using an analong meter for diy-wb

[bcroe at juno.com]

The Vout lead has a 1000 ohm resistor (R15)  to minimize 
the chance that a mis connection to it will blow something
up.  For a 10,000,000 ohm meter the 1000 is invisable.  But
for 200,000 ohms you see 1/2 % error.  You could calibrate
your meter to have an extra 1000 ohms outboard to cancel 
the error.  In any case the output current is limited to 2 or 3
ma, but taking out the resistor would bring you up to the 
amp (U4b pin 7) output limit of at least a dozen ma.  A
"pullup" resistor of 470 ohms, from +8 volts to the Vout 
could help the amp double that again to at least 24 ma.

Bruce Roe

On Thu, 14 Feb 2002 09:04:40 -0500 Brian Renegar
<thomas.renegar at nist.gov> writes:
> I was wondering about this.  On the main DIY-WB page it says a high 
> impedance device is required to read the output.  So 200,000 ohms is 
> the 
> limit then??  Why so high?  If you're reading stock NB O2 sensors, 
> then 
> obviously you need a high impedance device.  But this uses a heater 
> and 
> control circuit.  I figured that would supply enough current that 
> any type 
> of device could be used.  Am I wrong??
> 
> Brian
> 
> >If the meter input impedance is less than about 200,000
> >ohms, you might want to reduce R15 to zero.

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