Driving a 12V-24V 20 ohm solenoid from a 24V 0.5A input signal

Question

I had a question about trying to get a solenoid with 20 ohms resistance to power up from a 24V Power supply limited to 0.5 amps.

I tried connecting a 10 ohm resistor in series, to drop the driving voltage to 16V, but I ended up burning the resistor. The valve did work properly before the resistor burned up though. Is there a higher amp resistor part-number search I can get somewhere?

I know I should use a relay and drive this as an alternate circuit, but this is a field application, and I don't have the resources to create even a tiny panel atm... Any help would be appreciated, the goal is to get it working :D

The application is driving a valve which is connected to the SMC EX600 valve pack DYPB output card. Manual here.

Answer 1

Check the maths: V = 24 V, R = 20 Ω. If you hook the two together you get \$ I = \frac V R = \frac {24}{20} = 1.2 \ \text A\$ which is too much for your PSU.

If we run it at 12 V we'll get 0.6 A which is still too much for your supply. If you want to chance it then you'll need to add a 20 Ω resistor in series. The power rating of this resistor will be given by \$ P = I^2R = 0.6^2 \times 20 = 7.2 \ \text W \$. You're going to need a 10 W wirewound resistor mounted on a suitable heatsink.

As a quick hack you could try wiring a 6 W, 12 V tail-lamp in series with the solenoid. Measure the current and monitor the PSU temperature for a while to satisfy yourself that you're not going to start a fire.

Answer 2

If you actually have a supply limited to 500mA you need to beef up the supply.

It is extremely unlikely that a valve designed to draw 1.2A will be reliable at less than half the rated current.

Your proposed solution both overloads the power supply by 60% and under supplies the valve by 33%.

Answer 3

Try this circuit. It will drive the solenoid at 12V average, with 0.3A (+ a tiddly bit for the IC) from the supply. I didn't specify a FET number as any FET rated at least 1A & 30Vd and a Vg max over 12V will do. I set the frequency at about 1kHz. You could try other frequencies, but don't go too low or C1 won't filter the load on the supply as well. Lower frequencies will reduce eddy current losses in the core, but reduce the reactance, and higher frequencies will increase coil reactance, but also eddy current losses. It is not critical, but 1 kHz is probably about the best. If you have a digital multimeter that doesn't measure DC (of either polarity) on the AC range, put it in series with the coil (after the diode). Select the 2A AC range. It should read a small ripple current. Adjust the frequency so the ripple current is minimum. If not, measure the 24V supply current. It should be just slightly less at the most efficient frequency. That could be as low as about 100Hz or less.