4 Channel electronic load

[gelbanana]

Here's my attempt to build a 4 ch electronic load. I'll be using IRFP240 Nch mosfet. Each could dissipate 150w of heat @ 25°c.
Specs: Should be able to sink at least 5A per ch. Able to combine all 4 ch to aggregate into a single high current ch.

Mounted

Testing current measurement with an INA169 high side shunt sensor. I might use the ACS712-20A hall effect sensor instead.

High side shunt test

Li-ion batts makes great power source. They can easily source high current. These are sanyo cells I've salvaged from a lenovo laptop battery pack.
Test @ 12v,5A
Ambient temp 28°c
Case temp 85°c
Heatsink temp 40°c

Load test

Task left
- Op amp loop
- Sort out the current sense
- Get a DAC for current set
- Get a microcontroller
- Get some lcd for display
- Write the code (ughh)
- User interface
- Somesort of data logging sd card perhaps?

[Alex555]

This is very cool, I have been wanting to make one of these for a while. Do you have any schematics available?

[gelbanana]

No finalized schematics yet. But it's based from Dave Jones design.
Currently, using paper cad, still mix and match some of the parts.

[Alex555]

I have some IRF3205 n channel mosfets. The datasheet says they are good for 80 Amps continuous. Would these be compatible?

[gelbanana]

That is when the fet is fully on, or in the fully saturation region.
What we are interested is when the fet is beginning to turn on/conducting. So you should check how much power it can dissipate since it is now acting as a resistor.
Also take note of the derating factor and the Vgs max

[Alex555]

The datasheet said 20 volts for vgs. It can dissipate 200 watts, with proper heat sinking. 200W/5A=40V, so 200 watts should be more than enough for a twenty volt supply at 5 amps?

[gelbanana]

That's 200 watts @ 25°c at the Tj. I pass 12v5A the case shoots up to 100°c under a minute even with that heatsink due to thermal resistance. Take note that the Pd will have a negative coefficient with temperature.
So it's a good idea to leave some headroom. 20v 5A = 100w that should be ok.

[Sleepwalker3]

If I get a chance I might take some photos of my setup with passes 20A+ and has been in service for over 10 years. It's somewhat more elaborate in construction that this, but doesn't bother with things like Micros, etc. it's purely analogue.

[Sleepwalker3]

Don't want to poor cold water on your idea, but isn't this more or less just a variation of the DP electronic load? If you wanted variations or extra features, you could probably start with DP's code and work from there, assuming you used a compatible processor.

[gelbanana]

Don't want to poor cold water on your idea, but isn't this more or less just a variation of the DP electronic load? If you wanted variations or extra features, you could probably start with DP's code and work from there, assuming you used a compatible processor.

At first I thought of just using 4 pots. It makes the design much more simpler. But I want to try using a uC so I can learn how to write the codes.

[gelbanana]

I've build a little op amp module

These are the values I got after some testing.

I've drew up a diagram of the system. This is what I had in mind currently.

Did a quick pcb layout, 5cmx5cm. It have a lot of empty space left. Should I add the op amps here as well?

[matseng]

No trimmers, so you'll do the calibration in the firmware?

It's a bit hard to see on the pcb layout, but do you filter the DAC power supplies locally? And do you have a filtercap on the output of the DAC to reduce the noise there?

[gelbanana]

No trimmers, so you'll do the calibration in the firmware?

It's a bit hard to see on the pcb layout, but do you filter the DAC power supplies locally? And do you have a filtercap on the output of the DAC to reduce the noise there?

Haven't put much thought on software cal. Is it hard to implement?
I did but a 0.1uf bypass cap for each IC. There's a filter cap on each DAC output.
Also, is it a good idea to add a filter cap on the board supply line?

[matseng]

Haven't put much thought on software cal. Is it hard to implement?

Nah, it's rather simple to do. Unless you want an exact unit match between the DAC value and the amperage - like value 5 to the dac gives 50 mA and dac=1000 gives 10Amp - so you can step in precise 10mA steps with a 10 bit dac.

If you calibrate with firmware your steps will become not so nice and round. If dac=100 should give 1 Amp output but due to component tolerances it actually output 1.05 Amp instead you need to divide the dac value by 1.05 giving 95.23 (rounded to 95) and output that to the dac instead.

This will make the step size more or less 10mA per step on average, but not exact. By having a DAC with more bits than 10 you can get the steps closer and closer of the desired 10mA/step but you can never there exactly even with a 24 bit DAC. With a 10 bit dac and a trimmer pot you can adjust the analogue output to give an "exact" step of 10 mA.

But unless you have precision meters to measure the current it really doesn't matter. I'd probably go for a 12 (or possibly 14 bit dac) and do firmware calibration and be happy with it.

(Dividing by 1.05 can be done without floating point by multiplying by 100 and then dividing by 105. )

[gelbanana]

Finally I've got some more pictures.
The original case was from a 3u rackmounted building automation gear which my dad gotten from his work place. What I like about the case is that it can be completely dismantled.
I've replaced the sides, front and rear panels with custom cut acrylic. I've also reduce the height to 2u.

The front panel is currently temporary as I've to get it operational for the time being. Using scarp acrylic exposes the messy cables. Currently, it uses pots for Iset on the 4 channels. No voltage or current sensors at the moment, we're flying blind here.

The aluminum profile uses standard M3 machine screws to secure the front panels. This allows on the fly panel customization. The vacant spot there is where the 20x4 lcd will go. The pots will be replaced with a 4x4 matrix keypad.
The opamp board is freely moving around. I cant find anything to secure it to.(Disaster waiting to happend.)

The alu plate was salvaged from the original unit's power supply. I've build my own custom power supply. The psu rectifies and filters 12vac from the transformer and step down to 3.3v 5v and 9v rails. It the fan headers are connected to a mosfet which will be speed controlled by PWM via the microcontroller.
I didn't have threadlocker at the current time. So I used Uhu glue on the screw threads instead.

Did some cable management. Currently the fans are secured by poster putty aka blutack.

Finally, passed the smoke test.

That's all for now. Will post more pics if i get the chance to take them.