Couple of thoughts -
Does the Temp meter run off of the pulsing voltage
regulator too? If so then
those calculations might be off since the voltage
might be averaged in the 10
Volt range, as well as the baseline resistance of
the sender itself should be
factored into the entire draw at worse case 24
ohm thermistor + your resistor
PLUS whats is in the meter. The drop across
your resistor would have to be
calculated seperate to find its dissipation.
I would be very surprised if the temp gauge has much draw at all, and what is
likely in the temp sender is a termsistor and it can't draw much current at
all
or it would self heat and not be something that would work as a
temperature
measuring device. If all of this math is too messy, the easiest
way to check
would be to hook up you multi-meter in series in current mode
and measure (At
temp when reisistance is lowest), I'll bet a 1/2 watt to 1
watt resistor is
fine. As mentioned you need to derate the capability of the
resistor if under
hood, double wattage would be about right on. Now saying
all that, this is Lucas
stuff so might run the generator through the temp
sender to save some wiring so
you may see a bit more draw....
Sandy
________________________________
From: Tod Brown <todbrown@roadrunner.com>
To:
"Smit, Theo" <Theo.Smit@dynastream.com>
Cc: "Carmods@aol.com"
<Carmods@aol.com>; "tigers@autox.team.net"
<tigers@autox.team.net>
Sent: Thu,
October 21, 2010 6:50:15 AM
Subject: Re: [Tigers] Engine Temperature
Looking
at Stu's data, the temp gauge reads 248 F at 24 ohms, so we can
calculate the
power consumed from P = V*V/R = (12 v)*(12v)/24 ohms = 6
watts, so a 10 watt
resistor should do the job. I mounted the resistor
under the bonnet, near
the sender. I have not had any melting of the
plastic coating. Using a
resistor with a low power rating risks
overheating it and frying it, just
like a fuse.
Tod
On 10/21/2010 9:17 AM, Smit, Theo wrote:
> Hi all,
> As Tod
noted, resistors used in the temperature sender or fuel tank sender
>circuit
can pull quite a bit of current and they should be appropriately rated
>or
else they'll get too hot.
>
> I forget offhand what the minimum resistance in
the gauge circuits is, but I
>think that the maximum current can be on the
order of 1/4 to half an amp. In
>that case, a 4 ohm resistor would dissipate
one watt (1/2 x 1/2 x 4 = 1). That
>doesn't mean that a one-watt resistor is
sufficient power rating, since that
>power rating is typically obtained when
the case temperature of the resistor is
>75 degrees C or higher (that's like
160 F or something). You should probably use
>a 2, 5, or 10 watt resistor to
keep the temperature rise under control and
>prevent inadvertent burns or
melting of other underdash parts.
>
> Cheers,
> Theo
>
________________________________________
> From: tigers-bounces@autox.team.net
[tigers-bounces@autox.team.net] On Behalf
>Of Carmods@aol.com
[Carmods@aol.com]
> Sent: Thursday, October 21, 2010 7:06 AM
> To:
todbrown@roadrunner.com; tigers@autox.team.net
> Subject: Re: [Tigers] Engine
Temperature
>
> Tod,
> I have used a resister approach to calibrate the
temperature gauge like you
> did but I found that if I put shrink wrap on
resistors it may burn off so
> now I leave resistors exposed to air.
>
> John
Logan
>
>
>
>
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