diy uego

Robert R Ducker bducker at txdirect.net
Wed Jan 17 01:41:10 GMT 2001


At 06:38 PM 1/7/01 -0000, Phil Martin wrote:
>   Has anyone else built a diy uego circuit yet? If so  any hints or useful
>changes to the circuit on the diy-efi site would be useful.     Thanks 
>Phillip Martin 

     I haven't built the circuit, but did enter the 2 schematics into
Circuitmaker, a simulator. I had to drop this project for awhile, but did
come up with the following. If you want to skip the technical stuff, then
my conclusion here is that the circuit will probably work as advertised!
    The heater driver circuit works as a rough current limiter until the
heater reaches operating temp. The wiper arm of R1 should be set at about
500mV and the wiper arm of R2 set at about 4.5V, in order to limit power to
the heater during warmup to around 16W and voltage after warmup to about 10V. 
I think these values are about right, but maybe not. The positive
temp/resistance ratio of the heater then provides the needed regulation of
the sensor temperature. Q7 functions as a simple switch, the Vss line
switches from "high" to "low" when the sensor has warmed up.
     The "controller" part of the circuit uses PID feedback between the
sensor and the ion chamber. When using supply voltages of +5V and -5V, it
has an overall gain of about -2 for the proportional part, -5 per 5mS for
the integral part and no derivative part. In other words, if the sensor
input moves by +100mV from its nominal 450 mV, then the output accross the
ion chamber moves -200mV, plus an additional -500mV is ramped in every 5mS
as long as the +100mV input offset stays.
     If the ouput voltage swings too much to read on an ordinary voltmeter,
then maybe the PID constants are a little off.  Other effects are possible,
an output waveform from a woking version could tell the story. I would like
see one if anyone has it, a hand-drawn version would work! 

Thanks,
Bob D

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