Relay EMF killing LEDs? - Page 2 - Electronic Circuits Projects Diagrams Free

Mr RB · 26th June 2009, 06:40 PM

Put the diode across the LED itself, in parallel reversed, and stick a big cap on the LED too like 100uF. That will save your LEDs, and provide snubbing, and still provide fast relay release.

Actually with a 100uF cap on the LED you may not need the diode at all, the energy in the relay coil is not likely to move that voltage much as the cap will be at 1.7v and the back emf will just drop that cap voltage a fraction, maybe drop 1/2 volt?

If I had a spice thingy I could test that...

chand · 28th June 2009, 04:44 PM

1. connect a diode 1N4007 in reverse direction across the relay coil.
2. LED in series with 2k2 means higher current through LED. it looks it is more than the rated current. Hence increase the resistor to about 4k7.

All the best.

colin55 · 28th June 2009, 07:21 PM

Two points:
Firstly the diode does not have to be 1N4007. The reverse voltage is only 24v.
Secondly, the current through the 2k2 is about 10mA. So the resistor does not have to be changed.

A simple 1N4148 across the LED is all you need.
You can add a 100n across the LED too.

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26th June 2009, 06:40 PM	#16
Mr RB	Put the diode across the LED itself, in parallel reversed, and stick a big cap on the LED too like 100uF. That will save your LEDs, and provide snubbing, and still provide fast relay release. Actually with a 100uF cap on the LED you may not need the diode at all, the energy in the relay coil is not likely to move that voltage much as the cap will be at 1.7v and the back emf will just drop that cap voltage a fraction, maybe drop 1/2 volt? If I had a spice thingy I could test that...

28th June 2009, 04:44 PM	#17
chand	1. connect a diode 1N4007 in reverse direction across the relay coil. 2. LED in series with 2k2 means higher current through LED. it looks it is more than the rated current. Hence increase the resistor to about 4k7. All the best.

28th June 2009, 07:21 PM	#18
colin55	Two points: Firstly the diode does not have to be 1N4007. The reverse voltage is only 24v. Secondly, the current through the 2k2 is about 10mA. So the resistor does not have to be changed. A simple 1N4148 across the LED is all you need. You can add a 100n across the LED too.