Adjustable Current Sink

[Adamey]

I want to build a current sink that can sink from 1A - 15A at 12V. I've seen a few circuits online, but I'm looking for something that can work with a PICAXE so that I can control the sink via software. Problem is all the circuits I seem to find are analog using an OP-AMP to drive a transistor to sink the current.

The brute force way I thought of was to have 4 load resistors corresponding to 1,2,4 & 8 amps with 4 logic-level MOSFETs. It's then a simple matter to activate the appropriate MOSFETs to get any value from 1-15 in 1A increments. But a better solution might be one large transistor to sink all the current and a method to drive it (and monitor via PICAXE) to set the current to a value I like.

Has anyone ever done something similar to this?

 

[boriz]

180 Watts ! EEK!

Is it a room heater?

I like your 4 resistors idea. Prolly how I'd do it.

 

[Adamey]

180 Watts ! EEK!

Is it a room heater?

I like your 4 resistors idea. Prolly how I'd do it.

Well, 180W if it's at maximum. And duty cycle is low so average dissipation would never exceed 20W. In use likely closer to 10W.

 

[premelec]

I agree with Boriz - your weighted resistor idea is fine if you can stand the step wise nature of the load - otherwise it's back to PWM controlling MOSFETs with current sense resistors going to READADC and such - or using a digipot or DAC to control the MOSFETs...

 

[John West]

I like Premelec's idea, because you can then program any current load you want, (right down to a few mA's,) not just the steps your resistors allow. A programmable load-box is a very handy lab tool for your workbench, particularly if you ever need to check battery capacities.

 

[BeanieBots]

The switched resistor method would certainly be the easiest and there is no reason that resolution could not be improved by using more.
Alternatively, use one of the analogue designs you've found and use PWM from the PICAXE to set the demand current.
It really depends on what characteristics of the 'constant' current are important to you.

 

[fernando_g]

A constant current sink -as you may have already seen- is nothing more than a NPN or N-channel transistor, whose current is sampled and fed back to an opamp. The other opamp input is fed a variable voltage.

So, to control this voltage with a Picaxe, you have four options:

1) PWM control + low pass filter
2) Internal DAC (for applicable Picaxes)
3) External DAC.
4) Digi-Pot, with an I2C, 3-wire, or other interface.

It all depends on the required resolution and the interface protocols you are most comfortable with. I once built one using a 256 step digipot, using the 3-wire interface.

One word of advice: Once that you go over 100 steps, it pays to have an external, stable voltage reference, like the ubiquitous TL431.