Problem with testing WB (heater circuit)

[bcroe at juno.com]

I would suggest you use the 10 ohm, 5 ohm resistor test
for the heater ckt I previously described.  With a 10 ohm
in place of the sensor heater (at terminals J1, J2) the
LED should come on.  If it does, you should see about 
10.4 V across the resistor.  It will heat really fast unless 
you have a 10 watt resistor.  

Correct voltages but no LED could be reversed LED.  
Vout will never vary from 2.5V until the LED is on.  If 
the voltage from the junction of R7-Q1to gnd rises to 
near battery V but no LED, check voltage across LED.  
More than 2V means its bad or reversed.

If that works, put the sensor heater back and monitor the
voltage across it.  It should rise to 10.4 V in 1/2 minute 
and the LED come on.  During this same time, voltage
across R4 should be 1.25 V, decreasing slightly as the
LED comes on.  If J1-J2 voltage is way low, look for a 
short (C3 backwards?).   If you are close to correct, its 
possible your sensor needs a bit more than 1.5 A, which 
the WB can't deliver.  Get back with test results, we could
do a work around to save an extreme sensor.

Bruce Roe

On Mon, 28 Jan 2002 13:02:55 -0500 Brian Renegar 
<thomas.renegar at nist.gov> writes:

> Started testing my WB circuit yesterday, and have run into a 
> problem.  First a run down of the specs....

> Power source from battery on running car (13.50V)
> 
> The problem is, the sensor heats up and appears to be 
> working, but the LED never comes on.  I have been 
> monitoring VOut, and it never deviates from 2.50V.  The 
> current to the sensor started at about 1.33A.  Reading 
> through previous posts, I see it should be just over 
> 1.4A.   So I added several resistors to R4, one by one.  
> Eventually I got the current up to about 1.5A, which I 
> thought was more than adequate to properly heat it.  
> However,  within a minute of connecting the power, 
> the current slowly goes down to about 1.23A.  Is this 
> supposed to happen?  I'm afraid to add any more 
> resistors.  I don't want the current to go too high on 
> initial turn-on.
> 
> Can someone please give me some ideas as to what's going on?  I 
> don't know 
> where to begin testing.  BTW, I built two boards, and they are both 
> exhibiting the same behavior.
> 
> Thanks,
> Brian

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