I've been dabbling with electronics for the last month or so and came across a project that I wanted to attempt.

I want to use a PWM with adjustable frequency set at about 20Hz-50Hz frequency on a 30A solenoid.

After a bit of research I found a schematic that I thought would work.

I was able to wire up the schematic and even test it on an LED and it worked, however once I tried it on the solenoid it did not work.

I bought a NE555 Nano Pulse frequency Duty Cycle Adjustable Module Square Wave Signal Generator with adjustable frequency (1Hz-50Hz) from ebay. (I'm not sure if I am allowed to post the link to the NE555 on here or not)

The solenoid draws 30A and needs to be pulsed anywhere from 20Hz-50Hz.

I used the following schematic. (see attachment)

For the transistor I used a 2N3055G
The resistor is a 10K
The Diode is a 1N4007

When I tested it on an LED it worked real good, I could change the frequency and the duty cycle and the LED would do as the module would direct it to.

However, once I tried the PWM on the solenoid the LED would turn-off, but the NE555's built in LED was still blinking telling me it was working, the LED though that I connected after the transistor would turn off.

I believe perhaps the transistor perhaps is to small for the solenoid, but at this point I am not sure. So I would like to get other opinions before continuing on this project.

Also would it be better to use another PWM as opposed to the NE555? I've been reading up on the Arduino and it looks promising as well.

Thanks in advance for any help that someone may be able to provide.

 

I want to use a PWM with adjustable frequency set at about 20Hz-50Hz frequency on a 30A solenoid.

Why would you pulse a solenoid? Can that solenoid complete its mechanical cycle that quickly?

Why would you expect to switch a 30A inductive load with a 4A rated transistor?
Oh wait, I see yo used a different transistor. It is still be too small (rated to 15A continuous), but anyway you'd need a much higher base current - about 3A - to have a chance of switching the 30A load with that transistor.

A MOSFET would be a better choice. What voltage is the PWM signal?

 

Why would you pulse a solenoid? Can that solenoid complete its mechanical cycle that quickly?

Why would you expect to switch a 30A inductive load with a 4A rated transistor?
Oh wait, I see yo used a different transistor. It is still be too small (rated to 15A continuous), but anyway you'd need a much higher base current - about 3A - to have a chance of switching the 30A load with that transistor.

A MOSFET would be a better choice. What voltage is the PWM signal?

The solenoid will not entirely complete its cycle but that is ok. As a matter of fact that is what I want it to do which is where the duty cycle would help me fine tune it...I think...

I'm not sure how to read the base current on transistors. But I am a quick learner and open to suggestions.

I'm powering the PWM with a 12V source.

Which MOSFET would you recomend ? Would it be wired the same?

I was also thinking about using two transistors in parrelel would that work?

 

It is possible to parallel transistors but a single MOSFET could handle this job easily. A regular BJT needs a base current of about 10% of the load current in order to be fully saturated - fully "on". The base current is limited by the base resistor, but it might also be limited by the PWM signal itself, which is unlikely to be capable of 3A.

A MOSFET on the other hand, needs only a voltage on its gate (at this low frequency) of ~10V to be fully on. You PWM signal will be fine, as is. A MOSFET will typically also have a lower on resistance, and will thus need less attention to heat dissipation. (A 30A load still calls for a heat sink, though.) You can use the MOSFET in place of the BJT, base=gate, collector=drain, emitter=source. Find an N-channel MOSFET rated to at least 40A.

 

It is possible to parallel transistors but a single MOSFET could handle this job easily. A regular BJT needs a base current of about 10% of the load current in order to be fully saturated - fully "on". The base current is limited by the base resistor, but it might also be limited by the PWM signal itself, which is unlikely to be capable of 3A.

A MOSFET on the other hand, needs only a voltage on its gate (at this low frequency) of ~10V to be fully on. You PWM signal will be fine, as is. A MOSFET will typically also have a lower on resistance, and will thus need less attention to heat dissipation. (A 30A load still calls for a heat sink, though.) You can use the MOSFET in place of the BJT, base=gate, collector=drain, emitter=source. Find an N-channel MOSFET rated to at least 40A.

So a MOSFET sounds like it would be ideal then.

Also the PWM will only be on for about two seconds at a time.

I had been reading up on MOSFETs yesterday and ordered two of these.

FQP30N06L

I believe they are good for 30A,but I am not sure if I read the data sheet correctly.

Would this one work? If not which one do you suggest?

 

Even with the NFET, you will still need to bolt it to a huge heat sink. You might need an mica insulating washer kit.

 

I'd put both of those MOSFETs in parallel. They're rating is just barely enough, so using just one would be borderline.

How square is you PWM signal? It's not good for a MOSFET to be in between full on or full off. That's where heat is made.

 

Even with the NFET, you will still need to bolt it to a huge heat sink. You might need an mica insulating washer kit.

Would the heat sink still be required even though it will only be on for about 1 second every hour when in use?

What heat sinks work well with the MOSFETs?

 

I'd put both of those MOSFETs in parallel. They're rating is just barely enough, so using just one would be borderline.

How square is you PWM signal? It's not good for a MOSFET to be in between full on or full off. That's where heat is made.

I found a MOSFET that I had on another junk board that is good for about 60A but its a P MOSFET not an N MOSOFET.

I don't think that one would work. When I hooked it up I just kept the LED on without pulsing it.

It think it may be too square...its on for a half second off for a half second, but I can adjust it to blink faster if needed.

 

You need to put a P on the high side of the load. You can't just replace the low side N switch.

Another problem is the PWM peak voltage MUST get close to the load's supply voltage in order to turn off the gate of the MOSFET. If the PWM is a few volts below the supply voltage (as it will be for the output from a typical 555), it may not turn off.

 

You need to put a P on the high side of the load. You can't just replace the low side N switch.

Another problem is the PWM peak voltage MUST get close to the load's supply voltage in order to turn off the gate of the MOSFET. If the PWM is a few volts below the supply voltage (as it will be for the output from a typical 555), it may not turn off.

So I am better off sticking with the N MOSFETS that I ordered and run them in Parallel?

I should be receiving before Friday. Will both the N MOSFETS need a heatsink even though it will only be on for one second every hour or so?

 

I am also curious why you need to PWM a solenoid, what is its primary function?
Is it a proportional valve maybe?
Max.

 

So I am better off sticking with the N MOSFETS that I ordered and run them in Parallel?

We really need a schematic to get much farther. A high-side switch using your P-channel might be just fine. I would use at least a small heat sink, one that screws to the tab.

 

Even with the NFET, you will still need to bolt it to a huge heat sink. You might need an mica insulating washer kit.

Here's one that'll likely do fine without a heat sink:

http://www.digikey.com/product-detail/en/AUIRFB8409/IRAUIRFB8409-ND/4162209

 

the 1n4007 diode is way under rated, a solenoid with 30 amps current through it when switched off will have a much greater reverse curent spike than the 1 amp it will handle.

 

the 1n4007 diode is way under rated, a solenoid with 30 amps current through it when switched off will have a much greater reverse curent spike than the 1 amp it will handle.

In fact, it'll be 30 amps.

 

Yes, isn't the rule of thumb that the diode should be rated to at least the solenoid's current rating, and at least double the operating voltage?

 

Yes, isn't the rule of thumb that the diode should be rated to at least the solenoid's current rating, and at least double the operating voltage?

Not generally a problem, since the reverse breakdown voltage of (generic) high current diodes tend to be in the high dozens to hundreds of volts.

 

In fact, it'll be 30 amps.

But only for ~20mS each time the solenoid is turned off. A 1A diode is too light for PWM. It would be fine for a manual switch. The 30A 20mS surge once in a while is well within the one-cycle surge rating when rectified ac charges a filter capacitor in a power supply. I would use 5A rectifier...

 

But only for ~20mS each time the solenoid is turned off. A 1A diode is too light for PWM. It would be fine for a manual switch. The 30A 20mS surge once in a while is well within the one-cycle surge rating when rectified ac charges a filter capacitor in a power supply. I would use 5A rectifier...

Yes...and, for the record, I was only commenting on the magnitude of the "spike", not the duration. I made no allusion as to whether the chosen diode was appropriate or not.