# Diode and circuit problem

1. Nov 4, 2005

### QueenFisher

in the diagram below, the voltmeters are as near to perfect as possible, the cell has negligible internal resistance. the red blob is a conducting diode, and the green blob is non-conducting. the coloured boxes are resistors. in order to work out the current through the red diode, is it ok to redraw the circuit as shown on the left? i figured that as the voltmeters have very high resistance, there'd be virtually no current through them, and none at all through the green diode. is this an ok way of looking at it?

http://home.ripway.com/2005-7/361234/circuit.bmp

2. Nov 4, 2005

### WalterContrata

Negligible Conductances

I think you are on the right track. The conductance of Voltmeters can be neglected if they are in parallel with a much lower resistance element. Likewise, the current through the "non-conducting" diode can be neglected if it is much less than that through the reddish rectangle. If it is an ideal diode, then the current under reverse bias is zero, and your approximation is exact.

3. Nov 4, 2005

### QueenFisher

yes my teacher told me to take the resistance of the reverse-biased diode as zero, so i hope it will work.

thanks for checking it!

4. Nov 4, 2005

### WalterContrata

I think you are all set, but didn't your teacher mean that the current of the reverse biased ("non-conducting") diode is zero, or the conductance of the reverse biased diode is zero, or the resistance of the reverse biased diode is infinite?

Best Regards,
Walter

5. Nov 5, 2005

### QueenFisher

sorry 'bout that, i typed it wrong. thanks for noticing.

6. Nov 5, 2005

### WalterContrata

Don't mention it -- I thought it was a typo, but wanted to make sure.