# Is there a formula to calculate Protection diodes for relay coils?

Discussion in 'General Electronics Chat' started by Feign, Mar 31, 2009.

1. ### Feign Thread Starter Active Member

Mar 30, 2009
50
2
*Try number two, I keep timeing out typeing for too long*

Subject says it all...

(a second post to follow)

2. ### Feign Thread Starter Active Member

Mar 30, 2009
50
2
Given a relay with a 12-14.4V DC, 77ma, 155 Ohm rated coil.

How would one calculate the needed protection diode?

A pointer to the right tutorial if you know it off-hand, would be great.
I think I am looking for something about coil field collapse?

3. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Take the current of the coil, and make sure the diode can handle that as a peak current.
Even the 1N400x series can take a 30A peak pulse (8.3mS non-repetitive single half-sinewave)
A 1N914/1N4148 switching diode can handle a 1A pulse for 1 second, or 4A for 1uS.

4. ### Feign Thread Starter Active Member

Mar 30, 2009
50
2
I think it violated the power dissipation rate for the chip footprint.

P = IE
I <= ~77Ma
E <= ~14.4V

0.077 * 14.4 = 1.1088 watts
0.077 * 12 = 0.924 watts
(Is that right? yeah thats right 1000ma is 1A, it's been awhile)

So before it was a little burnt piece of carbon I "think" it was a SO-70/SOT-23 or similar. The one left on the board is about 2.5 mm (Ican't read it's rating it got flux blobbed.

The data sheet I have here for a temp sensor of similar size lists the power disipation as less than .8 watts at -40C for a SOT-23, Yikes this thing needed another temp sensor on the board to go with the one that monitors the motor.

EDIT: Another temp sensor would not have help after more thought on the subject.. Don't turn me off unless it's sub zero doesn't make much sense. more time on means more heat build up..

Last edited: Mar 31, 2009
5. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Well, you must realize that a reverse-EMF protection diode will only be conducting for a very short period of time. If it fried, it probably failed shorted, which caused it to conduct a lot of current through not only itself but the transistor driver as well, destroying both.

6. ### Feign Thread Starter Active Member

Mar 30, 2009
50
2
I already swept up and the transistor went into the trash bin, I am relativly certain it was cracked along the collector side, but it was conducting without current to the base. And from memory there was no voltage detected on the base pin.

How concerned should I be about the microcontroller that was driving it? I still have not located the data sheet for it.

The diode was carbon as mentioned before(?) and the transistor itself was cracked and a showed abit of grey smoke residue.

7. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
513
A good rule of thumb for the protection diode is 'at least 3 and preferably 5 times'.

That is the current rating should be at least 3 times the relay current and the voltage rating at least 3 times the supply.

The 1N 4000 series easily ( and cheaply) copes with this requirement in your case.

8. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
The uC should have been protected by the transistor's base resistor.

9. ### Feign Thread Starter Active Member

Mar 30, 2009
50
2
I don't see any SMT diodes here in the parts bin. Do I want the little glass ones(germanium?) or the big blacks ones. If I remeber correctly I had better luck finding datasheets for the former.

Yeah I could go visit radioshack in a few hours but where is the fun in that!

Mar 30, 2009
50
2