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3 volts is way too much Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Fri, 2 Jan 2009 07:23:41
In Reply to: milliamp draw vs voltage draw...., saaboc, Thu, 1 Jan 2009 16:44:54 Members do not see ads below this line. - Help Keep This Site Online - Signup |
The basics are simple V=IxR; the voltage is equal to the resistance times the current. Take a connection with one ohm of resistance, and if you measure one volt across it, there is one amp of current flowing through it. Two ohms, and one amp will give two volts.
The hydraulic analogy is water pipes and pressure. A narrow pipe has more 'resistance' than a fat pipe. If you have a flow of water (current) through a thin pipe (high resistance), you'll get a big pressure difference (voltage) between the ends. A fat pipe (low resistance) gives a lower pressure difference (voltage drop).
The other fun Ohm's law calculation is power; Power = Current squared times resistance. Or Voltage squared divided by resistance; the algebra is all the same.
Measuring resistance with voltage drop is actually a very good thing. Most meters can't really measure reliability below 1 ohm - 1 ohm looks the same as 0.1, or 0.01 ohms. It takes very special meters to read accurately below 1 ohm. OK, so what? Take a typical brake light cluster that pulls about 5 amps of current. Assume the resistance of the ground connection is 0.01 ohms - that would be a 50 millivolt (0.05) volt drop. No big deal. But assume a 1 ohm drop - at 5 amps, that is 5 volts. So what? Remember the power calculation? At 13 volts, with a 0.01 ohm resistance, you've got 12.95 volts to play with. That works out to about 65 watts of power to the bulbs. With a 5 volt drop, you're down to 8 volts across the bulbs. But power increases (or drops) with the square of voltage - you've only got 24 watts at the bulb; less than half the brightness. If it's a window motor, that 8 volts may not allow enough current for the motor to even start turning.
A three to four volt drop is a very big drop in a car. A car's electrical system runs at around 13 volts (12 volts is a nicety, but the sign of a bad battery and a dead alternator). Take away three or four volts, and you've only got 9 or 10 volts left. That's a lot less current available to turn motors, and a lot less available for the electronics.
Remember the bulb example? 5 amps at 1 ohm was a five volt drop? If the current increases to 10 amps, you've got a 10 volt drop. You'll never pull more than 13 amps - you can't drop more voltage than the battery supplies. 13 amps - that's a fuel pump, way less than a blower motor or radiator fan, and way, way less than a starter. So if you measure 3 or 4 volts, it all depends on the current being pulled.
For example, a ground can seem just fine if there is no current going into it. Turn on some lights, and the current goes up, and so does the voltage drop.
I consider any ground to be bad if it has more than a 0.1 volt drop.
posted by 76.221.218...
, Fri, 2 Jan 2009 07:23:41 <-- Viewing This Message
, Thu, 1 Jan 2009 13:21:30
, Wed, 7 Jan 2009 15:06:07
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