<VV> Bulb over Meter WAS Late model headlight switch - problem solved

David B. Neale david.neale3 at ntlworld.com
Thu Nov 18 13:40:44 EST 2010


"I have thought about that over the years.  Can it be possible for modern meters sense the presence of voltage / potential without any current draw?  I don't own anything sensitive enough to find any amperage at all through the volt meter.
Ken P "

Modern meters still have to draw a current, Ken, but they draw such a tiny current that, to all intents and purposes, they are, in effect, drawing no current at all. In some circuits, such as radio receivers, modern meters can measure voltages around, say, a transistor or valve, (tube), without changing the circuit's operation.

If a voltage is applied across a resistor, and the current through that reistor is extremely small, there is virtually no voltage drop.

Imagine trying to measure the voltage across a resistor of 200 ohms, using a meter which itself presents a resistance of 200 ohms across its probes.  We now have, in effect, two 200 ohm resistors in parallel; in other words, the circuit now has a 100 ohm resistor where it before had a 200 ohm resistor.  Now, half the current will flow through the original resistor, and half through the meter .... the voltage shown by the meter will be only half the true value. This scenario would obtain if one used a very old, very insensitive meter.

With a modern electronic, (digital), meter, its internal resistance is so extremely high, usually many millions of ohms,  that the current drawn is incredibly small; so small that there is, in effect, no voltage drop due to the presence of the meter. Ideal for certain applications, but not for others.


In your Corvair, as in the case of a defective but visually-perfect fuse, grossly misleading results may obtain from using a modern meter if one doesn't have some understanding of the issues involved.  Many people get wholly misleading results from simply sticking a meter across two points and believing that they are measuring the true voltage.

Fault-finding and voltage tracing in your Corvair will be much, much easier, and the indications be true, if you use a light bulb/buzzer device; with the added benefit that, especially with the buzzer, one doesn't have to look at, and especially importantly, interpret what a meter is trying to tell you.  If the bulb light, and the buzzer buzzes, you have voltage there!

Sorry this was so long ...

David in Blighty



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