# Which Diode is best?

Hey there,

Im currently working on my third year pneumatic project and i need to invest in some diodes for my solenoids, these are the ones i have ordered..

http://www.ebay.co.uk/itm/2-Position-5-Way-Air-Pneumatic-Solenoid-Valve-Kit-DC12V-4-8VA-IN-OUT-1-4-PT-/360595841291?pt=LH_DefaultDomain_3&hash=item53f52fe10b

Is there any chance anyone could give any advice on which diodes will be best to use?

Thanks! Kayla

According to the spec on the page it's a 12v solenoid and draws upto 4.8VA Therefore less than half an amp. So really any diode that would be able to handle 1/2 an amp but with a fairly hefty voltage (to be on the safe side)

I'd suggest something like a 1n4003 and it'll be a fit and forget item.

KenF: with a fairly hefty voltage (to be on the safe side)

That is terribly muddled thinking.

The only reverse voltage that the diode ever experiences is the forward voltage of the solenoid.

Paul__B: That is terribly muddled thinking.

The only reverse voltage that the diode ever experiences is the forward voltage of the solenoid.

Had to think about that for a second, but you're correct. In normal operation the voltage on the coil is what appears across the reverse biased diode. Assuming the diode does what is required when the drive voltage is "released" the forward voltage will only be around 0.7 volts. It is the current carrying capability of the diode that is the important factor. Assume up to 10 times the normal solenoid current, but only for a few milliseconds and even that depends upon the Q factor of the coil (resistance versus reactance) Wikipedia has a decent statement under "Flyback Diodes"

Paul__B: That is terribly muddled thinking.

The only reverse voltage that the diode ever experiences is the forward voltage of the solenoid.

Who said anything about REVERSE voltage?

So now stop being a dickwad and try to make a useful contribution.

jackrae: Assuming the diode does what is required when the drive voltage is "released" the forward voltage will only be around 0.7 volts.

Before the diode has done its job it has to survive the spike that is thrown at it. Put one of those tiny glass signal diodes on there if you like and see how it holds out.

for the simplicity of not bothering with any logical or engineering that seems to be this thread.....

for the very general application of a relay or solenoid, one will get a back EMF spike of not more than 5 times the forward voltage. sorry, but you cannot discuss a coil and EMF without including reverse voltage. it is the only thing the diode is there for.

as for current, the back EMF spike will also not exceed 5 times the current from a relay or solenoid coil.

the reason is that the design of the relay or solenoid coil is such that the force needed are not that great.

so, in very simple applications, select a general purpose diode of not less than 5 times the forward voltage and 5 times the current.

there are lots of sites that you can use for calculations. however, you will need to know the data of the coil. unfortunately, for many hobby parts, that information is not available.

YES in practise the back EMF are about 5 times, the solenoid are not a perfect choke.

The back EMF diod are reversed when the coil are on, in this case 12 volt. When turned off current flows in the forward direction thru the diod and the voltage vill be about 0.7 volts.

An inductance vill keep the current flow. When turned off the same current vill flow thru the diod. Diod peakcurrent vill be exactly the same as the solenoid current, not 5 times.

Kaylaa, use 1N400X (X=2,4,5,6,7), easy to find and cheap

Pelle

We have used literally 1000s of 1N4007 diodes for purposes like this with power switching relays and contactors.

I rest my case.

Paul__B: I rest my case.

That's useful.

@Paul__B, help the original poster or move on.

@KenF, stop feeding the troll.