Discussion in 'Homework Help' started by debbie, Jan 17, 2007.

1. ### debbie Thread Starter New Member

Jan 14, 2007
2
0
God this makes me look thick!
What would happen if a moving coil meter with fsd of 10mA had a 1A circuit momentarily connected to it? I'm not sure of this or the suitabilty of such action

2. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
That's 100 times fsd, so it would attempt to move to a point 100 times farther than fsd. Well, not really. It would simply move very hard and fast to wherever the magnetic fields become parallel

Mechanical stops (the little peg) would prevent this, of course. This is colloquially known as "pegging the meter."

Now... if we assume that the meter coil can handle 120% to 200% of fsd current - what happens when we send 10,000% fsd current through said coil?

3. ### debbie Thread Starter New Member

Jan 14, 2007
2
0
I'm thinking it would blow the meter

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Hi,

A quick application might not. The pointer should make a loud tick against the stop. I'd want to check response afterward.

5. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
"Blowing the meter" typically means burning off the insulation from the coil. How hot the coil needs to be for this to happen depends on what type of insulating varnish is used. (They have different ratings.) How fast the wire gets to the bad temperature depends on how much current goes through it, how big around it is, and what the ambient temperature is.

How many seconds in "momentarily" or "quick?"

6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Hi,

Actually, there's another aspect to "blowing the meter". I got one wet and, thinking it was low pressure, tried to blow the water out with an air hose. The air pressure blew the coil into a long knot. Didn't hurt the coil, though.