# Diode question

Discussion in 'Homework Help' started by thrival, Jul 1, 2009.

1. ### thrival Thread Starter New Member

Nov 18, 2006
7
0
When a diode is being sold, typically there's a voltage rating not to be
exceeded, i.e. 10kv. I presume that's the reverse bias rating. Also say
that voltage rating applies for say, 10mA of current.

But what about forward bias? How much current/voltage can the diode
withstand? Would it be safe to compute the reverse bias ratings in watts
and stay within that parameter, even though voltage and current may vary
in the forward bias mode, i.e. so long as said watt rating isn't exceeded?

Reason is I need to run more current through a given diode than it's rated for.
Voltage will be much less than it's rating in the forward direction. Some heavy-
duty (microwave) diodes have large R drops, which are unacceptable for my
application, hence the need for a ligher duty diode to handle more current at
less voltage.

2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
In the reverse direction a diode can withstand large voltages at very small leakage currents until breakdown occurs.

In the forward direction, at forward voltages above 0.6 to 0.7 Volts the current is unlimited. That is why you will always see a resistor in series with an LED to limit the exponential rise in current associated with small changes in forward voltage.

Schottky diodes have a lower threshold, typically 0.2 to 0.3 Volts.

Forward and reverse characteristics are essentially unrelated. You can draw no conclusions about the one from the other.

3. ### David Lewis Member

Jun 29, 2009
12
0
A heatsink or cooling fan may help the diode carry more current. Resistivity decreases rapidly as the junction gets hotter (high thermal coefficient of conductivity), so stay in the safe operating area to avoid thermal runaway.