beefing up a constant load tester

[Jbates58]

hi all

i have stumbled across a DIY kit for a constant load tester, and upon reading the instructable, i found it was also featured on mailbag.

i am interested in making this unit, but want to know if i can beef it up to handle more power? i understand that a bigger heatsink will be needed, and i can handle that easily enough, but i am wanting to know if i use the supplied PCB layouts from the GITHUB, can i modify it slightly, and just add in another MOSFET and then just insert it in paralell with the other?

or am i not on the right path?

i am looking to use it for various automotive loads and what not so am looking for a fairly high current sinking capability for testing various things.

http://www.instructables.com/id/Arduino-Programmable-Constant-Current-Power-Resist/

https://github.com/wigman27/Arduino-Programmable-Constant-Current-Power-Resistance-Load

thanks all

Jason

[sleemanj]

No you can not just parallel another fet, when operating in the linear region (as a dummy load will by definition) a mosfet will not "self regulate" and therefore one will almost assuredly take all the current, until it burns out.  If you want to use multiple fets, they must each have thier own current sense, and gate driving circuitry.  It is simpler to just pick a single fet that can handle the current you require, given a sufficient heatsinking.  There are fets that can handle a lot of current, if you can get the heat away quick enough.

[alm]

Finding affordable high-current FETs with a specified SOA for linear operation may be less straight-forward, however. Most of the high-current FETs are only specified for switching (PWM) applications.

[Kleinstein]

Direct parallel connections of MOSFETs in linear (non switching) mode is tricky. It can work if the source resistor is large enough and the FETs are reasonably good matched. A relatively low voltage also helps.

In addition to the load sharing problem, 2 MOSFETs in parallel also have a higher capacitance and thus might need a new adjustment to the frequency compensation (e.g. slightly slower response). This problem also applies to most larger / higher power MOSFETs.

The more practical part is to duplicate the whole part with the OP and shunt.

[Jbates58]

ok, seems like its not a simple as i had hoped. i will simply just make 2 of the circuits then.

thanks all.

Jason

[Jbates58]

dos anyone know of a DIY style 600W minimum tester, good for 600W at about 30VDC? so about 20A.

im having a search, but not turning up alot. would perferto just have it in one unit but if its to im-practicable then i will just use multiple units.

Jason

[Cliff Matthews]

600 watts? Hope you have about 6 hours.. At very least, this 7-part series (or design progression) covers a wide gamut.

[Jbates58]

winner winner.

you sir, have the goods im after. haha. im about 1/3 way though the videos.

thanks mate, i think its exactly what im after.

[Jbates58]

OK, so after watching the videos, I like the product. However, it seems that he has it locked down to about 100w, however the components are capable of much note than that, just wondering if I  mossing something, or if I remove the Max limit, and can sink away the heat fast enough, will it work for what I'm chasing? I can't see why it won't, as the FETs are 100v and well over the current rating I am after at 30v.

EDIT:

i wrote that at work, i got home and looked at the specs for the fets, and they are capable of 350W each, but mentioned in the videos is that they will de-rate to aprox 1/2 - 2/3 of that value when near full load and temp, so assuming that i can dissipate the 600W+ of heat wouldn't that mean that i can have it safely pulling approx 2/3 of total rated capacity? so about 900W? give or take a bit.

I also noticed that he paralelled up 4 fets, by using a low value resister on the drain pin of the fets, which was my thought, as wouldn't the electron balance themselves out amoungst the fets?

i hope that makes sense.

Jason


Jason