Relay substitute for low voltage low current application

[Wildgoose]

Hi,

There is a thermocouple in parallel with an inductor (that's the WHOLE
circuit!), the thermocouple generates 35mV @ 0.5uA when hot and 0V@0A when
not. Currrently between the + pole of the theormocouple and the inductor I
have a relay that will open the circiut in certain conditions, otherwise it
stays closed and so "invisible". How can I wire up a transistor or
transistors or scr in such a way that when biased properly the device is as
"invisible" as a closed relay switch, so that the 35mV @ 0.5uA is absolutely
unaffected while the device is properly biased?

Best

Wildgoose

 

[CFoley1064]

Subject: Relay substitute for low voltage low current application

From: "Wildgoose" [email protected]
Date: 4/27/2004 5:12 PM Central Standard Time
Message-id: <[email protected]>

Hi,

There is a thermocouple in parallel with an inductor (that's the WHOLE
circuit!), the thermocouple generates 35mV @ 0.5uA when hot and 0V@0A when
not. Currrently between the + pole of the theormocouple and the inductor I
have a relay that will open the circiut in certain conditions, otherwise it
stays closed and so "invisible". How can I wire up a transistor or
transistors or scr in such a way that when biased properly the device is as
"invisible" as a closed relay switch, so that the 35mV @ 0.5uA is absolutely
unaffected while the device is properly biased?

Best

Wildgoose

The standard answer would be to look at an opo-FET like the Fairchild H11F1

http://www.fairchildsemi.com/ds/H1/H11F1.pdf

It has an Rds(on) of a couple hundred ohms, and an Rds(off) of 300 Megohms
plus. For your 0.5 uA current, that will give you a voltage burden of 100
microvolts, which might be adequate.

If this is a home brew project, you might want to take a look at getting relays
which are meant for the job -- they might give you a more accurate and reliable
solution. You need to specify a "dry switching" reed relay with low thermal
offset. A good first choice might be the COTO 3602 series, which can have from
10 uV to 0.5 uV thermal EMF voltage.

Good luck
Chris

 

[CFoley1064]

Subject: Relay substitute for low voltage low current application

From: "Wildgoose" [email protected]
Date: 4/27/2004 5:12 PM Central Standard Time
Message-id: <[email protected]>

Hi,

There is a thermocouple in parallel with an inductor (that's the WHOLE
circuit!), the thermocouple generates 35mV @ 0.5uA when hot and 0V@0A when
not. Currrently between the + pole of the theormocouple and the inductor I
have a relay that will open the circiut in certain conditions, otherwise it
stays closed and so "invisible". How can I wire up a transistor or
transistors or scr in such a way that when biased properly the device is as
"invisible" as a closed relay switch, so that the 35mV @ 0.5uA is absolutely
unaffected while the device is properly biased?

Best

Wildgoose

The standard answer would be to look at an opto-FET like the Fairchild H11F1:
http://www.fairchildsemi.com/ds/H1/H11F1.pdf

It has an Rds(on) of a couple hundred ohms, and an Rds(off) of 300 Megohms
plus. For your 0.5 uA current, that will give you a voltage burden of 100
microvolts, which might be adequate.

If this is a home brew project, you might want to take a look at getting relays
which are meant for the job -- they might give you a more accurate and reliable
solution. You need to specify a "dry switching" reed relay with low thermal
offset. A good first choice might be the COTO 3602 series, which can have from
10 uV to 0.5 uV thermal EMF voltage.

Good luck
Chris

 

[Tom Bruhns]

35mV/0.5uA implies that you have 70kohms in the series loop. A mosfet
with 5 ohms resistance should loop pretty invisible: less than 0.01%
of the total. 2N7002?? Just pick one with low "on" resistance and
very low gate leakage current, so that the gate leakage is also tiny
compared with the loop current in your thermocouple/inductor loop.
Then you need to be able to apply a voltage between the loop (the FET
source, in particular) and the FET gate to turn it on Any bipolar
transistor will introduce too much offset. The other effect you will
see is capacitive coupling from the gate to the loop, so there will be
an impulse when you turn it on and one (in the opposite direction)
when you turn it off.

Cheers,
Tom