1N4001 vs 1N4148 for EMF suppression

Thread Starter

tracecom

Joined Apr 16, 2010
3,944
I recently looked at a commercially available circuit that uses a 555 to operate a 12V relay. The relay draws about 40 mA. The circuit included a 1N4148 across the coil of the relay, but I have always used a 1N4001. Is a 1N4148 suitable, and what specifications does a diode need for this purpose?

Thanks.
 

ErnieM

Joined Apr 24, 2011
8,036
Don't use a LED. LEDs have a very limited reverse voltage rating, and 12V is greater then most.

Either of those diodes should be fine, though the 1N4001 is a better choice as it is the "beefier" device: the 4001 is a 1 amp rectifier and the 4148 is a 100mA small signal device.
 

Ron H

Joined Apr 14, 2005
7,014
I recently looked at a commercially available circuit that uses a 555 to operate a 12V relay. The relay draws about 40 mA. The circuit included a 1N4148 across the coil of the relay, but I have always used a 1N4001. Is a 1N4148 suitable, and what specifications does a diode need for this purpose?

Thanks.
If your coil only draws 40mA, then that will be the peak current through the diode. Reverse voltage will be the supply voltage for your relay.
 

takao21203

Joined Apr 28, 2012
3,695
Don't use a LED. LEDs have a very limited reverse voltage rating, and 12V is greater then most.

Either of those diodes should be fine, though the 1N4001 is a better choice as it is the "beefier" device: the 4001 is a 1 amp rectifier and the 4148 is a 100mA small signal device.
It is OK for small relays. Or 1n4148.

The critical voltage can never be reached, since the LED starts to conduct much earlier.

With a large relay, eventually you would be able to burn out a LED.

But that way, you really know the coil is energized.
 

kubeek

Joined Sep 20, 2005
5,718
The critical voltage can never be reached, since the LED starts to conduct much earlier.
You mean the LED will prevent the relay from turning on? It will just rapidly heat up because the reverse current will rise very high and then it will release the magic smoke.
 

crutschow

Joined Mar 14, 2008
24,725
.................

The critical voltage can never be reached, since the LED starts to conduct much earlier.

With a large relay, eventually you would be able to burn out a LED.

But that way, you really know the coil is energized.
The diode or LED is in connected in reverse bias across the coil (cathode to coil plus side) and thus must withstand the full relay voltage, which it typically cannot do. An LED is in forward bias when it is ON.
 

takao21203

Joined Apr 28, 2012
3,695
The diode or LED is in connected in reverse bias across the coil (cathode to coil plus side) and thus must withstand the full relay voltage, which it typically cannot do. An LED is in forward bias when it is ON.
It is OK for small 3V relays.

They don't break down from 3V in reverse and not from 4V, but I would not really use it much for 5V.

More kind of a curiosity, the LED was old + 20mA type.

You don't need to worry about a small pulse from a relay coil damaging 1n4148. These even can be used for some dc/dc converters.

A really large relay maybe is not well suited for 1n4148.
 

ErnieM

Joined Apr 24, 2011
8,036
..is a rectifier and zener as described here.

Agreed. An LED would be a bad choice for this application.
DING DING DING

I've actually worked on the suppressors used by this, erm, I mean "some unnamed major relay manufacturer" and they spec'ed in a 36V zener.

Plus a 1,000 volt diode. Oh and a fuse too. Did I mention the two fly leads?

Sits on a board .09 by .22 inches.


One thing to note is when using a zener to increase the discharge voltage this voltage ADDS to the power supply voltage. So if you have a 28V relay and a 36V zener your switch sees 64 volts. Actually a bit more.
 

timescope

Joined Dec 14, 2011
298
I prefer to use 1N4001. If this diode short circuits, the transistor will be destroyed attempting to switch the supply to ground.

Timescope
 

timescope

Joined Dec 14, 2011
298
Sorry, poor choice of words. I was referring to 1N4148. I do not feel comfortable using a 100mA diode at 40mA for reliability reasons.

Timescope
 

panic mode

Joined Oct 10, 2011
1,829
Sorry, poor choice of words. I was referring to 1N4148. I do not feel comfortable using a 100mA diode at 40mA for reliability reasons.

Timescope
????
:confused:

why not use alternator diodes, they are rated for few hundred amps and should be able to handle peak current of 40mA :rolleyes:

1N4148 is a good match for 40mA relay with plenty of margin. it is rated for 100mA continuous, rain or shine. and several times more in pulse operation (few hundred mA). the transient when relay turns off is very short.
 
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