Re: A Quick Question

"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.


.

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