# Circuit analysis with diodes

1. Feb 16, 2009

### dancavallaro

How do I do basic circuit analysis of circuits that have diodes in them? For example, if I have a basic circuit consisting of just a voltage source, a resistor, and a silicon diode, how would I calculate the current in the circuit?

2. Feb 16, 2009

### Mapes

For basic analysis (and assuming a silicon diode), assume a constant 0.6-0.7 V voltage drop at the diode.

3. Feb 16, 2009

### Staff: Mentor

Here's a recent Homework Help thread discussing analyzing a circuit with diodes in it. The thread starts off assuming "ideal" diodes with Vf = 0V, but does address real diodes later with Vf = 0.6 to 0.7V, etc.

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4. Feb 16, 2009

### dancavallaro

Ah, thanks for that link! What happens if there's no resistor in the circuit and it's just a diode hooked up to a voltage source?

5. Feb 16, 2009

### Redbelly98

Staff Emeritus

6. Feb 16, 2009

### dancavallaro

I found the diode equation, but I think it's overkill for my needs. Mapes, and the link berekman posted, helped a lot for my purposes. The only thing I don't get now though, is what happens when the diode is hooked up with no resistor. Is the current 0, or is it infinite? I'm inclined to say that it would be "infinite", but is that right?

7. Feb 16, 2009

### Redbelly98

Staff Emeritus
If the diode equation is overkill, then the simple answer is:

The current is 0 when V is below the diode-drop voltage (about 0.7V).
The current is infinite when V is above the diode-drop voltage.

8. Feb 17, 2009

### Staff: Mentor

For a real diode, you will probably burn it up. Look at the plot of the diode equation -- the current increases quickly for voltages above the forward threshold voltage. And since power P = V * I, the power dissipated increases quickly as well. Real diodes have limits on how much power they can handle...