Hi all

I am looking to control the current going to a 12v DC motor ( Mocal Electric Oil Pump EOP2 from Merlin Motorsport ) using the following SSR EL100D20-05 Sensata-Crydom | Relays | DigiKey .

I don't need to accurately control the speed I am just essentially using the SSR like a variable resistor to limit the power of the motor and it will run at this power until the next test.

My current idea is to use an arduino to output a PWM signal to the SSR using analogWrite. I am then running a series of tests where for 1 test I might want half the power for the whole test then 3/4 for the next and so on.

I would just like to know if there are any considerations I need to bear in mind, I have read about using a protective diode so I will install this and also I am concerned about the switching time, the SSR I have quotes a max switching on time of 1msec and switching off time of 300usec. Is this sufficient to allow the motor to run 'fairly' smoothly. It is only a pump so it doenst need to be running at a perfectly smooth speed.

I would buy some permanent resistors and change them out each test but I don't know how many different tests I will need to do or what type of increments I will need to change the resistance by yet. Also I do feel a motorcontroller would be better suited but I already have this SSR and I have a rather imposing deadline so would just like to know if anything here could potentially break. in summary it doesnt need to be the optimal way of doing it as long as it will do the job.

 

It's a DC relay with ON/OFF control so you must use PWM to limit/set the power to the load.
https://www.phidgets.com/docs/Solid_State_Relay_Guide

 

Is this sufficient to allow the motor to run 'fairly' smoothly. It is only a pump so it doenst need to be running at a perfectly smooth speed.

The rotation will likely be smooth enough, but there may be audible nose at the PWM switching frequency.

 

That switching time is way too slow because it will spend too much time in between on and off and overheat. Use a MOSFET to switch it. They will switch in less than a microsecond. There is no advantage to the SSR except isolation, and you would still be better off with a MOSFET and opto-coupler if isolation is necessary.

 

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That switching time is way too slow because it will spend too much time in between on and off and overheat. Use a MOSFET to switch it. They will switch in less than a microsecond. There is no advantage to the SSR except isolation, and you would still be better off with a MOSFET and opto-coupler if isolation is necessary.

The SSR is a mosfet output one so I believe it is using a mosfet. The 1ms is just the max switching time I assume it can do it quicker but it doesn't quote it on the datasheet annoyingly.

 

The SSR is a mosfet output one so I believe it is using a mosfet. The 1ms is just the max switching time I assume it can do it quicker but it doesn't quote it on the datasheet annoyingly.
View attachment 319619

Don't assume the datasheet is lying to you. I usually double things like that for a added design safety factor.

 

Do you need isolation? If not, the SSR is just a very expensive, very slow MOSFET.

 

I already have this SSR and I have a rather imposing deadline so would just like to know if anything here could potentially break. in summary it doesnt need to be the optimal way of doing it as long as it will do the job.

it is not ideal but it can do the job. you may want to keep the frequency low... (50-100Hz).

 

Do you need isolation? If not, the SSR is just a very expensive, very slow MOSFET.

Yeah I'm controlling 12V at around 5A so I need to isolate my control circuit from that.

 

Yeah I'm controlling 12V at around 5A so I need to isolate my control circuit from that.

That does not follow.

You might need two supplies with a common ground, but not necessarily isolation. This is done all the time without isolation. Isolation is needed when the grounds cannot be common.

 

That does not follow.

You might need two supplies with a common ground, but not necessarily isolation. This is done all the time without isolation. Isolation is needed when the grounds cannot be common.

As you've probably guessed I don't have a great deal of experience with this so I'm not too sure on the technical terms. By isolated I just mean that my pump is running off of 12V power supply so it uses that ground and then my control is a 5v arduino so I'm using the arduino ground pin for my low voltage side stuff.

 

As you've probably guessed I don't have a great deal of experience with this so I'm not too sure on the technical terms. By isolated I just mean that my pump is running off of 12V power supply so it uses that ground and then my control is a 5v arduino so I'm using the arduino ground pin for my low voltage side stuff.

Then all you need to do is connect the two grounds together and you can control a simple MOSFET directly from the Arduino. For faster switching a MOSFET gate driver is needed, but even without, the MOSFET can switch way faster than the SSR.

 

Then all you need to do is connect the two grounds together and you can control a simple MOSFET directly from the Arduino. For faster switching a MOSFET gate driver is needed, but even without, the MOSFET can switch way faster than the SSR.

Ok thanks for the help I'll look into it.

 

An example of where you would need isolation is if the pump ran off the AC mains.

 

You may not need two separate power supplies, although I recommend it when driving a noisy load like a motor or solenoid.
The Arduino has an onboard regulator and will run off of 12V in its Vin pin.