Current Source Update

So I managed to breadboard up a simple constant current source from a schematic and CAD up my enclosure (whilst at the same time learn how to use Autodesk inventor) and make some end covers for it (also at the same time train up on works laser cutter).

The initial test proved my design did need some tweaking, and thanks to ease of CAD, these where amended on the schematic easily. Also to weeded out some crap components!

The Circuit

 So the circuit is pretty basic but enough to constitute a constant current source with a little overload alarm/indicator.

Brief Run-down of the specs

1. Must be mostly constructed of recycled, scavenged or spare parts found lying around in spares boxes/collections.
2. Must be mostly THP components and populated on a single sided PCB 
3. Must manage up to 7A load current at 30V maximum input voltage. Current must be fully adjustable from 0 to 7A.
4. There must be a display showing the set and driven current to a resolution of 0.1A (100mA)
5. must have over-temp warning for the driver circuit either audible or visible
6. must have an set/enable switch for turning the load on and off prior to adjustment or for quick disengage of load (in those situations where you realised you overloaded your DUT!)
7. driver electronics must run off a minimum of 12V supply, with input protection against reverse polarity supply and fused
8. DUT input must be protected against over-voltage (greater than 30V DC)
9. Spend no more than £20 on any parts that must be bought.

First iteration

I got the inspiration for this design from PauloRenato - its a pretty good design though, and also employs the same ethos as the design I want to construct.
Below is the first schematic revision and highlighted circuits are fitted to the breadboard:

So I used an adjustable reference source (TL431 adjustable voltage reference) to set a voltage of about 7.0V to the Current Set pot(goes off board on J2). The reason for a voltage ref? well why not, if you have it use it! I could have used a 9V1 Zener, but that would require potting it down and then there's the voltage/Temp issues with a Zener (as with any semiconductor junction), so with a voltage ref, there's more stability.

J3 & J4 are for the SET/ENABLE switch, but for testing on the bread board I just connected the wiper of the set pot to the non-inverting input of U1:A (first op-amp on U1). U1 is a LM324: cheap, readily available and are pretty much here to stay.

U1:B is a buffer for the LCD DMM. The LCD I chose was the DVM-210, main cos it had no units on the display meaning the reading could be translated into amps without confusion, it was small and had a backlight, which I intended to use as a over-temp indicator. It required a 0-200mV input for the DMM and required 9V to operate. So the output of the buffer had to be potted down to give a 0-200mV output from a 0-7V output, with the ability to trim/zero the meter reading.

I omitted VR2 and connected a 1R 5W resistor where J1 pins 2 and 3 are. This acts as the load resistor. Q1 & Q2 are the driver MOSFET's. They are FDP6670AL N-channel MOSFETS with some pretty good specs, albeit are they are becoming harder to find. The reason for the MOSFET pair is that the current load is split between the 2, thus halving the amount of power each FET is trying to dissipate.

U4 is a 555 timer which flashes the back light on the LCD when the over-temp detector circuit activates it. Its a slow flash at about 3Hz.

Also the input power protection was not needed and I also hit my first design snag. I had no 7809 voltage regulators (D'oh!) so I had to tweak the design and utilise an LM317 and  a few resistors to get 9V. Below is the second revision of the schematic:

Rev02 of schematic - 9V reg from LM317 highlighted in green

Testing

Testing the voltage reference output was the easy bit. Turn on power and stick a volt meter across its output and GND: 6.99V - Spot on! Job's a good 'un!

Next I tested the 9V out of the LM317: 9.05V - Good enough!

I tested the balance of load through each MOSFET to ensure each one was getting their equal share of the load. I placed an ammeter in series with the drain of Q2 and hooked it up to load a bench PSU. I set the PSU voltage to about 3V and tweaked the set-point pot till the ammeter on the PSU read about 1A, which means you would expect 0.5A through the ammeter. And you would be right, except after a while the current started to go down slowly! After some head scratching I put this down the the combination of the voltage lost over the meter impedance and the Vds of Q2, along with the affects of temperature due to power dissipated in Q2. So it did work, but it's worth noting the affect of this measurement.

Then things started to go a little awry from this point! I hooked 9V power the display and tried out the alarm circuit by connecting the reset pin on the 555 to Vcc & GND and sure enough the backlight flashed. 

However I could not get the blasted thing to give a sensible reading from the DMM, I kept getting negative values. At full scale on the set-point I got -7.00 on the display. As I approached 0A on the setpoint I got 0.56?! I tried zero it using RV1 and trying again but still negative, I thought I must have hooked it up the wrong way, but swapping the leads made things worse, and the voltage between the + and - DMM terminals remained at a steady 1.0V! WTF?! In the end I ditched it, not before dismantling it to see if there was anything stupid going on and then chucking it. I still have no idea what was going on there, the manual for these DMM's are pants and offered no detailed explanation of the internal circuitry, so I went on amazon and found me a china special 0-99V display for the low low price of £2.17! - and guess what? With VR1 & R6 removed, along with removing the LM317 circuit and hooking the display directly to Vcc, the bloody thing worked! up yours VoltCraft!

All I had to do to get it to flash was connect the power for the display to the output of the 555 timer to turn it on and off. simple and effective, the 555 can easily source the current required for this display. It did flash a bit fast now so I had increase C7 to slow it down to a little under 1Hz. There is a very small delay when the display is powered on and showing the set current, something I can live with.

I then proceeded to test the accuracy of the display against a Fluke 77 (old fluke!). I set a bench supply to a low-ish voltage to minimise heat from power dissipation (about 2V). I set the setpoint to 200mA on the display and took a reading of the voltage across the 1R shunt resistor and the current off the bench PSU:

OK so it's about 37mA out, but with the 0.1 degree of resolution and a single turn set pot I can't conclusively say that it's bang-on. Another at 0.5A (500mA):

This looks promising! The needle is pretty much half its full deflection of 1A and the Fluke says 526mV across the shunt! One more at an 1A:

That's pretty good! For now I'm satisfied that the display is not so far out for 100mA resolution. It's not uber precise or accurate, but best I could have hoped for from mostly scavenged and collected bits!

 CADding!

With the basic circuit done, it needs something to live in. I found an old extruded aluminium enclosure in my spares box. It used to house some control electronics for a pump. its perfect for PCB's as its has guide slots. All it needs is some end panels for the front and back.

New job new things to learn so I decided to  CAD up the enclosure shell into Autodesk Inventor CAD:

The best and simplest way to CAD this was to measure up all the dimensions of the profile of the enclosure, and then extrude them out. This way if I were to use the same enclosure I can just call up the outline and extrude to whatever length I want! Be useful for work related projects!

The end plates were a simple case of drawing them directly as part of an assembly so they fit the contour of the enclosure and then cut through the holes for fixtures and engravings:

From these parts I exported some DXF's and managed to find some scrap 3mm acrylic and cut them out on the laser cutter, plus some self tapping screws to bolt them to the enclosure and hey pesto:

I know the rating on the front is wrong, but I was between projects and got my spec's mixed up! The grill on the front allows ample air flow for the fan on the rear. there is a section of the fan grill where the centre of the fan blocks the centre of the grill and also doesn't match the CAD image, this is cos I amended the CAD after I realised the redundant cut outs and didn't bother re-cutting the panel.

What's next

Well the rest of the circuit! Testing the Fan control circuit, the temp control circuit and sourcing some more parts as well as getting the gerbers for the PCB done and milled out on the isolation router.

Amongst some of the parts I found these heatsinks, which'll be perfect for this design and enclosure.

Must Crack on!