WTB (UK) Electronic DC Load

[Flump]

I am looking to buy an electronic dc load which is variable from around 1-40 amps
I would need it posting to me as I can't travel (live in Hertfordshire)
Budget around £100, maybe more for something superduper 

[EHT]

Electronic Loads are expensive pieces of kit, perhaps partly because they're a lot less common than PSUs so 2nd hand ones are hard to come by. However, there is this kit made by someone facing the same problem: http://www.arachnidlabs.com/reload-pro/ Disclaimer - haven't got one but was considering it. Anyone else have it?

[PlainName]

Yes, got one and it does what it says on the tin. Which is nowhere near 40A.

[pqass]

What would the max. voltage (and thus wattage) be required?
Do you need a current, voltage, and wattage display?
Is energy tracking (ie. time) or a dynamic load profile (changing/spiking current) needed?  eg. for testing battery capacity or testing power supplies, respectively.
Is remote control and/or data logging needed?
Portable or lab use only?  ...and many more questions.

Maybe a simple analog (potentiometer) controlled load will meet your needs. If you are capable, it is fairly easy to build with junk-bin parts. Even for high powers; which can be scaled-up with multiple lower-power loads in parallel.

The most basic of loads are just a heat-sink, a MOSFET, a shunt, an op-amp, and a potentiometer.
See:

After seeing Dave's video above, I've made my own with a surplus CPU heat-sink, an IRF540, some nichrome wire (shunt), an LM358, and a potentiometer.  I've tested it to ~100W@8A intermittently although I mostly use it for <4A most of the time. 

If I were to design one to handle 40A @1000W I'd build it around several of the most robust TO-247 package MOSFETs I could find. Like this one: http://www.kerrywong.com/2017/01/15/a-400w-1kw-peak-100a-electronic-load-using-linear-mosfets/
No need to find an expensive heavy heat-sink. Just attach the MOSFETs to one side of large metal plate and submerge the other end in a bucket of water.  It won't be pretty but it will be functional.  And cheap too. 

[Flump]

Thank you all 

Just a basic mosfet on heatsink would do, I have meters for volts and amps
I will scour the web and look for a suitable schematic.

[pqass]

Here's a simple circuit and good writeup:  https://www.smbaker.com/raspberry-pi-controlled-dc-load
Just look at the Basic Analog Load schematic and add ganged MOSFETs as shown in the next section. Nevermind the DAC and RaspPi.

You need to start with deciding what shunt you have available, then determine its voltage drop at your max. current requirement (40A).  That will determine the max. voltage at the wiper output of the potentiometer so you'll need to add a appropriate resistor on Vcc side of the pot to your chosen regulated Vcc.   eg.  if you'll be using a 0.1ohm shunt, then at 40A, it will drop 4V (at 160W!). This is your max pot wiper output.  To  get 4V at the pot wiper at max., the positive fixed leg of the pot can't be more than 4V. If your chosen regulated Vcc is 12V, and you need to get to 4V with (say) a 10K pot, using resistor divider arithmetic, we have 4V/12V=.33 ratio, so 10K / .33= 30K (of which 10K is the pot), therefore, you'll need 1*20K resistor or 2*10K resistors between Vcc and the fixed pot leg.

The other issue is power dissipation and keeping the MOSFETs from blowing up.  You'll need to know what your max voltage requirement is.  All you've written so far is that you'll need to sink 40A but a what max voltage?  If it's 12V, then that's 480W and I don't think a single MOSFET is going to cut it.  You're probably better off paralleling several MOSFET/shunt/op-amps until it meets your design requirements.  eg. 4, 10A loads in parallel; but now the shunt voltage drop is only 1V and you'll have to change the max pot wiper voltage to 1V too however each shunt power rating goes down to 10W!    4 * 10W is much better than 1 * 160W shunt!


I'm a little fuzzy on how to determine the safe operation of a MOSFET in such cases. I believe it starts with the chosen transistors junction-to-case (RthJC) and case-to-sink (RthCS) thermal ratings (in the case of an IRFP250 in the link example https://www.mouser.ca/datasheet/2/427/91212-1768624.pdf) it's 0.65 and 0.24 degreesC/W, which combined is 0.89.  The object is to keep the die under 150 degreesC (Tstg).  So the question to be answered is what is a max (realistic) heat sink temp that you can keep it to such that the wattage going through the MOSFET doesn't bring the junction above 150 degreesC.  Say, with water cooling, you think you can keep the heat sink below 40 degreesC.  150-40 = 110 degreesC delta, therefore, at 0.89 degreesC/W, we can't pump in more than 123 W.  But 123W / 40A gives us only 3.075V as our max input voltage.  If we need to handle more voltage, then we can double, etc.  eg. for 12V we'd need 4 MOSFETs (each pumping 123W; totalling 492W).  These are at the limit, so I'd derate further.   

I think this is a start.  Someone smarter than me can confirm.  If in doubt, just add more stages and ramp-up the current slowly.

[pqass]

Thinking about this further, as an exercise, I wrote a preliminary schematic for a modular, high current, high power load based around the IRFP250. I'm not sure if I can push more than 100W per module but you can just add more modules until your current, voltage, and/or power needs are met. 

Just stack (parallel-up) multiple modules across same POSITIVE and NEGATIVE banana jacks, I SET (P2.pin1), and P2.Vcc, P2.Vee, P2.GND pins. P2.pin2 and P2.pin3 are for future Arduino/controller monitoring of actual current and heat sink temperature.   

However, be careful with cranking the pot to max. Make sure you don't exceed 100W per module limit.  For example, if you parallel 2 modules,  you will be able to pull 40A combined @ a max of 5V (for a total of 200W).  If you have a 10V input, then the pot better not be cranked beyond half way (each module: 10V * 10A = 100W).

Good luck with yours.

EDIT: Made some corrections to the attached schematic for future readers.
IN-LINE ATTACHMENT REMOVED. SEE EDIT2 BELOW.


EDIT2: Due to some screwup with the forum software, when clicked, the above attachment is replaced with an image belonging to a later post of mine.  I'm leaving the above attachment as is for the admins, if they wish, to investigate.  In the mean-time, I've re-attached the correct image at the end of this post.

[Flump]

Wow thank you pqass, I do have a few Raspberry Pi's not in use at the moment so will have a good lock at this     

[pqass]

I've updated the schematic once again (to r1.3) because I forgot to mention that the MOSFET MUST BE ELECTRICALLY ISOLATED from the heat sink (for safety).  And that you should attach a GND wire from the heat-sink to the negative banana jack (also for safety).  The temperature sensor may be un-isolated since its tab is at GND potential anyway.


The RaspPi will need ADC inputs and/or DAC output via daughter card(s) as I don't think they can be found on the pin header.  But I'd be wary of using a RaspPi unless it's for monitoring only (ADCs for current read-back and temp). You don't want the I SET input control voltage to flap-around in the breeze while it's booting!   That's why I recommeded an Arduino (or other microcontroller) to set the current control voltage, add a display, and/or remote connectivity (via USB, or RS232) as its programming is more fail-safe since it's not running an OS.