very very basic help with diode problem

Thread Starter


Joined May 24, 2011
As you can guess from the question I have no knowledge of Electronics but I do need help.

Are there different diodes for AC and DC?

I have a system (Irrigation)that worked on manual operation for years on 24VDC with no problems from the diodes.

Recently I changed it to 24VAC so that I could incorporate a programmable timer .

The diodes are used to stop current backfeed on the common pump start circuit and various warning lights.

The diodes function perfectly on 24VDC but fail(permit backfeed) on 24VAC.

The backfeed readings Im getting on the diodes are

24VAC input 24 VAC backfeed 13.7VAC
24VDC input 24VDC backfeed nil

These readings were from loose diodes (Straight out of the pack and not put in the circuits)

Can anyone help



Joined Mar 24, 2008
The ratings for diodes are PIV (peak inverse voltage, how much voltage they will take in reverse) and current. Both are important. You don't need to worry too much about AC or DC as long as the ratings are equal to or exceed the two I just named.

Ron H

Joined Apr 14, 2005
A diode will only block voltage (and current flow) on one half of the AC cycle. I think you will need a different approach.

Thread Starter


Joined May 24, 2011
Thanks Ron.

Has anyone got any suggestions.

What I need is a device to stop the AC current from backfeeding in the way the diodes originally blocked the DC current.
The Diodes I am using are N4007 1000V 1A DO41
ZR1008.These diodes worked perfectly using the original 24VDC current. It was only when I changed to AC that the problem occured.



Joined Feb 5, 2010
Diodes block DC CURRENT

They cannot block AC CURRENT. One half of the AC voltage will always get through the diodes. You need to reconsider your choice to use AC voltage, and select a timer that functions on DC VOLTAGE


Joined Aug 10, 2008
If you need to block the positive alteration to the rest of the circuit, then you can put the diode in the circuit, with the cathode lead towards the ac source, that way when the AC goes positive, the signal is blocked, when the AC goes negative, you can direct it through a resistance to ground, or something like that.

It depends on what part of the AC you need to block.

If you want to block the negative side of the AC, then put the diodes with the anode towards the AC source.

But you cannot block both sides of the AC with diodes alone.

That would take some sort of filtering.
You may have to design a filter into your circuit, to keep the AC from interfering.

You could rectify and filter the AC positive alteration, to give a more steady positive voltage, then use that to run a part of your circuit.

There are ways to work this problem, you need to give as much info, about the circuit as possible for someone to come up with a solution.
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Joined Dec 26, 2010
I think that it is very unlikely that the OP can succeed in getting a system designed for DC operation with diodes directing current flow to work with AC.

The first priority for should be to describe the system as fully as possible, with a schematic to show what the diodes did in the original version. Until/unless this is done, guessing what might be done with diodes and filters may cause confusion.

Thread Starter


Joined May 24, 2011

Thanks Adjuster.

I have attached a very rough schematic.

Shows only 4 circuits but in fact there are 15 circuits all basically the same.

The Diodes were in the lines to prevent current backfeed from the individual power supplies. Both 24VAC. One for manual operation and one for automatic.

Now no matter what the power source, manual or automatic (Timer) both sets of warning lights come on instead of just one.

The manual on/off switches have the led warning light incorporated in the switch while the timer warning light is a seperate led.

Hope this will help you to give some advice




Joined Dec 26, 2010
I think you would need to change to a quite different system to use an AC output timer for this. In any case, aren't your existing pump relay and solenoids designed for DC? I agree with Kermit2's suggestion that you might be better off getting a DC timer.

Maybe someone else will have a better idea?