Hi, Darrell,
OK, the Electronics teacher in me just had to come out. I poked around
on the net a little and couldn't find many details about how an electric
gauge like this would work. My concern is about the power that a pot would
need to dissipate to simulate the sender. According to one description the
circuit in the meter applies a constant current through the sensor so the
meter is just basically an ohmmeter with the meter scaled appropriately. If
this is true of yours and the current is low enough you won't have any
problems.
Based on my reading on the net I don't think the following is an issue
but I'm going to mention it anyway:
On the other extreme if a full 12V is applied to the sender then at the
lowest resistance it would pull I = V / R = 12V / 85 Ohms = 0.14A. If you
use a potentiometer set to 85 Ohms then the pot would be dissipating P = V^2
/ R = 12^2 / 85 = 144 / 85 = 1.7 Watts. The two pots mentioned so far would
fairly quickly fry trying to dissipate this amount of power.
I did see one mention of 5V being applied to the sender. In that
instance the power dissipation would be 5^2 / 85 = 0.3 Watts. The pot that
Greg linked to is a 1/2 Watt unit so should would survive this treatment
undamaged IF you don't go much lower than that resistance (P = V^2 / R so R
= V^2 / P = 5 ^2 / 0.5 = 50 Ohms minimum).
Now you've got me curious about how an electric oil pressure system.
Blast - yet another thing to investigate ;-)
Hope this helps and doesn't confound the issue.
Mark Watson
1956 Daimler Regency Mk II '104' (stalled restoration :-(
1965 Ford Falcon - brake work
and various other uninspiring transportation pods
-----Original Message-----
From: Randall
Sent: Tuesday, March 12, 2013 6:43 PM
To: Shop-talk@autox.team.net Talk ; DarrellWalker
Subject: Re: [Shop-talk] Need a variable 85-285 ohm resistor
Radio Shack unfortunately doesn't carry much in the way of components any
more. You're lucky if they get even close to what you want.
But in this case, I think that
http://www.radioshack.com/product/index.jsp?productId=2062307
will do well enough. You won't be using its full range, but since it is 15
turns from 0 to 1K, you'll still get about 3 turns between 85 ohms and 285
ohms.
Randall
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