noob compatible 5A constant current supply

[helobin]

Hello everyone,
I've been struggling trying to get a constant current supply working for a specialised heating element- it's quite sensitive and works best if it can be set to specific current and held there to within around 10ma. The element is about 0.5 ohm cold and 1-2 ohms depending on operating temperature - ideally it would be able to be operated from 4A to 5.5 A continuous.

I tried a buck converter from eBay, but the current regulation seemed very primitive. It made a buzzing noise and when I got an opportunity to look at the output with a scope it looked like it was using some kind of really slow pwm to limit the current (300 hz or so). With the current limiting off the output looked OK, ripple around the 150khz switching frequency but that's OK for this application.

Is the primitive current regulation how these cheapo buck modules do it?

The whole project will be running of a micro when its done, so I was thinking I could sense the current and then control the voltage of the buck converter but it seems silly to be feedback looping around a device that must itself contain some kind of loop...

The options I can think of are:
- stick with the cheap buck module and add an external current sensing set up
- source a better buck module (seems like the super cheap ones are all that's out there in these modules!)
- try and build my own buck module -- this would be an interesting project but I've got a history of jumping into making what sounds like a simple thing to make and then getting caught up in the inevitable complexities of perfecting it and then it taking months to get working!
- try and get some kind of linear supply.... not sure where I would start with that

I couldn't find much info on adjustable current sources except for much lower output led ones. Does anyone know of something I could modify or adapt for my heater? What do people think of these cheap buck converters as far as reliability etc goes?

[uncle_bob]

Hi

Ok, so your output will be 5A times half ohm = 2.5V at cold and as much as 10V when the element is hot. The regulator will be putting out 12.5W at the cold end and as much as 50W at the hot end.

Most of the constant current stuff you see on eBay is designed for things like LED lighting. They are not a good match to your needs. If 50 W is indeed correct for the max load, a 90% efficient supply will be dumping 5W. That's doing quite well at max load and it's going to require some level of heat sinking.

Do I have this right so far?

Bob