Low voltage diode needed solinoid project

Thread Starter

Mike N Julie Nyman

Joined Feb 20, 2016
7
Looking for some advice for the correct diode. I have a solinoid that is controled via a circuit that should apply 12v for 1.5 to 2 seconds to "energize" and then drop to 2v to "hold" this a factory made unit and there are no diagrams avalabe for it. The 12v energize function has failed but the 2v hold function is working. I wish to put a 12v feed to the solinoid via a momentary push button and use a diode to stop the 12v flowing back to the controller board. Im concerned that the volt drop across the diode will make the hold voltage to low.

Energize volt - 12v to14v dc
Energize current - 700ma or about 8w
Hold volt - 2v
Hold current - 300ma
I will post a diagram of my plans.

Any advice appreciated.
Mike
 

ian field

Joined Oct 27, 2012
6,539
Looking for some advice for the correct diode. I have a solinoid that is controled via a circuit that should apply 12v for 1.5 to 2 seconds to "energize" and then drop to 2v to "hold" this a factory made unit and there are no diagrams avalabe for it. The 12v energize function has failed but the 2v hold function is working. I wish to put a 12v feed to the solinoid via a momentary push button and use a diode to stop the 12v flowing back to the controller board. Im concerned that the volt drop across the diode will make the hold voltage to low.

Energize volt - 12v to14v dc
Energize current - 700ma or about 8w
Hold volt - 2v
Hold current - 300ma
I will post a diagram of my plans.

Any advice appreciated.
Mike
The 0.7V of a standard silicon diode is insignificant in the greater scheme of things - a Shottky-barrier diode drops only about 0.2V, but you have to watch out for their low reverse breakdown voltage - they generally start at 20V and start getting expensive over 60V. There is the possibility of back emf from the solenoid, so you may have to select the diode PIV accordingly.

There are various circuits floating about the web for relay economisers. Usually you start with a supply rail that's adequate to hold the relay in but not pull it in. In the resting state; a transistor and a diode network arrange for a boost capacitor to charge from Vcc, The signal to actuate the relay pulses the transistor to shift the capacitor into series with the coil to make sure the relay pulls in. The capacitor is shifted into parallel with a bypass diode, so when its charge decays the coil is still fed by Vcc to hold it in.
 

tracecom

Joined Apr 16, 2010
3,944
Looking for some advice for the correct diode. I have a solinoid that is controled via a circuit that should apply 12v for 1.5 to 2 seconds to "energize" and then drop to 2v to "hold" this a factory made unit and there are no diagrams avalabe for it. The 12v energize function has failed but the 2v hold function is working. I wish to put a 12v feed to the solinoid via a momentary push button and use a diode to stop the 12v flowing back to the controller board. Im concerned that the volt drop across the diode will make the hold voltage to low.

Energize volt - 12v to14v dc
Energize current - 700ma or about 8w
Hold volt - 2v
Hold current - 300ma
I will post a diagram of my plans.

Any advice appreciated.
Mike
I suppose you have tried to fix the 12V energize feature?
 

ian field

Joined Oct 27, 2012
6,539
Good choice.

ak
Mixing Shottky-barrier diodes and inductive loads can be risky - AFAICR; that has a PIV of only 40V.

A zener in parallel would protect it as long as Vz is just under PIV. The zener has about the same Vf as a regular diode, so the SB would always forward conduct first.
 

AnalogKid

Joined Aug 1, 2013
8,229
Sorry, missed that the new diode would disconnect any protection already in place.

Mike, you need two of those diodes, one in series with the controller and one in parallel with the solenoid coil. Even with only 2 V across the coil when it is turned off, it can kick up enough reverse EMF to damage something. The controller probably has a diode across its output because of this, but that diode is disconnected by your diode.

One diode as shown on your drawing, and a second one with its anode to GND and cathode to your first diode's cathode.

ak
 

Thread Starter

Mike N Julie Nyman

Joined Feb 20, 2016
7
Sorry, missed that the new diode would disconnect any protection already in place.

Mike, you need two of those diodes, one in series with the controller and one in parallel with the solenoid coil. Even with only 2 V across the coil when it is turned off, it can kick up enough reverse EMF to damage something. The controller probably has a diode across its output because of this, but that diode is disconnected by your diode.

One diode as shown on your drawing, and a second one with its anode to GND and cathode to your first diode's cathode.

ak
Thanks for your time guys and excuse my atrocious spelling in y first post as it was 3am in Australia and didn't have spell check on.
Is this diagram Solenoid Diag02.jpg what your thinking?
 

Thread Starter

Mike N Julie Nyman

Joined Feb 20, 2016
7
If you're in Australia, does that mean that both diodes are upside-down?

ak
Yer Mate, always a problem. Electrons all over the floor and lead acid batteries are a real problem.. Didgerydiodes are great but they are a bit noisy.
Oh, and we use Crocodile clips instead of Alligator clips
 
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Thread Starter

Mike N Julie Nyman

Joined Feb 20, 2016
7
So im not just copying your advice, can you give ma a quick explanation on the operation of the diode you suggested I add (Diagram Above)
Does it let the EMF drain back to ground through the solenoid coil?
 

AnalogKid

Joined Aug 1, 2013
8,229
Yup. The same thing that makes your spark plugs spark makes solenoids and relay coils dangerous for electronic components. When energized by DC which is then *rapidly* disconnected - as in electric current stops and the input goes to a very high/infinite impedance, the magnetic field in the coil collapses, inducing a current in the coil that manifests as a terminal voltage that is the opposite polarity of the energizing voltage. And large. It is easy for a 5 V relay coil to kick up hundreds of volts for a microsecond. Because of the polarity change it is easy to suppress with a diode. The diode acts as an automatically switched-in short circuit.

All of that is because of this: - <-- a minus sign

One of Maxwell's equations (actually Heavyside's reduction of Maxwell's equations) is a corrected version of Faraday's Law of Induction, and Faraday's Law has a minus sign in it. That minus sign is the reason a generator is harder to turn the greater the electrical load on it is, the reason it takes huge nuclear power plants or structures like Hoover Dam to get generators to turn.

ak
 
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