Diodes for 12v

[scarecrow55]

I'm not an electrician so bare with me.
I'm designing a simple circuit to operate warning lights and hydraulic valve solenoids and the circuit will only operate together if there is some sort of non return "valve" (diode?) in the line.
Question - will a diode work for a 12v circuit, ie allow current to run one way then if another part of the circuit is energised, will stop that current bypassing the diode?

 

[Barry1961]

Can you use a relay that is always energized when the main power is on to isolate and switch to the alternate supply?

Barry1961

 

[scarecrow55]

I didn't want to go to relays if I could used something simple like a diode. Thanks

 

[alexit]

A 12V diode will work for a 12v circuit, however you may desire a fail-safe solution (if the diode fails will the reverse current indicate everything is fine when there is really a problem?)

In our systems each sensor activates a relay (as Barry said), the normally open contacts of which are all series-wired together. In this fashion the "good" light is on only when all sensors report everything is fine. From the normally closed pole of the relay we activate the "bad" light for each sensor to help operator find problem area.

 

[CJCPE]

Diodes are generally rated 50v and up. A 12v rating would be needed as a minimum and you could easily have more than 50v spikes from inductance of solenoids. There will be about a 1v drop across the diode regardless of voltage rating or current conducted.

 

[buzzp]

Thats what diodes are for. From a hydraulic point of view, to block flow, is called a check valve. Current will be allowed to flow towards the arrow of the diode symbol, from anode to cathode.
Putting diodes across solenoids also affects the release time of the solenoid but is generally a necessary evil. Other schemes can be used to reduce the release time of the valve vs. just a diode. The affect is relatively small, usually around 20-100ms.
Don't spec something elaborate like a 12V diode because it is to small. The inductive kick will produce a large voltage at the time of de-energizing. This can be up to 20 times the voltage of the coil as a worst case. I would use the common 1N4004 which has a rating of 400V and they are cheap. A faster acting diode could be used as well but probably unnecessary.
Don't forget about the voltage drop across the diode in your application. The diode across the coil of the solenoid is not a concern here but diode used to block flow in one direction will be. It will be around 0.6-1V drop depending on the current going through it. If this is a concern, you can use diodes with a little less drop. Here is a data sheet link:

http://www.fairchildsemi.com/ds/1N/1N4004.pdf

 

[jtkirb]

Diodes work just fine, but make sure you buy one with the right current level and physical layout to make mounting easy.

I used some from International Rectifier that were rated for 70A as battery isolation for multiple generator batteries. The diodes supplied the "best battery" to the gear and kept from backfeeding the other systems. Slick.

Remember reverse voltage rating (higher the better), amp rating, and voltage drop across the diode.

JTK

 

[scarecrow55]

Thanks for all your help guys, I now have an understanding of what to look for.

 

[waross]

Switching off any inductive dc load will cause inductive currents that may destroy the diode when the equipment is de-energised. A basic protection circuit can be made with a second diode.
Take two identical diodes. Connect the anodes together.
Connect this common point to one coil terminal. Connect the cathode of the first diode to the other coil terminal. This is the surge prevention diode. Apply the operating voltage to the cathode of the second diode. The current caused by the inductive discharges through this diode and the energy is disipated in the coil winding resistance. The current at the moment of de-energisation will be the current that was flowing in the coil. The voltage will be the voltage that was present at the time of de-energisation. The diodes do not have to be physically mounted on the coil, or even both in the same place, but I find this a good way to describe the proper polarity of the connections.
Both current and voltage will decay to zero, but the decay will not be instantaneous, and this is what causes the slow release of the solenoid.
The surge prevention diode may be rated the same as the operating diode. This is a conservative rating.
Other protection schemes allow a higher surge voltage which decays faster, giving a faster release time for the solenoid.
good luck.