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VK6JY > QRP 25.09.02 09:20l 76 Lines 3212 Bytes #999 (0) @ WW
BID : 20482-VK6JY
Read: DB0FHN GUEST DG9NBR
Subj: Power FET Tester (ASCII)
Path: DB0FHN<DB0ZWI<DB0HOT<DB0ERF<DB0FBB<DB0GOS<ON0AR<ON0AR<ZL2BAU<VK6HGR<
VK6BBR<VK6RAW<VK6PBV<VK6JY
Sent: 020925/0717Z @:VK6JY.#ALY.#WA.AUS.OC #:52896 [Albany] FBB7 $:20482-VK6JY
From: VK6JY@VK6JY.#ALY.#WA.AUS.OC
To : QRP@WW
Test Power MOSFETS at Home. VK6JY
ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ
These devices look like open circuits between all terminals when
tested with conventional DVMs or analog multimeters. A good / bad
decision is not possible without a better test jig - hence this
circuit suggestion.
It needs a metered power supply capable of fixed or preferably
variable regulated DC from typically 10 to 20 volts and providing
0.5 to 2 amps where volts and amps can be read off a meter. High
precision isn't a big factor here. If you have no metering built
in, I would expect a tail light globe from a car will make a fair
indicator by using that in place of the 22 ohm drain load shown -
go for a 21 watt 12 volt type or a bit heavier.
Gate volts can be followed on a normal multimeter clipped from the
pot wiper to the negative rail. I assume an N-Channel device
throughout. If the odd P-Channel FET shows up, obviously the power
supply can be reversed but the scheme would work as outlined.
POWER MOSFET TESTER VK6JY
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄo
ÚÁ¿ ÚÁ¿
10k³ ³ 22R ³ ³ +
³ ³ 20W ³ ³
ÀÂÙ ÀÂÙ 12V to 20V
³ ³
³ D³ REGULATED
ÚÁ¿ ÉÍÍÍÏÍ»
³ ³ ÚÄÄÄÄ¿ Gº MOS º DC
5k ³ <ÄÄÄÄÄÄÄÄÄÄ´ ÃÄÄÄÄĶ FET º
³ ³ ÀÄÄÄÄÙ ÈÍÍÍÑͼ
ÀÂÙ 10k S³ -
³ ³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄo
MOUNT DEVICE ON SUITABLE HEAT SINK
The idea is to preset the pot to minimum volts, then apply power
with moderate volts, say 10 or 12 at first. If the FET has no
short from the source to drain, there should be negligible current
flow. Slowly rotate the pot and about half way you will see a
beginning of drain current that will cook the FET unless on an
adequate heatsink. Touch test to check and don't leave it up for
too long if heating is excessive. Check the gate can be wound back
below the point where drain current ceases - maybe around 2 volts
on the gate. If you are sure the FET max ratings are okay and the
heatsink can cope, apply more volts and explore the Id current
control via the gate pot. The smaller VN10 types may not survive
high gate volts - check their max rating ... 5V ?
It is the lack of that forward gate bias that makes DVM testing
useless. That's it folks -- certainly a correct "good / bad"
decision is likely. Also matching pairs may be possible, with due
diligence. Have a go!
The drain R used here is a set of four 100 ohm 5 watt power
resistors in parallel giving 25 ohms at 20 watts. This is not
critical. As I said above a lamp will be fine and will vary in
resistance with its positive temperature coefficient. The other
resistors are 1/4 watt types.
73 de John, VK6JY @ VK6JY Albany, Western Australia
ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ
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