Thermopile Switch

R

Radim S

Guest
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.
 
On 2008-12-11, Radim S <radim100@yahoo.com> wrote:
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 ?
Click to expand...
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?

> Thanks Radim.
 
"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:ght5bh$5vd$1@reversiblemaps.ath.cx...
On 2008-12-11, Radim S <radim100@yahoo.com> wrote:
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 ?

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?

Thanks Radim.
Click to expand...

Can't believe how this ng has improved. Lots of good stuff and helpful
problem solving ideas. No flames, spam or interminable OT threads.
 
On Fri, 12 Dec 2008 00:54:25 -0700, Jasen Betts <jasen@xnet.co.nz> wrote:

On 2008-12-11, Radim S <radim100@yahoo.com> wrote:
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 ?

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?

Thanks Radim.
Click to expand...
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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"Radim S"
I have only 2.5-3.0V from control circuit to drive gate of mosfet .
Click to expand...

** 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
 
Phil Allison wrote:
"Radim S"
I have only 2.5-3.0V from control circuit to drive gate of mosfet .


** 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
Click to expand...
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...
 
On Sat, 13 Dec 2008 21:07:08 -0700, Andrew McMeikan <andrewm@iinet.net.au>
wrote:

Phil Allison wrote:
"Radim S"
I have only 2.5-3.0V from control circuit to drive gate of mosfet .


** 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





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...
Click to expand...

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. This should work OK with may requiremnts .
(http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ZXMN2B03E6CT-ND).
Now I have 1 thermopile driving solenoid valve ( 0.5-0.7V @ 400-6000 Deg
C ) . Internal resistance is 2 Ohm so 0.075Ohm will still only marginally
effect current throught solenoid ( 0.023Ohm - need 250mA to hold ON valve
.. ) 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 res
.. ) 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.




--
Using Opera's revolutionary e-mail client: http://www.opera.com/mail/
 
"RAdim Stepanik"

Phil Allison wrote:

I have only 2.5-3.0V from control circuit to drive gate of mosfet .


** 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.
Click to expand...

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.


(http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ZXMN2B03E6CT-ND).


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




...... Phil
 
On Tue, 16 Dec 2008 18:27:50 -0700, Phil Allison <philallison@tpg.com.au>
wrote:

"RAdim Stepanik"

Phil Allison wrote:

I have only 2.5-3.0V from control circuit to drive gate of mosfet .


** 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.



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.


(http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ZXMN2B03E6CT-ND).


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




..... Phil
Click to expand...
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 for
"basic design" questions ....




--
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