Electronic constant current DC load

[ax2013]

For mosfet, I assume IRL540N (LogicLevel) is just fine?

Ax

[ax2013]

Martin, from where did you get that enclosure from? I can only find closed Hammond cases (=not removable from one side).

Ax

[MJLorton]

Hi Ax,

I got it from RS Components in South Africa....I've just gone online to check and they don't seem to stock it any more. I'll see if I can dig up a part number...

Cheers.

[ax2013]

Thanks. Found it, built it.

I found out that the highest current load is 2.4A. I used LM324 with IRL540N. Am I right if I assume it's because LM324 has top rail voltage Vdd-1.5. For 2.4A load gate voltage is 4.7V which is all LM324 can drive using 6V Vdd.
Hmmm should I use higher Vdd for the controlling circuit to get 3A or so...?

Ax.

[ax2013]

Hmmm... I opened IRL540N datasheet and 4.7V is enough to drive approx 45A. I can't understand what limits the current as low as 2.4A.
btw: potentiometer has "dead zone" beyond 4.5V due to LM324 Vdd-1.5V issue. It is more or less cosmetic issue but can be fixed by two resistors to lower 6V.

Ax.

Edit: easy fix for potentiometer can be done with voltage divider of two resistors: 4k7 and 10k.

[dr_p]

Are you realy sure the MOSFET's Vgs is at 4.7V? Because I'm thinking maybe you're measuring the voltage from GND to Gate, not from Source to Gate. What I mean is there is a voltage drop on that sense resistor, and as current increases, in can be substantial.

[Paul Collins M0BSW]

Martin, from where did you get that enclosure from? I can only find closed Hammond cases (=not removable from one side).

Ax

I've seen them on E-Bay, however there even expensive on there too,£55:00p UK that's a lot of money.

[Fernando]

Hi, before anything I would like to thank in advance for the helpful videos! I'm building the de load project. I'm using the LM358 and the mosfet is a IRFZ44N. I would like the circuit to draw at least 3A over a maximum of 15V. On the circuit the mosfet gets very hot and I'm wondering how hot is normal. Supplying the circuit with 9V, drawing 500mA over 6V gets the mosfet build up about 80° C rapidly. (Without sink). What would happen at higher voltages or amps? Is it the wrong mosfet? The data sheet is very similar. On the oscilloscope reading the gate I get oscillations above 300ma.

[SeanB]

0.5A at 6V is 3W of power, and this has to be dissipated somewhere, and in this case that place is the MOSFET> 15V at 3A means you need to dump 45W of heat, so the heatsink is definitely needed, as without the heatsink the Mosfet is only really going to be able to dissipate 1W of power at best.

[RonaldFT]

Hi Folks / hi Martin,

I would like to built Martins electronic load by myself but I do have trouble
in getting the BUZ31LH.

All places / shops gave me the answer that this thing will not be produced
anymore and is already sold out.

Does anybody know a replacement for the BUZ31LH?

And if yes, do I have to change the circuit at some point?

Thank you very much in advance for your support!!!!

And thank you Martin for your wonderfull youtube chanel. It brought me back
to the electronic hobby.

Regards from Germany

Ronald

[Bzzz]

I'm using the IRFP240 (TO-247) as a replacement in my smallish test unit, which is available for just 1,40€ a pop from Reichelt. It worked with the circuit shown in http://www.mjlorton.com/forum/index.php?action=dlattach;topic=106.0;attach=264;image and it also does work fine with halved source resistors (so 2x 1 Ohm in parallel). It even does this with two MOSFETS and the LM324 part doubled, which is already very powerful.

For the final unit I'm however thinking about using the IRFP250MPBF (from Mouser, no sources in Germany I guess) and again smaller resistance values. Dunno if it works to the specs I'm aiming for, but you'll never know until you've tried... 

[RonaldFT]

Thank you very much for your reply!!!!