Re: A Quick Question

David T. Ashley wrote:
"Mark Olson" <olsonm@xxxxxxxxxxxx> wrote in message news:CoydnQ-8MKI6vM3VnZ2dnUVZ_tHinZ2d@xxxxxxxxxxxxxx

TomO wrote:


What I'll probably do is wire in a pot with a couple of clip leads to determine what value actually satisfies the sensor circuit, then pull the proper resistor out of my all-too-large supply of spare parts.

I spent many years in the consumer and industrial electronics repair field, so I'm quite familiar with how to figger these things out, but thanks for the quick reminder.

What does putting a tap into a cell of a multi-cell battery look
like? It probably looks like a voltage source more than it looks
like a resistor. So what I would do is put a voltage divider
across the battery and hook up to that. Apparently tying the
sensor input to battery voltage through a series resistor works
too, but the sensor is almost certainly designed to measure voltage
and compare it to a threshold.


I think it is potentially slightly more complicated than that.

A voltage divider will have a characteristic impedance, i.e. it can be modeled as a Thevenin source with a resistor:

http://en.wikipedia.org/wiki/Th%C3%A9venin's_theorem

As you pull current through the divider, the voltage will drop.

It depends on what assumptions the designers made about the characteristics of a probe in electrolyte.

The open-circuit voltage isn't the whole story. A voltage divider with two 1K resistors isn't the same as one with two 10K resistors. Depends on how the input circuit in the electronics loads it.

DUH. I would use a couple of relatively low value resistors
(hundreds, but not 10s of Ohms) hooked up to a switched voltage
source so that the sensed voltage is a stable fraction of the
battery voltage. Without having measured it, my guess is that
the input impedance of the sensor is relatively high (10s of K
Ohms at least). But I would do the pot thing just to find out
where the trip points were and set my divider so that I was
right in the middle of the safe zone (with the sensor hooked
up, naturally, so there's no error from input bias) to minimize
the chance of a false error light. You can certainly do that
with discrete resistors or a decade box but it's not going to
be as quick as using a pot.

Thanks for the refresher on EE 1001, David. You do it your
way and I'll do it my way, and Thevenin will be happy either
way. But my way will give *me* a nice warm fuzzy feeling that
your way won't.

--
'07 FJR13AW '99 EX250-F13
OMF #7
.

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