Heater Test of DIY-WB

[bcroe at juno.com]

Working out solutions to practical issues like this makes
these projects take longer than you think.  Another route
would be to put 15 resistors of 15 ohms 1/4 watt 1% in
parallel.  Cost from DigiKey $1.62.  Now you have accuracy 
without a calibration procedure, but you are using some 
board space instead of a heat sinked resistor.  A 5 watt
1 ohm 5% is only $.41

My feeling is the current limiting isn't that critical to sensor
life, since you will running in the voltage limit mode most
of the time.  On the other hand, dropping out of voltage
mode is bad.  I say go for a current high enough to get
warmup in 45-60 seconds.  Maybe a better test is the 
current after full warmup.  If it is close to current limiting,
you need to increase the current limiting value.

Bruce Roe

On Thu, 16 Aug 2001 18:03:25 -0400 rr <RRauscher at nni.com> writes:
> 
> Yes it should be paralleling R4. Only needs to be a 
> 1/2W resistor.  The value of this resistor will vary 
> depending upon the real value of R4 and the desired 
> warm-up time. The resistor R4 is the current sense 
> resistor for maximum heater current.
> 
> I tend to use easy to obtain parts and fudge from 
> there. I've used a 10% tolerance resistor for R4 and 
> then find it may need trimming. On the perf boarded 
> unit I ended up with a 6.8 ohm across R4. For the 
> PCB unit I just built I ended up using a 10 ohm resistor 
> across R4. Both times I used a 10% 1 ohm main 
> resistor (0.9 to 1.1 ohm)
> 
> There is still some dispute as to what the absolute 
> maximum current should be. On the low side of ~1.3A 
> it has been found that the heater takes over 2 1/2 
> minutes to warm up. At that current it sometimes 
> won't reach the 10.5 V limit required to enable the 
> UEGO controller. In other  words the sensor never 
> reaches full temperature.
> 
> Empirical evidence shows that warm up times of 30 
> to 45 seconds is common for WB units. In order to 
> obtain this warm up time a higher heater current is 
> required. Hence the trimmed current sense resistor. I
> try to shoot for a current of a little over 1.4A or 
> ~40 seconds warm-up time from a cold sensor.
> 
> BobR.
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