12v pull and hold circuit for solenoid?

Thread Starter


Joined Jun 15, 2022
I have a Line Lock Solenoid (normally open) that requires 12v to activate. But the manufacturer only recommends to use for 60 seconds at a time, I've learned that you can hit a solenoid with 12v to pull it closed but then you can lower the voltage to say 5v to hold it closed (I will do testing to see what the actual hold voltage is). Greatly increasing the run time before it gets too hot. This is going in a car. The simpler the better

I have contacted the manufacturer and apparently they have no data sheet for this solenoid...

Part Number

So my question is, How do I design a step down circuit that hits it with 12v initially but then drops the voltage afterwards for continuous use until turned off?

Or is there an off the shelf option that I cant find. I've only found step down circuits that are always at the lower voltage :/

Also this would be safe to have 1 circuit feeding 2 solenoids? I'm planning to wire it as 12v source > on\off switch > this step down circuit > both solenoids > individually grounded to the chassis. If need be I;m happy to make 2 circuits

I would also like to add I'm not fussed about wasted energy or making sure it runs *perfectly*. The solenoid doesnt have to stop at certain points. It just needs to stay fully closed.

Thank you in advance.


Joined Jun 8, 2013
I think a solenoid driver would circumvent the safety of the solenoid.

"WARNING! The Aeroflow launch control is designed primarily for use on high performance vehicles used in drag racing. IT IS INTENDED FOR SHORT DURATION USE (MAXIMUM 60 SECONDS) to lock the front wheels while staging the vehicle for Drag Racing. IT IS NOT INTENDED FOR BURNOUT COMPETITIONS and or to be used as a long-term brake holding device or in place of a driver depressing the brake pedal. It should only be used on passenger cars and light trucks, for example 750kgs or ¾” tonne or less using a standard hydraulic brake system that is in good safe operating condition."

For example, what if a short circuit caused the solenoid to engage while accelerating?



Joined Mar 14, 2008
Below is a solenoid-hold circuit using an added relay and capacitor to momentarily short the voltage/current reduction resistor (R1) during the initial actuation of the device.

The value for R1 (and its wattage) will depend upon the solenoid's actual resistance, and its minimum hold current (voltage).
(For the simulation, arbitrary values for R1 was 8Ω, and the solenoid was 4Ω).

The initial pulse length is determined by the value of C1 and the actual relay coil resistance (60Ω was used for the simulation).
C1 can be increased if a longer pulse time is needed.

The relay can be a 12V SPST automotive type.

The two solenoids could be placed in parallel, with the appropriate change in R1's value.

Last edited:


Joined Nov 6, 2012
The Solenoid pulls approx. ~3-Amps / ( 4-Ohms ),

R-1 should be 2, 10-Ohm/10-Watt Power-Resistors wired in parallel.
DigiKey p/n HS1010RF-ND, ~$3.24 each.
Power Resistor .PNG
These Resistors will get extremely HOT, ( but this will not damage them ),
mounting them to a sheet-metal surface is recommended.
Insure free Air-Flow around the Resistors.
Ideally, mount them to a 6" X 6" piece of Aluminum.

C1 can be reduced down to ~4700uf.
DigiKey p/n 1189-1732-ND ~$1.65


Joined Nov 6, 2012
The manufacturer recommends a "4-Amp-Fuse",
and simply based on experience, and the specs of similar devices,
it probably pulls ~2.5 to ~3-Amps.

The objective with the "Time" calculation is only
overcoming the Inductance of the Coil,
plus the inertia of the moving parts.
There is no need to keep full Power applied to the Coil after the Valve has been fully seated.
~50ms is plenty of time.
With half the Capacitance,
the Relay should stay closed for around ~100ms,
( according to your graph, which is showing around ~200ms ).

The reason why it was designed to "draw-too-much-Current" for continuous operation is
specifically to "speed-up" the closing of the Valve, and slam it hard into it's seat.

Just in case the "Hold-Current" needs to be higher ........
buying 3 or 4 of the same part-number Power-Resistors
will allow increasing the Current to suit the actual situation,
by simply adding-on 1 or 2 more Resistors in parallel with the first 2 Resistors.

The objective is to keep the Solenoid's Temperature under about ~150C.
This can be easily verified with a hand-held Infa-Red-Thermometer,
available from Harbor-Freight cheap.


Joined Mar 14, 2008
There is no need to keep full Power applied to the Coil after the Valve has been fully seated.
~50ms is plenty of time.
Don't really agree with that.
Why push that time nearer to the limit for no good reason?

My simulation (below) with 4700µF pulse-width capacitor, shows that the solenoid current barely reaches 3A before it starts to decay.
That may be due to my assumption of a coil inductance higher than the actual but, given the large unknown circuit and solenoid values/tolerances, I see no reason to push the design margin to save a few cents by using a smaller capacitor.
Certainly the solenoid operation will not be adversely affected by the longer time.