Choosing a surge protection diode for a relay coil?

MarkS

I'm working on a temperature controller for a reptile enclosure and need to be able to handle quite a large load. I'm going to use this relay: T9AS1D12-5 RELAY PWR SPST-NO 30A 5VDC PCB. The coil uses 5V DC. What diode should I use to protect against voltage spikes?

Mike Green

The datasheet from DigiKey's website shows a coil current of 200mA. I'm assuming that you are using some kind of transistor to switch this. Any small power rectifier should work for the diode. The 1N4001 is rated at 50V and 1A. Anything similar should be fine.

MarkS

Thanks!

Beau Schwabe

What Mike says is correct...

Typically you want to select a diode with the same or greater current rating as you are supplying the inductive load with, since it is the collapse of the magnetic field within the inductive load that you want to clamp.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

MarkS

How is it oriented in regards to the power supply? I know it goes in parallel to the coil, but should it be oriented as normal, with the positive voltage going to the anode or reversed?

Also, can someone explain to me how a diode in parallel does anything anyway? It would seem that the current and voltage would take the path of least resistance and by-pass the diode completely.

RobotWorkshop

Here is a nice site which talks about diodes:

http://www.kpsec.freeuk.com/components/diode.htm

In particular the paragraph from the site above probably has the answer you're looking for:

"Protection diodes for relays

Signal diodes are also used with relays to protect transistors and integrated circuits from the brief high voltage produced when the relay coil is switched off. The diagram shows how a protection diode is connected across the relay coil, note that the diode is connected 'backwards' so that it will normally NOT conduct. Conduction only occurs when the relay coil is switched off, at this moment current tries to continue flowing through the coil and it is harmlessly diverted through the diode. Without the diode no current could flow and the coil would produce a damaging high voltage 'spike' in its attempt to keep the current flowing. "

RobotWorkshop

Here is another link that will help:

http://www.st.com/stonline/books/pdf/docs/3569.pdf

MarkS

Thanks for the links! I thought it was connected in reverse, but it's been so long I was no longer sure.