Thanks Peter!

The heater wire you mention is probably Nichrome (also available from used
toasters) with a thermal resistance gradient of about 150 ppm/0C.  Not too
bad.  As I mentioned I ended up using Constantan wire (the red wire from a
Type J thermocouple) which is probably called constantan because it's
resistance is nearly constant with temperature (+/- 20ppm/0C).  Copper
actually makes a pretty good thermistor(or a really bad shunt), having a
coefficient three orders of magnitude higher than constantan and two orders
higher than nichrome.

Dan says he is shipping Pic-O-Botboards today, so the brains part of the
robot is soon on it's way!

Lawrence Lile

----- Original Message -----
From: "Peter L. Peres" <plp@ACTCOM.CO.IL>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, May 19, 2001 3:26 AM
Subject: Re: [EE]: NiMH batteries


> Next time when you run out of $3000 meters for making shunts do it my way:
> Set benchtop PSU to current limit 1.00A, clamp 1 crocodile to start of
> shunt, clamp DVM probe to same crocodile, do the same with the other croco
> and DVM probe and then measure out wire until you read as many mV as
> mOhms you need. And since you are at it pick heater wire (usually robbed
> from new 500W space heater elements bought on purpose). This is rated 2+A
> (220V countries) and fairly high R*l. You only need a few cm to make a
> shunt usually and the temperature coefficient is near zero (!). In case of
> need twiddle several strands together (but by this time you can use a
> beefier PVC clad cable as is). A 20A shunt for a SLA battery is mostly a
> 20 cm long battery cable with a thin measuring wire attached at either end
> just at the PVC sleeve.
>
> If you worry about the precision, get a 0.1% 1Ohm resistor put it in
> series with your $3000 (x 10^-4) shunt meter and set 1.000V with the
> current limiter control on it (using the SAME DVM after disconnecting the
> probes). You want to check before and after because benchtop PSUs are not
> known for being precision sources (they drifet as they heat up). If you
> plan to do this often replace the current setting pot on the PSU with a 10
> turn precision Spectrol or Bourns pot.
>
> If you really worry about precision buy a Vishay or other four-point
> shunt. These come in a special case with four terminals (so you need to
> define a footprint for your layout). Not very expensive but hard to get at
> short notice in small quantity.
>
> Peter
>
> --
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>
>

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