For some time now, I’ve been thinking about creating better tools for my own use here at Jee Labs. One of them would be a simple power supply with adjustable voltage and current limits. Nothing fancy, even 1 Amp is really overkill. But with a finely adjustable current limit and a continuous readout of the actual voltage and the current draw would be really nice.

Yes, there are lots of laboratory supplies. Cheap ones even, with such capabilities. I’ve got one. But they don’t measure or limit well in the milliamps range I’m interested in. And they are all so… b u l k y !

So the idea of a “JeeSupply” was born: a little unit which runs off any simple power brick, and which uses a JeeNode to perform all the control tasks. Why a JeeNode? Well, because then I could display the readout on the PC/Mac screen, and do it wirelessly without cluttering up my desk. And set it wirelessly as well, of course. It wouldn’t take much to turn it into a stress testing rig, gradually raising the voltage and plotting the current draw over time, or lowering to see when the circuit fails. Auto power-off. Watt-metering. Motor/servo test rig. Lots of extensions come to mind once it’s under computer control.

As I said, I don’t really need much power. Even just going up to 12 V @ 500 mA would be quite useful. Especially if the whole unit is so small that you can place it on the table next to the circuit it’s feeding.

Here’s a first design for it:

Click here for a larger PDF.

It has two switching power supplies. One produces 5V and powers the JeeNode and some of the other chips, the other is based on a L6902D and generates the requested voltage. This particular switching regulator has built-in current limiting. Normally, this would be done via a fixed current sense resistor, but in this case there’s a “high-side current sensor” chip involved which converts the current to a 0-based voltage level – this level is read out by the JeeNode, and also attenuated by a digital potentiometer to adjust the actual current limit. A second digital potentiometer is used to adjust the output voltage. This is all done via an I2C bus.

The digital pots have 257 steps, which is a bit limited at the end of the V and I ranges. For this reason, I added a second pair of pots in parallel to be able to tweak the values a bit more. My hunch is that the non-linearity of this setup may work out nicely, but I haven’t thought it through yet – these extra digital pots are entirely optional anyway.

The JP1 .. JP4 connectors on the left are connected to the port headers on a JeeNode. All pins have been assigned to some function, just to see how far one could take this.

There are two more connectors: one brings out the I2C bus to drive an LCD display, the other allows connecting some LEDs, buttons, and a rotary encoder to adjust settings directly. Both are optional – with remote control, this JeeSupply would need no other connections than a power jack and the two output power lines. It could all be built very compactly – not much larger than the JeeNode itself:

The above circuit can deliver up to 25 V or so, and can probably handle 1 amp. But I really don’t care – any 8 .. 30 V power brick will drive it. I just want it to work well in the 2 .. 5 V range with a few hundred milliamps of current at most. And because both supplies are efficient switching types, there will be little heat involved.

I haven’t built this circuit, I haven’t even tried it on a breadboard. But I sure could use a pair of these in my daily lab work. Something like this would definitely help detect and debug short circuits and other common electrical wiring mistakes in all those little projects sprawling around here. It might even save a chip from frying, occasionally.

What do you think?