I have a 120VDC solenoid that I want to use in 480VAC application, i.e., rectified ~680VDC. The signal to activate the solenoid will be given by a micro. The solenoid won't be continuously activated, it needs just a pulse of about 25 ms. The resistance of the coil measured with a meter is 290 ohms. Is there a way, maybe using a MOSFET, to operate the coil at this higher voltage? Maybe the mosfet with the help of some resistor reduces the voltage stress on the coil?

 

Can you not use Half Wave Rectifier, this will give you 240AC rectified to DC, and a Series resistor of 330 Ohms..?

 

I have done a similar thing, albeit a slightly lower AC voltage, but the principle is the same, this is by using a bridge with a series resistor and zener diode.
Max.

 

Can you not use Half Wave Rectifier, this will give you 240AC rectified to DC, and a Series resistor of 330 Ohms..?

Do you mean using an extra half wave rectifier? The circuit has a full wave rectifier and I can't change that.

 

No just use a single diode, but as you can't change the circuit, forget it...

 

No just use a single diode, but as you can't change the circuit, forget it...

To give a little bit of more insight, the full wave rectifier is for a SMPS, which then generates 12V for the control and micro. As the input voltage max is 480V three phase, and my solenoid is 120VDC, I'm looking for a way to actuate the solenoid from a GPIO port.

 

So if you went across a phase and Neutral you would get 270VAC, rectified to DC with a full wave bridge would approx 268 V.

You can use a relay to switch the Solenoid on!

 

So if you went across a phase and Neutral you would get 270VAC, rectified to DC with a full wave bridge would approx 268 V.

You can use a relay to switch the Solenoid on!

The design has no access to the neutral. Seems like I'm doomed.

 

Then you will have to use a Transformer .

 

Can you not use Half Wave Rectifier, this will give you 240AC rectified to DC, and a Series resistor of 330 Ohms..?

That doesn't change the voltage, only the ripple.

 

The design has no access to the neutral. Seems like I'm doomed.

A resistor in series with a high-voltage MOSFET should work but you would need to opto-isolate the signal to the MOSFET.

 

A resistor in series with a high-voltage MOSFET should work but you would need to opto-isolate the signal to the MOSFET.

You mean something like this? See attachment.

 

Yes with a Series resistor in the opto led.

 

You mean something like this? See attachment.

After looking at the resistor power dissipation (≈500W when on) the resistor size could be very large, depending upon the duty cycle.
That could be reduced, if the solenoid can be momentarily overdriven.
How often will the solenoid be energized?

 

After looking at the resistor power dissipation (≈500W when on) the resistor size could be very large, depending upon the duty cycle.
That could be reduced, if the solenoid can be momentarily overdriven.
How often will the solenoid be energized?

The solenoid can be momentarily overdriven. This is for a protection relay where you give a brief pulse to the solenoid until the energy is removed. Ideally I want to control the on-time with the micro so I don't necessarily have to remove the power supply to avoid damaging the coil.

 

I want to control the on-time with the micro

You didn't answer my question about how often the solenoid is energized.
How long between "on-times"?

 

You didn't answer my question about how often the solenoid is energized.
How long between "on-times"?

There's no an exact response to that question since the solenoid will be energized only when there's a fault sensed by another circuit. So the process is: The micro senses a fault, trips the solenoid, and stops the current flow through it. So I'm not gonna turn the solenoid again if there's no a fault. I have to make sure the FET turns off quick enough to avoid damage to the solenoid.

 

There's no an exact response to that question

I don't need an exact value, just the minimum expected time between consecutive operations of the solenoid.

But after further looking at possible circuits, it will still require quite high power resistors, even with a low duty-cycle, so you may have to consider adding a transformer to drop the AC voltage down to about 90V to generate a rectified 120Vdc.

 

My experience has been that the performance of a solenoid designed for DC will not be satisfactory on AC, rectified or not. With a resistance of 290 ohms and 120 volts DC the intended current is about 0.4 amps, and so you need a resistor to drop 360 volts at 0.4 amps. Then, to make the solenoid not buzz horribly you need two diodes back-to-back with the solenoid connected across one of them, The diode across the coil allows current to keep flowing when the supply diode cuts off. I used that trick first a very long time ago. But the diodes need to be rated well over the line voltage and at twice the load current, at least. AND you need a series fuse just in case.

 

No one has asked the obvious question
Is the coil 120Vdc OR is it that the contacts are only rated for 120VDC
120VDC for a coil is very unusual.