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Electronics => Beginners => Topic started by: bollio on July 09, 2016, 02:52:20 am

Title: Feedback/Question about electronic load design...
Post by: bollio on July 09, 2016, 02:52:20 am
Okay, so like every other noob out there, I'm building my own electronic load.  Loosely based off good ole' Dave's lm324 design, but I'm trying to make something of my "own."  You know, learn a little bit as I make mistakes.  I've got a couple questions and/or requests for feedback.  Essentially I'm looking for something with mA accuracy and nothing more.

First, attached is my rough chassis mock-up for the electronic load... a little bit of a Frankenstein/50's vibe and I've got a vintage Bakelite radio tune dial coming in the mail I'll be mounting on the top to adjust the load.  The heatsink is recycled from an old 80's receiver... anyone have any rough ideas as to what kind of wattage (estimate of course) this thing could handle?  It's got a .5" heatpipe mounted in an aluminum block (not pictured but it's about 1" x 4" x 3") and each of the plates are probably 1mm thick and 4" square.  Figured that this would be kind of important, given I'm planning on using a a pair of mosfets capable of 190watts each.  Meaning, I believe that the limiting factor on what kind of amperage the load can handle will be the heat sinks ability to dissipate the heat!

Also, some input on the circuit would be nice as well.  I've come across some op amps that I'm thinking of using, but am unsure as to their suitability.  They're not bipolar op-amps, but rather CMOS op-amps.  LMC662 rail to rail op amps to be exact.  Now from everything I've read, it seems as though it'll "work," but CMOS op amps tend to be a bit noisier, suckier at lower bandwidths, and have worse input offset voltage.  That said, it's a home made electronic load and most likely won't be a precision piece of gear... will these issues be overly detrimental to my build?  I do have some TL082's or TL741's (I realize everyone seems to hate these anymore) that I could use instead.

Also, I've read quite a bit about running multiple mosfets to share the load and will probably be using the aforementioned tandem mosfets.  Some sources say parallel is bad idea, others say just match the mosfets, many say each needs it's own op-amp.  I was planning on running each off their own op amp, sense resistor sets, and a tuning pots for each to even out any differences in output, but I'm not really sure that it matters all that much as long as I can see the total combined load.  I guess I'm a bit hung up on the question as to if it's more precise to replicate a single load on a power source or parallel loads on the power source.
Title: Re: Noob Project Feedback...
Post by: Atree on July 10, 2016, 05:24:08 am
fans make a hell of a difference. just keep in mind

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Title: Re: Noob Project Feedback...
Post by: bollio on July 10, 2016, 03:08:11 pm
Yea the chassis will have a couple of 80mm fans in taking from the back.  I figure it'll cause enough positive pressure in the chassis to creat airflow over the heat sink... We'll find out I suppose!