Protection diode for relay, 1N4001 or 1N4148? - Page 1
Forum section: electronics / beginners

author: Mint.
author_rank: Frequent Contributor
timestamp: July 17, 2012, 08:22:29 am
content: Quick question. So I need a protection diode for a relay that is going to be receiving 12v and outputting 12v, which of the two diodes should I use 1N4001 or 1N4148?

author: Psi
author_rank: Super Contributor
timestamp: July 17, 2012, 08:24:19 am
content: The voltage on the relay switch contacts has no relation to the diode specs.1N4001-4007 is fine.In fact for your typical sized relay pretty much any diode will work but i tend to avoid small diodes like that 1N4148.If you had a massive relay, like the size of a D battery, then you might want a larger than normal diode to handle the kickback.

author: Simon
author_rank: Unknown
timestamp: July 17, 2012, 08:31:36 am
content: It should never see more than the supply so 100V + is fine. More importantly check it can take high peak amperages.

author: Mint.
author_rank: Frequent Contributor
timestamp: July 17, 2012, 08:32:50 am
content: Quote from: Psi on July 17, 2012, 08:24:19 amAhh perfect, thanks!Is there any simple maths involved in this? e.g. How am I able to find the kickback voltage from the relay?

author: Psi
author_rank: Super Contributor
timestamp: July 17, 2012, 08:35:17 am
content: Quote from: Simon on July 17, 2012, 08:31:36 amYes good point, that might be a catch for new people.As the voltage goes negative the diode conducts at ~0.8V (~0.4V for schottky) so the voltage never gets to rise much above this.Maybe another V0.3 as the current peaks.So even through the back EMF might be 100V or so without a diode the fact that you've added one means the voltage never gets anywhere near that.So you don't need to spec the diode to handle the back EMF voltage.As Simon said, it's the high peak amperage from the induced current that the diode needs to handle.Quote from: Mint. on July 17, 2012, 08:32:50 amI'm sure you could calculate the back emf voltage but you'd need to know the inductance of the relay coil, maybe the number of turns, wire thickness and core material as well as the load resistance.As i said above, the voltage isn't important, as adding the diode stops the voltage from risingEven then, the protection diode specs isn't something many people bother to calculate. If you really wanted to check it you could use a current probe to see what sort of current flows through the diode and confirm it's within the diode specs.The general rule of thumb is that any typical diode like the 1N4007 will work fine for any typical size relays.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 17, 2012, 08:55:50 am
content: It is pretty simple: The diode reverse voltage must be able to handle the supply voltage since it is reverse biased during operation. The diode peak forward current equals the coil operating current. The reverse voltage is almost never an issue, and generally neither is the current. I tend to use 1N400x diodes -- they are robust, cheap, and there is no need for great speed. Honestly a 1N4148 will work for a lot of relays 200 milliamp @ 12 volt is a fairly big coil.

author: David_AVD
author_rank: Super Contributor
timestamp: July 17, 2012, 09:00:31 am
content: I use 1N4007 mostly, but have used 1N4148 (or 1N914) for very small / reed relays.The only reason we use 1N4007 for relays (instead of 1N4004, etc) is that they are the same price as 1N4004, so no point stocking two parts.

author: Mint.
author_rank: Frequent Contributor
timestamp: July 17, 2012, 11:31:47 am
content: Perfect!Thanks to everybody who contributed. I am going to use 1N4001 as my diode for the relay. Then when I run out I will buy the higher voltage model like David_AVD suggested. My forward current of my relay is very small, about 40mA so I don't think that I will have any problems using both, just wanted to double check and get more knowledge. Cheers!

author: SeanB
author_rank: Super Contributor
timestamp: July 17, 2012, 03:35:34 pm
content: Just a quick add on is that using a diode does increase the turn off dely of the relay somewhat. It might add 20mS or so to the value specified for the relay. You only have to worry about this if the timing of the switching has to be fast and is time critical. For that you normally use a transistor rated for high voltage, like 100V or so, and a 100V zener diode across the transistor to protect the transistor. Then the turn off time is about as fast as it can be.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 17, 2012, 05:42:15 pm
content: There is usually no need to go to 100 V. A ~12 V zener will make the turn-off time approximately the same as the turn-on time for a 12 V coil while clamping the inductive spike to something more manageable. You can also use a resistor + diode or RC snubber, but this will have inferior performance -- for a given voltage they will switch slower than a zener since the current falls exponentially instead of linearly.If you really need every last fraction of a millisecond and you absolutely need a relay, then sure, go for 100 volt, but generally that is not needed.

author: madires
author_rank: Super Contributor
timestamp: July 17, 2012, 06:04:00 pm
content: Quote from: Mint. on July 17, 2012, 11:31:47 amThe recurrent peak forward current for a 1N4148 is 400mA. No need for over-engineering :-)

author: Psi
author_rank: Super Contributor
timestamp: July 18, 2012, 04:11:31 am
content: There's always a need for over-engineering