leak much! But U can see the 100Hz FM
bands on a 13cms CCTV Rx.
³³¬ Wave
COOLINGÚÄÄÄÄÄÄÄÄÄÄÄÄÄÙ³RF Seal
FAN ³ Magnetron |
³ Radiator o |see through
³ |mesh &
³ platter |glass door
³ =========== |
ÀÄÄÄÄÄÄ×ÄÄÄÄÄÄ¿³¬ Wave
Motor ³³RF Seal
I use a commercial microwave RF sniffer to make sure. It is crude thing
with a 10GHz diode & a meter.
HAM USE
I have seen a design for a 13cm ATV Tx based on a magnetron. A highly dangerous
project!
Many hams think the small but powerful 900W mains transformer could be useful
for valve linears. It can be done, but the trouble is one side of the 2.5kV AC
is wired to the transformer's case earth & even if that end was lifted off
earth, to feed a bridge rectifier to give a 3.5kV HT, the insulation to earth
of that end of the winding is poor & will not readily handle the 3.5kV peak! If
done a single 900W transformer should make a 800W continious PSU or 1600W peak
CW/SSB mode PSU OK provided it is fan cooled as in the oven.
Another approach is to use a half wave rectification for 3.5kV DC PSU. But this
will DC saturate the transformer & reduces the rating by about a factor of 3,
making a 900W transformer only safely give 300W continious with this amount of
core satuation & the much reduced inductance on the mains. So 2x identical 900W
transformers wired in antiphase will be OK for QRO CW/SSB use. eg. 600W
continous load or 1.2kW intermittant.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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