I am searching for the easiest and cheapest way to vary the speed of a 12VDC motor (1/4 hp) with a 4-20 mA signal.

 

It is hard to commbine the cheapest way and the easiest way. Built a PWM circuit combined with a power MOSFET circuit may be the cheapiest way. The easiest way, you can buy the driver.

 

I'm working right now on a similar assignment.
I have to vary the current trough a DC-coil
0-100% = 0-2 Amps, with a 4-20mA signal.
I have a pretty good concept in my mind, and within a week or so, I can tell if it works.
Right now I'm setting it up on my breadboard.

If this principle works, you could adapt it for your application, the only thing you have to worry about is the current controller, this must be capable of handling the currents you need, for this specific motor.

Right, this was the reply to the "easy" demand. If you decide to use off the shelf devices, you would not be able to meet the "cheap" demand, I can assure you that.

Best regards,
Flyer (always open for debates)

 

Dan Clark is not reading the replies anymore. Probably he solved the problem. If not contact me at: elaborated2freemail.be

bye