Re: Relay Suppression Diode Failure
- From: nospam <nospam@xxxxxxxxxxxxxx>
- Date: Mon, 18 Jun 2007 22:16:09 +0100
Mark P <marketmark@xxxxxxxxxx> wrote:
I trying to determine the cause of a relay suppression diode failure.
The diode is built into the relay can (T-05 type) and is a standard
switching type diode - according to the manufacturer. The relay coil
specs: L=600mH and R=850 ohms. The relay was driven with a 2 second
ON pulse followed by 2 seconds OFF. This duty cycle was continued for
three minutes. The relay was driven with a transistor switch on the
low side. The voltage on the coil was 13V. The relay failed and after
it was opened up you could see that the diode was cooked.
When the transistor switch turns OFF the diode suppresses the voltage
transient. Is there sufficient energy dissipated in the diode over 3
minutes to cause it to fail? Is there a way to calculate or estimate
the diode junction temp rise? The manufacturer has no thermal data on
the part.
The energy stored in the relay inductance is i^2L/2.
13v and 850 ohms gives 15.3mA.
15.3mA in 600mH gives 70uJ of stored energy.
If the diode had to absorb all the stored energy (which it doesn't) the
power dissipated would be 70uJ every 4 seconds or 15.5uW.
The estimated junction temperature rise from dissipating 15.5uW is zero.
More likely you had transient voltage spikes on the 13v supply (possibly
caused by whatever you were switching with the relay) which caused the
diode to break down and then cook because it and the transistor were
shorting the 13v supply. That or you had the coil wired the wrong way
round and were trying to short out the supply with the diode from the
beginning.
--
.
- References:
- Relay Suppression Diode Failure
- From: Mark P
- Relay Suppression Diode Failure
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