Posted by Jim Wilkins on July 11, 2009, 7:10 pm
> Ulysses,
> If I remember correctly, rheostats where used as light dimmer knobs on
> old cars, maybe you'll run into one sometime?
> Curbie
Rheostats that will handle 3 to 5 Amp field current aren't cheap or
easy to find surplus.
Be careful of both the wattage and the current ratings. When they are
set near their lowest resistance the current and thus local power is
quite high in a small section of the element and you can burn out the
wire or the sliding contact without exceeding the device's total power
rating.
For 0.5 to 3A field current:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=D225K25R=
E-ND
http://www.ohmite.com/catalog/pdf/rheostats_wirewound.pdf
http://www.ohmite.com/catalog/pdf/210_series.pdf
jsw
Posted by Curbie on July 12, 2009, 12:06 am
Jim,
>Rheostats that will handle 3 to 5 Amp field current aren't cheap or
>easy to find surplus.
Thanks for the correction, didn't mean to lead anyone astray.
Curbie
Posted by ghio on July 12, 2009, 8:28 am
snip
> Yes, the voltage regulator is disabled. It is simply bypassed and the
> connection to the field coil is made directly via the brushes.
> I'm just using some nichrome wire (it's what I had on hand) and I *think*
> it's about 14 ohms. A rheostat would be a neater way to do it and would be
> simple to fine tune. The combined total power consumption of the field
> coils and the resistor on my setup is somewhere around 175 watts so the
> rheostat would need to be sized accordingly. The field coils are powered by
> the output of the alternator from the ground connection and the positive
> output and connected directly to the brushes. The 80+ volts that I'm
> getting from my setup are reduced, via the nichrome resistor, to somewhere
> in the neighborhood of 12-14 volts (I have not measured this voltage for a
> long time but it's one of those things that's consequencial--we are more
> concerned with the output voltage in this case). I'm using the internal
> diodes for rectification and, from what I've read, as long as I don't exceed
> the 63 amps they are rated for they should hold up.
The Nichrome wire should be long enough so you can connect one end to
the field and using an alligator clip connected to the positive on the
alt allows you to fine tune the field by sliding the alligator clip
along the Nichrome. I then found a rheostat on an old spot welder
going for scrap.
When Rimmer (aka wayne) sticks his nose in you can safely disregard
anything he says.
snip
> > Curbie
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> If I remember correctly, rheostats where used as light dimmer knobs on
> old cars, maybe you'll run into one sometime?
> Curbie