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How to make a shunt Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Wed, 4 Sep 2002 09:34:29
In Reply to: Any improvisations/substitutes for big ammeter?, Dorsal1, Tue, 3 Sep 2002 18:22:18 Members do not see ads below this line. - Help Keep This Site Online - Signup |
A shunt is just a low resistance resistor. Yes, you can buy a high precision unit, but you can also make one yourself cheaply and easily. It won't be high precision, but good enough.
18 gauge wire has a resistance of 6.385 ohms per 1000 feet. 16 gauge is right about 4 ohms/1000 feet.
Get yourself 56 inches of 18 gauge wire. That's a 0.03 ohm resistor. Don't bother trying to measure that with your ohmmeter - it takes a special ohmmeter to measure that low a resistance. But stick with physics - just 56 inches of 18 gauge wire. Insulated wire is probably best. Wind it around a stick. This isn't for anything other than to keep almost 5 feet of wire from tangling up with everything. Open the circuit, and connect your shunt in series with the load. Use the millivolt reading of your voltmeter to measure the voltage across the shunt.
One amp across one ohm produces one volt. So one amp across your 0.03 ohm resistor will be 0.03 volts. (30 millivolts). So 25 amps will be 0.75 volts, 20 amps will be 0.6 volts. To measure amps, just measure volts, and divide by .03 to get amps.
You would get more voltage with a longer wire. But then you'd be 'using up' your voltage in the shunt. You don't want to drop more than 0.75 volts or so in your shunt, so keep the wire short.
My guess is that you'll be seeing less than 10 amps in the fan. When you first start the fan, there is a big current spike - that's the energy needed to get the fan spinning. Once it gets spinning, the current requirement drops a lot. You may or may not see that current spike with a digital voltmeter, because it should be very short.
Remember, fuses aren't sized for the load - they're sized for the wire. What? The fuse is there to keep the wire from getting too hot, not for protecting the fan.
When Saab (or anyone) decides what wire size to use, they look at the current through it. That's the load. Thinner wire is better - it's cheaper and ligher. But from making a shunt, you now know that you drop voltage across a wire. So fatter wire drops less voltage. So the designer makes a decision on the thickness of the wire to drop an 'acceptable' amount of voltage, and for a wire that won't get too hot under the normal operating load. There are tables for this.
The fuse is to keep the wire from getting too hot if there is a short circuit. So for every size wire there is a corresponding fuse size. So a big fuse only means fat wire. Yes, fat wire usually means a big current load, but don't assume that a 25 amp fuse means a 25 amp load.
Good luck!
posted by 140.157.4...
, Wed, 4 Sep 2002 09:34:29 <-- Viewing This Message
, Wed, 4 Sep 2002 09:20:09
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