Thermopile Switch

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
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.

Re: Thermopile Switch
Quoted text here. Click to load it



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?

Quoted text here. Click to load it


Re: Thermopile Switch

Quoted text here. Click to load it


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




Re: Thermopile Switch

Quoted text here. Click to load it

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

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

Re: Thermopile Switch

"Radim S"
Quoted text here. Click to load it


** 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





Re: Thermopile Switch
Quoted text here. Click to load it

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...

Re: Thermopile Switch
On Sat, 13 Dec 2008 21:07:08 -0700, Andrew McMeikan < snipped-for-privacy@iinet.net.a=
u>  

Quoted text here. Click to load it

Quoted text here. Click to load it

Quoted text here. Click to load it


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
@ Rds3D%0.075Ohm. This should work OK with may requiremnts .  =

(http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name3D%ZXMN=
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.




-- =

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

Re: Thermopile Switch

"RAdim Stepanik"

Quoted text here. Click to load it


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






Re: Thermopile Switch
On Tue, 16 Dec 2008 18:27:50 -0700, Phil Allison < snipped-for-privacy@tpg.com.au=
Quoted text here. Click to load it

Quoted text here. Click to load it

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 ....




-- =

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

Site Timeline