CC Source Project update

General Update

So: I have moved into the new house and all (but a few boxes of crap) have been unpacked.

I made a start on my new work space and already I am getting my hands dirty; The Mini is getting its gearbox removed again and am sending it off to a company that specialises in gearboxes! They actually quoted me a reasonable price if I dropped the box myself! Will mention them if they do a good job!

At least this time my tools are steps away rather than 100m away and don't have to lug them from the top of the garden to the end of the driveway! Also: nice smooth tarmac rather than bloody pebble driveway, bliss on my back! So given these benefits, this took me 2 hours! Plus I knew where everything went!

CC Project update

So the next logical step after simulating the circuit was to build it up and test it:

I butchered the old breadboard of its parts and plonked a new one on the side and began to build the main CC source part of my design:

This schematic has the changes I made from test findings

U5 output was connected directly to a 10k trim pot instead of all the pin headers for the SET/EN switch. The wiper of the trimmer was then connected to all of the non-inverting inputs of U3:A through D.

The black a red wires dangling off to the right are load supply connections and also the varistor is not fitted for this prototype test.

I cut a piece of 2mm aluminium and bolted the MOSFETs to them each with their own insulating pad and colet. I wanted the MOSFETs to be a thermal equilibrium with each other so one didn't run away with itself.

As such, I wasn't going to exceed a load current of 1A or a load voltage of 5V, I wanted to keep the power as low as possible. I just wanted to see if it worked first before punishing it!

 Testing

Upon initial power on I found that I had the LT1014 op-amp wired in the wrong way! Idiot! No harm done, just got a little warm. Fixed that.

The other think that was getting warm was the emitter resistors: R25 - R28. As there was no load supply attached, and therefore no voltage feedback, the transistor was being driven fully by the op-amp and thus nearly the full supply was across the 100R resistors. Although not an issue for the small load currents for this test, but definitely and issue when it comes to testing bigger currents! I worked out they need to be 2W or at least 1.5W. Another change to the list!

A steady 2.5V out of the voltage reference: Good stuff!

I then did a load test on a bench supply: 3.3V and set the current limit to 250mA. The idea was to slow wind the trimmer up till I reached the limit, least then know it worked. But it didn't: power on, instant supply trip. I had wired the FETs in wrong now! Facepalm 2.0!

Re-wired the FETs right way this time, repeated above and same result...ok so one of the FET's are possibly poorly? removed the shunt resistors one by one till the fault cleared and replaced that FET.

The next day I came back, different work station (single supply not dual supply this time) so I hooked the supply for Vcc to the load and expected it to work as expected: nice linear increase in current as I increased the voltage at the non-inverting inputs of the op-amps: it did and didn't

I decided to wind the current limit up to 1A for this test. The Vcc was taking 400mA (quite high), so I wanted to allow some head-room. This supply also had a moving needle meter. It went up to about 200mA then jumped up to about 500mA, crapping my pants thinking I had accidentally induced some runaway, I back the trimmer down and yet it started to go down linearly, yet when it reached approx. 200mA, it stayed there, even as the trimmer reached the end of its travel. OK weird! I decided to leave it and come back later.

I then went to another dual supply, MNM display again. hooked up the Vcc (12V) and hooked the other supply up to the load source and this time set the voltage to 3.5V and wound the trip all the way past 1A. I basically wanted to repeat the last test and rule out using the single supply, I suspected this was the cause.

Powered on both supplies and observed a nice steady linear control of the current on the load supply. I set the load so the display read 200mA and sure enough the shunt voltage on each was within at least 1mV of 50mV.

50mV over 1R = 50mA; 4 x 50mA = 200mA!

Bang on!

I then wound the trimmer up so that there was 250mV across one shunt resistor and sure enough the needle on the display deflected toward 1A nice a smoothly throughout!

Conclusions and Plans

So it works at a glance. I'm going to do some more accurate testing:

1. observe how linear the current increases (assuming the trimmer I have is not log! Best check that first)
2. introduce a method of observing the transient response as in the simulation and observe the transient current.
3. observe the emitter voltage as well and see what if there's anything interesting I can see.

So far then I am pleased that it works up to this point. After the testing above I'm planning to then move onto developing the over-temp protection and physically sizing the heatsink by buying it in!