Thermopile Switch

Hi, I have following problem . The Thermopile probe ( 0.5V output @ 600 Deg C with internal resistance 2 Ohms ) drives gas valve solenoid with very low resistance ( approx. 0.023Ohm ) . Approximate holding current is 250mA . So there is only about 5mV on solenoid . I need to cut off this circuit with some kind of FEt , MOSFET or opto MOSFET . External circut to control this switch can have common ground with above circuit. Cannot use relay - external circuit does not have enought current to drive relay. I see problem with relativly low switched voltage ( only 0.5V ) and low resistance of fet needed . Any ideas what could be good solution ? Thanks Radim.

Reply to
Radim S
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Mosfets are available with very low on resistances, eg IRF1324S has 0.001 $5 ea at digi-key. you'll need 10VDC to switch it - is that available?

Reply to
Jasen Betts

Can't believe how this ng has improved. Lots of good stuff and helpful problem solving ideas. No flames, spam or interminable OT threads.

Reply to
MoiInAust

I have only 2.5-3.0V from control circuit to drive gate of mosfet . This voltage would be against common ground with Thermopile source . Will this MOSFET or similar be able to switch only 0.5 0.7 V between S-D junction ? Thanks for your reply RADIM

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Reply to
Radim S

"Radim S"

** So you need a DC voltage booster to drive the gate of the MOSFET.

Try "sci .electronics design " - someone there may spoon feed you basic design stuff.

.... Phil

Reply to
Phil Allison

You could try a different way, if what you want to do is to shut off the valve, why not steal enough current so it does not operate. Shunting current may increase your options. How much drive current can your 3v signal supply?

cya, Andrew...

Reply to
Andrew McMeikan

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Thanks everybody for help. But I thing I have found solution. Digi-key has Zetlex ZXMN2B03E6 =

N-channel MOSFET with very low VGS =3D 1.2V minimum @ Rds=3D0.075Ohm. This should work OK with may requiremnts . =

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2B03E6CT-ND). Now I have 1 thermopile driving solenoid valve ( 0.5-0.7V @ 400-6000 D= eg =

C ) . Internal resistance is 2 Ohm so 0.075Ohm will still only marginall= y =

effect current throught solenoid ( 0.023Ohm - need 250mA to hold ON val= ve =

. ) Additional part of this circuit is fed from 2 of the same ( but =

separate from sol. valve ) thermopiles in series . These two (2) ( 1.0 - 1.5V Dc out of them 4 Ohm internal re= s =

. ) thermopiles power flame sensing circuit ( Silicon labs C8051F931 MCU= ) =

. This MCu can run from 0.9 to 1.8 signle cell voltages and generates 3.=

3V =

for output I/O . One of those I/O will drive gate of above MOSFET . I am= =

getting some parts tomorrow so I will let you know how it worked out . Thanks for help again. Radim S.

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Reply to
RAdim Stepanik

"RAdim Stepanik"

Thanks everybody for help.

But I thing I have found solution. Digi-key has Zetlex ZXMN2B03E6 N-channel MOSFET with very low VGS = 1.2V minimum @ Rds=0.075Ohm.

** Reads as 1.5 volts GS when I looked.

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.

** So what are you going to do with the other 2999 you don't need ?

..... Phil

Reply to
Phil Allison

MN2B03E6CT-ND).

Thanks Phill for 1.5V versus 1.2V Vgs minimum correction . I will have 3= .3 =

V available so even 1.5V should work . At 3.3V Rds should be around

0.05 Ohm. These MOSFETS are available in single Qty - please read link =

above . I will have 10 of them tomorrow on my desk , they shipped this =

afternoon .

Regards Radim S.

P.S. Thanks to all for help and to Phill for his direction where to go f= or =

"basic design" questions ....

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RAdim Stepanik

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