MOSFET and residual current
0001T
Posts: 14
Hi, I need some advice.
I am making use of BS2 to control 4 mosfets (100V, 50A), which will in turn send·current (12V max, 2A max) to heat up 4 wires (DUTs).
I/Os from BS2 are wired to the gates of these mosfets, a 10k ohms resistor is wired in series between them.
Protection diodes are wired across the DUTs (wires).
The operation is cyclic in nature, i.e. 2 DUTs are heated up simultaneousy for 100 cycles.
However, it was observed that after about 15-20 cycles, there seems to be residual current even though the gates of the mosfet are opened (signal from BS2 are '0')! This was observed through some LEDS·which were dimly lighted.
What could be the cause of these 'residual current'?
It seems like these mosfets are acting somewhat like a capacitor. The wires are heated up for 3 seconds, then cooled for 3 seconds. Are these timing too short for the mosfets to discharge?
What could be wrong?
Thanks.
I am making use of BS2 to control 4 mosfets (100V, 50A), which will in turn send·current (12V max, 2A max) to heat up 4 wires (DUTs).
I/Os from BS2 are wired to the gates of these mosfets, a 10k ohms resistor is wired in series between them.
Protection diodes are wired across the DUTs (wires).
The operation is cyclic in nature, i.e. 2 DUTs are heated up simultaneousy for 100 cycles.
However, it was observed that after about 15-20 cycles, there seems to be residual current even though the gates of the mosfet are opened (signal from BS2 are '0')! This was observed through some LEDS·which were dimly lighted.
What could be the cause of these 'residual current'?
It seems like these mosfets are acting somewhat like a capacitor. The wires are heated up for 3 seconds, then cooled for 3 seconds. Are these timing too short for the mosfets to discharge?
What could be wrong?
Thanks.
Comments
What MOSFETs are you using? Are they getting hot? If they are not logic level MOSFETs, they may not be switching on fully with the logic high voltage output (roughly 5V) and thus heating up unnecessarily after a few cycles. That still doesn't explain the behavior you're seeing because a hot MOSFET should conduct less than when it's cool.
They are not getting hot during operation.
The mosfets worked fine during the initial 15-20 cycles, after that the LEDs will be observed to be dimly lighted.
Thanks
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- Stephen
If the mosfets are switching properly and not overheating then it's pretty safe to say they are logic level.
The LEDs that glow dimly... what are they connected to?
sorry
LEDs are connected to MOSFETs as well, paralled to the DUTs. They are meant to indicate that signal/ current is present.
-Phil
'Training Program for SMA wire
' {$STAMP BS2}
' {$PBASIC 2.5}
counter········ VAR··· Word
Reset:
DO WHILE IN3<1·············
LOOP
counter = 0················
DEBUG "Start Training", CR
PAUSE 3000
Heating:
IF IN3 = 0 THEN STOP
DEBUG "Heating wire 1",CR
HIGH 0·····················
PAUSE 3000·················
DEBUG "Cooling", CR
LOW 0······················
PAUSE 3000
DEBUG "Heating wire 2",CR
HIGH 1
PAUSE 3000
DEBUG "Cooling",CR
LOW 1
PAUSE 3000
counter = counter + 1
DEBUG ? counter
IF counter < 100 THEN heating
I'll try to load the schematic of the circuit.
anyway, yesterday i tried troubleshooting from the bs2 side. i realised that by removing the signal connection from bs2, only two of the dimly lighted LEDs remained lighted.
i even rewired the whole circuit
Post Edited (0001T) : 6/6/2008 12:53:04 AM GMT
-Phil
http://www.irf.com/technical-info/appnotes/mosfet.pdf
BTW, what's the resistance of your heater wires when they're cold?
-Phil
-Phil
tried my best to draw the schematic
all the resistors are 10kohms, diodes are 3A schottky diodes, mosfets are STP40NF10
See attached picture.
Post Edit -- "breadboard", hopefully not a "solderless breadboard"; it's not a good idea to pull Amps from/through such a thing.
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Post Edited (PJ Allen) : 6/7/2008 4:11:47 PM GMT
-Phil
I need higher voltage (current) to wire heater, stamp's supply is not high enough which is why I have two separate supplies.·5V to stamp and·a 12V to mosfets.·
-Phil