Re: neg voltage relay driver (for an idiot)

On 2 Sep 2005 15:04:02 -0700, crm0922@xxxxxxxxxxxxxx wrote:

>Ok. So the TL071 could work with a voltage divider calculated to go
>near to zero from the TL071's offset voltage.
>
>However, that doesn't explain why both cmprtr outputs are going high
>when Vin is 0V. Could it be because I am using 15k's instead of 10k?
>I've checked the wiring about 10 times, but I will check it again. Now
>I'm at home and I think I have some 10k's here, or I could pick some up
>tomorrow.
>
>Could the 0V input actually have a small voltage against ground,
>resulting in a small positive potential to ground? would that even
>turn on the top relay, were it the case?

---
After looking at the data *** for the TL071, I found that they
don't have rail-to-rail inputs (mea culpa for making that asumption)
but, rather, have an input common mode range of +/- 11V with a +/-
15V supply, which means that you have to stay 4V away from the rails
in order to have the thing work right. With a single 24V supply,
that means (I assume) that both sets of inputs should be between 4V
and 20V. The reference divider satisfies that requirement since its
high output is at +18V and its low output at +6V, but if Vin is
being fed into the working divider from a low impedance source, then
its output will go very close to the rails when Vin is either at 24V
or 0V. The easy way to fix that is to connect a series resistor
between Vin and the input to the working divider, but we need to
know what the source impedance of Vin looks like in order to
calculate the value of the resistance.

If you could set Vin to +24V and then then connect it to the
ungrounded ends of, first, a grounded 100k ohm resistor, second, a
grounded 10k ohm resistor, and third, a grounded 1k ohm resistor,
then measure and post the voltages read across the resistors I can
show you how to calculate the value of that series resistor if you
don't already know how.

--
John Fields
Professional Circuit Designer
.

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