# Which diode to use to prevent back EMF from a 12v mag lock

#### rickmcdonald83

Joined Jun 8, 2020
13
Hello all.

I am experiencing back EMF from a 12v mag lock. I have read that you can use a diode to help prevent this, however all of the 12v diodes that I'm finding are 5 watt.

If I were to use a 60v - 5A diode, would this work? Would this cause any issues?

I am very new to this and still learning so I am open to any and all input. Thank you very much!

#### dl324

Joined Mar 30, 2015
11,245
Welcome to AAC!

The current in the diode won't last long and diodes with a lower continuous current rating could withstand significantly higher currents for short periods of time.

1N4001 will tolerate spikes of 30A:

If you don't want to do the calculations, use a diode with a continuous current rating the same or higher than the current in the device causing the back EMF.

#### rickmcdonald83

Joined Jun 8, 2020
13
Thank you for the reply Dennis!

I'll try the 1N4001

#### dl324

Joined Mar 30, 2015
11,245
I'll try the 1N4001
I wouldn't do that before understanding what the current is in the device causing the back EMF.

Using an inappropriate diode could cause it to fail and you'd be back to no protection.

#### rickmcdonald83

Joined Jun 8, 2020
13
Ahh good to know! Thank you!

I just looked up the specs. Here's what We have:

I have 2 - mag locks wired in parallel.

Voltage: DC12V
Current Draw: 0.11A ~0.15A

Do you think the 1N4001 would work for this?

#### peterdeco

Joined Oct 8, 2019
202
On very small current devices using low voltage, a 1N4148 often works. We also use 1N4001. On our larger 12V 5A DC motor we use FR603. High speed, high current.

#### dl324

Joined Mar 30, 2015
11,245
Voltage: DC12V
Current Draw: 0.11A ~0.15A

Do you think the 1N4001 would work for this?
If the current spec is for both devices, 1N4001 would be overkill. You could use a signal diode like 1N4148 or 1N914.

#### rickmcdonald83

Joined Jun 8, 2020
13
Thank you very much for the input!

Would there be any disadvantage to using the 1N4001?

Just trying to understand what the difference between that and the 1N4148 would be.

I have a few devices including a couple linear actuators I will be installing diodes on as well.

#### crutschow

Joined Mar 14, 2008
25,251
Would there be any disadvantage to using the 1N4001?
Not really.
The main difference between the two is current rating, and the 1N4148 has a sufficient rating for your purpose.
The only disadvantage might be a slightly higher cost and physical size for the 1N4001.

You do know that the diode should be connected across the solenoid coil (cathode to plus side), not in series with the coil. (?)

#### dl324

Joined Mar 30, 2015
11,245
Would there be any disadvantage to using the 1N4001?
The only thing I could think of is appearing to be amateurish.

Some conservative designers would always derate components by 50%. Real designers do the arithmetic and make more appropriate choices. But 5-10X is unreasonable.

If you were stranded on a desert island and all you had was 1N4001, that would be different.

#### rickmcdonald83

Joined Jun 8, 2020
13
You do know that the diode should be connected across the solenoid coil (cathode to plus side), not in series with the coil. (?)
Thank you for the info!

Please forgive my ignorance, I am very new to this. Here's what I know:

The mag lock has a red and black wire coming from it. I have connected the positive wire from my 12v adapter to the red wire, and the negative wire to the black wire. From what I understand, the diode should be connected to the red wire. Is that correct?

#### peterdeco

Joined Oct 8, 2019
202
On high current inductive loads, the speed of the diode is important. Years ago some "genius" that worked for our company used 1N4001 on the 12V 5A motor we use. It used to get red hot. One by one every circuit board it was on was returned N/G.

#### rickmcdonald83

Joined Jun 8, 2020
13
The only thing I could think of is appearing to be amateurish.
Thank you for the info! I am 100% amateur, so that's why I'm here asking questions. I appreciate all of your help!

#### dl324

Joined Mar 30, 2015
11,245
I have connected the positive wire from my 12v adapter to the red wire, and the negative wire to the black wire. From what I understand, the diode should be connected to the red wire. Is that correct?
A diode has two terminals and polarity matters, so your description doesn't give us enough information.

The cathode of the diode should be connected to the red wire and the anode should be connected to the black wire.

Even for simple circuits, most would prefer a schematic to a verbal description. There's less chance for confusion; assuming you draw the schematic correctly.

#### Alec_t

Joined Sep 17, 2013
11,511
The diode cathode is usually marked with a paint band.

#### rickmcdonald83

Joined Jun 8, 2020
13
This is how I currently have it wired. Please excuse my crude drawing.

#### dl324

Joined Mar 30, 2015
11,245
This is how I currently have it wired. Please excuse my crude drawing.
What else is in the circuit that makes back EMF a concern?

#### rickmcdonald83

Joined Jun 8, 2020
13
What else is in the circuit that makes back EMF a concern?
I also have a 12v video player connected to a TV on the same circuit. When the mag lock is released, it trips the video player.

#### dl324

Joined Mar 30, 2015
11,245
I also have a 12v video player connected to a TV on the same circuit. When the mag lock is released, it trips the video player.
That makes more sense. A snubber diode would limit the voltage spike on the power supply.