Author Topic: Another Dummy Load Project!  (Read 13315 times)

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Offline ZachFlem

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Another Dummy Load Project!
« on: June 23, 2012, 10:08:20 am »
Howdy Folks!

Now I know there are already a lot of threads here about dummy load projects, but I don't seem to get any definite answers on higher capacity projects.

I am looking for a unit to test battery banks over long periods of times with current draw of up to 100a for periods of 3-5 hours.

I would like to use something that will maintain a constant current draw (e.g. http://bit.ly/KTbscH) but have come to terms that the heat loading on the componants may be to great.

I have in the past used units (http://bit.ly/Nkpa3g) rated at 500a @ 48vdc, but for my current and forseable needs, such capacity is not warranted (I'm not working on battery banks that big any more!)

I have a range of fan units (both 240vac and 12vdc) at my disposal, but am limited somewhat by space (approx 200x200x400mm).

I have also used units that simply switch a series of 10a coils on and off depending on the required load (i.e. if you need 30a, flick 3 switches, 40a? flick 4)

Either solution will be suitable, but having something that maintains a constant load would be preferable.


Sorry if this doesn't make any sense, I'm far from experienced in these things, but my current situation dictates I adopt a more DIY approach to these things (read: my wallet somewhat empty)

Cheers in advance for your help!

Zach
 

Offline Rerouter

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Re: Another Dummy Load Project!
« Reply #1 on: June 23, 2012, 10:13:55 am »
hmm, seems i jumped the gun a little, i'll repost my solution to you from the other thread, :)

hmm, 25 of these in parrellel http://au.element14.com/international-rectifier/irlb8743pbf/mosfet-n-ch-30v-78a-to220/dp/1740785
each taking 4A, would dissipate ~47W (11.6x4) means they would be running 74 degrees above ambient (silicon is 1.11c/w and assume 0.5 for heatsink), or 90-100 degrees

your load resistances would be 10 of these in parrellel per mosfet http://www.jaycar.com.au/productView.asp?ID=RR1502&form=CAT2&SUBCATID=968#1
that will dissapate at half the resistors rating, 1.6W with a 0.4V drop

all mosfets must be insulated from each other by a sil-pad or similar, you will want a fan performing active cooling, but without selecting a capable heatsink, cannot calculate air flow rate,
the wiring between mosfets should be 2 heavy bus bars  such as http://australia.rs-online.com/web/p/busbars/4896568/
this particular one can be cut to size, so you can cut it into 4 stips of 15 (13 mosfet legs, 1 tie in and a negative sense line for op amps) and have 2 groups of mosfets, 1 of 13, 1 of 12,

note i realise it would be quite difficult to solder to these, but insulated spade terminals generally fit onto the tabs and a piece of wire connected to them is very easy to solder to your mosfets and resistors,

so far we are up to only
$29.50 (mosfets)
$11.52 (resistors)
$34.70 (busbar)
$75.72 + postage

then comes op amps which isnt all that bad, it would just depends on how fancy you want to get with them,

the rest of your price will be the heatsink (jaycar do have one that forms and air column of sufficient size but isnt on there site, believe $40-70)
the case, the fan and current measurment,

an idea for current measurement would be to use an op amp (your already buy a crap load of them) and set one up as a 25 way adder, this will give you effectivly 0-10V (0-5 if in a voltage divider) for 0-100A, (yes your lowest and highest may get funky due to the op amp, but say you drove it at 7V and used the spec sheet to allow a 0V input / output,) to a standard volt meter

that is my thought on how to approach a 0-100A constant current load in a realistic size footprint,
 

Offline ZachFlem

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Re: Another Dummy Load Project!
« Reply #2 on: June 23, 2012, 11:17:13 am »
Are you thinking of the fan Assisted tunnel type heat sinks from jaycar?

2x HH-8532 http://bit.ly/MD6Tkv - 254mm long, w/ dual fans (push/pull) has a rating of "0.30oC/w approx" according to my jaycar catalogue.

I'm no expert, so I'm wondering if that is enough?
« Last Edit: June 23, 2012, 11:27:27 am by ZachFlem »
 

Offline ZachFlem

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Re: Another Dummy Load Project!
« Reply #3 on: June 23, 2012, 12:24:02 pm »
sorry, I just re-read your reply, and I missed it the first time, but I think I'm reading it correctly now... 10 of the resitors PER MOSFET? as in, 25 mosfets, so 250 resistors?
 

Offline T4P

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Re: Another Dummy Load Project!
« Reply #4 on: June 23, 2012, 01:58:34 pm »
sorry, I just re-read your reply, and I missed it the first time, but I think I'm reading it correctly now... 10 of the resitors PER MOSFET? as in, 25 mosfets, so 250 resistors?
Yes he did, 250
 

Offline codeboy2k

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Re: Another Dummy Load Project!
« Reply #5 on: June 23, 2012, 03:28:17 pm »
With this design, or for that matter at anything near a kilowatt or above, I'd seriously start doing some thermodynamic simulations.
I just hate guessing at stuff like this.  You'll have a lot of heat sources inside an enclosed container... can you really get all that heat out?  It will feel good to know for sure.

you'll need to create models of your heatsinks, the heat sources, how many sources are on each heat sink, the air flows, the enclosing container, the exit ports,  the ambient temperature of the incoming airflow, etc.

Once you have the models, feed it all to Elmer.  http://www.csc.fi/english/pages/elmer/examples

Elmer is an opensource fluid dynamics simulation software.  Airflow is considered a compressible fluid flow.

Run some simulations.  You'll know if your design is thermally sound.  (and safe)



 

Offline ZachFlem

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Re: Another Dummy Load Project!
« Reply #6 on: June 23, 2012, 11:00:26 pm »
Someone has suggested I use resistive coils (apparantly found in old DC welding equipment) trimmed to length to get the banks I require? ie, a 120w coil, and run say, 10 of them in paralell to get the 1200w I need... the current will fluctuate a bit with heat and input voltage changes, but it will actually simulate what I'm testing for with more realism.

I'm looking at options, because Rerouters solution looks good, but I think it's getting a bit out of reach of my capabilities =)
 

Offline Rerouter

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Re: Another Dummy Load Project!
« Reply #7 on: June 24, 2012, 07:11:49 am »
yes zach that was the heatsink i was thinking of, 0.3C/W (twin fan assisted) means its down to 65 above ambient, which is far more realistic, (and yes 250 resistors, but they are only 1/4W so tiny)

note this is not something i need, just a way to approach it that seems feasable to me, the mosfets can handle up to 175 degrees max, so effectivly they would be running well under spec, so thermal simulations arent as important to the outcome,

also a probably layout for this should only be about 28cm long, with the heatsink, and fans governing the length,

12 (or 2 pairs of 6) mosfets mounted onto one side of the column, 13 (or 1 of 6 and one of 7) on the other, and a fan controller and thermal cut out would really not be a bad idea, (fan controller probably another op amp driving a higher power npn running a pair of high flow rate fans, with its input part of a voltage divider with an ntc thermistor,

now onto some of the math side of it, to successfully dump 1KW/s of heat, you need 1200L of air per second,  for a 1 degree temperature rise (this is put ambient for the heatsink) so now that we have freed up 10 degrees (lets aim for 20), that means 60L/s, still a tad unresonable for an 80mm fan and borderline for 120mm fans,

if someone good with airflow modelling would want to work out the flow rate loss from a 120mm fan to 80mm shaft, or if someone knows of a 120mm version of the heatsinks, they would be the best bet to realistically dump all of the heat, (inlet and outlet must be the same) with a realistice temperature rise,

alternativly you could use 2 of the columns, so 0.5KW per sink, 30L/s which is well in the scope of 120mm, and borderline on 80mm fans, for a 20 degree increase, bringing us upto 85 above ambient on the silicon junction, (still inside thermal limits)
 

Offline george graves

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Re: Another Dummy Load Project!
« Reply #8 on: June 24, 2012, 08:42:17 am »
subscribed!

Offline ZachFlem

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Re: Another Dummy Load Project!
« Reply #9 on: June 24, 2012, 08:59:27 am »
yes zach that was the heatsink i was thinking of, 0.3C/W (twin fan assisted) means its down to 65 above ambient, which is far more realistic, (and yes 250 resistors, but they are only 1/4W so tiny)

Please excuse my ignorance, but could a higher value resistor be sub'd in to reduce the number required? ie could one use 0.5w resistors and half the number to 125?

I'd be pretty keen to see your design come to fruition, as I really like the idea (it's much more elegant than my original heater/nichrome idea) I might need a little help with the technical stuff though =)

Cheers again for everyones input!
 

Offline Rerouter

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Re: Another Dummy Load Project!
« Reply #10 on: June 24, 2012, 09:48:20 am »
you could even get away with 5W 0.1 ohm resistors per mosfet, this way was just cheaper and a little more accurate,

to point out, this design should be able to give a variable load over the range, lower currents may be hard due to your choice of op amps, but using app notes it wouldnt be that unfeasable for it to be able to cope down to 1A or less,
 

Offline ZachFlem

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Re: Another Dummy Load Project!
« Reply #11 on: June 24, 2012, 10:53:02 am »
you could even get away with 5W 0.1 ohm resistors per mosfet, this way was just cheaper and a little more accurate,

to point out, this design should be able to give a variable load over the range, lower currents may be hard due to your choice of op amps, but using app notes it wouldnt be that unfeasable for it to be able to cope down to 1A or less,

Would using the 5w 0.1ohm resistors result in a smaller physical size?

And as for usable current range, I cant see myself using it for anything under 10a anyway...
 

Offline Rerouter

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Re: Another Dummy Load Project!
« Reply #12 on: June 24, 2012, 12:00:50 pm »
yep, you can use them in place and try to save a bit of space http://www.jaycar.com.au/productView.asp?ID=RR3206&form=CAT2&SUBCATID=968#1

however when i first approached this, the thought of a few mosfets running hotter than the others was my main concern, such as dave with his original use multiple resistors to get a more accurate value, my though was to get a better average value,

also if you wanted to pursue a single resistor, then i would reccomend using JB WELD from jaycar or a thermal epoxy and epoxying the resistors to the heatsink, that or cutting up a bar, and clamping them in place against it, and make sure to secure the wires in something flexible, some silicone gel that can handle decent temperatures, as on the 5 and 10W resistors its pretty easy to have your wiring work harden the legs to a point where they snap off,
 

Offline ZachFlem

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Re: Another Dummy Load Project!
« Reply #13 on: June 25, 2012, 02:18:31 am »
Ok, so lets go with the 250 resistors for better overall performance. Would the resistors be mounted on pcb such as in Dave's blog (#102) or because of the number required, would they go on say a sheet of acrylic or are they mounted on heatsinks?

I don't mean to be a PITA, I'm just trying to get an idea of how it would be assembled.


Cheers again for you help!
 

Offline Rerouter

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Re: Another Dummy Load Project!
« Reply #14 on: June 25, 2012, 02:33:45 am »
its no problem to me mate, :)

one thought i looked over last night is that most PCB fab houses give you 10 copies, so if your boards held 3 sets of op amps and load resistors, not only are you within thermal limits, but you end up with 1 board and 2/3rds of another as spares, in case you wanted to use it for 13.8V or 14V instead, or damage one in set up, that should fit on a 10x10cm board easily, maybe even smaller, (5x10 i believe was an option on some)

they are dumping a fair but manageable bit of heat, you only would need a tiny bit of airflow across them, (1.6x25 = 40W)
(little more than passive)

a dual sided board is recommended, as there is less worry of a trace pulling away later, you would have your negative connection come in, the mosfet drain leg connection in, on one end near the resistors

and the mosfet drain leg fed back into some high ohm (say 10K) output for your current, summing amplifier,  (or a 10K and a 10K to ground if you wanted 0-5V out)  and positive and control signal for your op amps in, on the other side

pick an op amp and what size board you can get and i may even throw together the design for you, but it would be in expresspcb  or image format and you would just need to re-copy it out in your desired pcb software,
 

Offline T4P

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Re: Another Dummy Load Project!
« Reply #15 on: June 25, 2012, 04:16:54 am »
I smell lots of fans ...
 

Offline Rerouter

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Re: Another Dummy Load Project!
« Reply #16 on: June 25, 2012, 07:09:34 am »
so far 4 fans, if a dual heatsink approach is used, if a 120mm varient of the heatsink can be found then 2, + for the control electronics perhaps a low speed silent 80mm,
 

Offline T4P

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Re: Another Dummy Load Project!
« Reply #17 on: June 25, 2012, 07:54:16 am »
so far 4 fans, if a dual heatsink approach is used, if a 120mm varient of the heatsink can be found then 2, + for the control electronics perhaps a low speed silent 80mm,
Or it can be easier to use any bog standard 80mm and drop the voltage down with a resistor
 

Offline Rerouter

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Re: Another Dummy Load Project!
« Reply #18 on: June 25, 2012, 08:27:32 am »
ok, i dont normally want to touch op amps, but this would do very nicely if your willing to try hand soldering a ssop package
http://au.element14.com/texas-instruments/tlv2774cn/ic-op-amp-quad/dp/1460036

your requirements are an input range of 0-4, and an output of 1.8 to 3.2V this covers both, you also end up with an extra op amp per board, so i dont know, could add some functionality on that? or have each module checking its own mosfets temperatures for more reliablility, i dont know :/

also its cheaper to buy 10 than 9, and that last one will come in handy as a current summing amp, and probably a voltage follower for your potentiometer, and also a 2 fan regulators,

daveXRQ yes true, suppose so, just thats even more heat to dump,
 

Offline T4P

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Re: Another Dummy Load Project!
« Reply #19 on: June 25, 2012, 09:07:29 am »
ok, i dont normally want to touch op amps, but this would do very nicely if your willing to try hand soldering a ssop package
http://au.element14.com/texas-instruments/tlv2774cn/ic-op-amp-quad/dp/1460036

your requirements are an input range of 0-4, and an output of 1.8 to 3.2V this covers both, you also end up with an extra op amp per board, so i dont know, could add some functionality on that? or have each module checking its own mosfets temperatures for more reliablility, i dont know :/

also its cheaper to buy 10 than 9, and that last one will come in handy as a current summing amp, and probably a voltage follower for your potentiometer, and also a 2 fan regulators,

daveXRQ yes true, suppose so, just thats even more heat to dump,

Hardly much more heat, let's say it draws 12V at 0.16amps so if you want only 0.06amps you only dump 1.2W via the resistor ... 1.2W is hardly alot
 

Offline ZachFlem

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Re: Another Dummy Load Project!
« Reply #20 on: June 25, 2012, 09:18:58 am »

your requirements are an input range of 0-4, and an output of 1.8 to 3.2V this covers both, you also end up with an extra op amp per board, so i dont know, could add some functionality on that? or have each module checking its own mosfets temperatures for more reliablility, i dont know :/


This may surprise you *sarcasm*, but that paragraph wen't COMPLETLY over my head!

I'm INCREDIBLY gratefull for all your help in throwing this together, but I think I'm out of my depth...
 

Offline Rerouter

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Re: Another Dummy Load Project!
« Reply #21 on: June 25, 2012, 11:49:20 am »
this would be your basic circuit, with all of your VIN lines tied together and fed 0-4V (0-4Amp per mosfet) and all of your VOUT lines tied together fed into an op amp in a summing configuration,
http://upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Op-Amp_Summing_Amplifier.svg/200px-Op-Amp_Summing_Amplifier.svg.png
where rf = 10K aswell,

The power traces are 2mm wide on both layers, and signal wires are 20 mil (yes i am mixing units,)
this would be for a 5cm x 5cm PCB, the 5V would be the supply to the op amp, with ground being common to the mosfets, all nice and pretty,

D1 is drain 1 on a mosfet, G1 is the gate, and ground goes to your ground bus bar, the large holes are 1.5mm with a 3.3mm pad feel free to change as required,
the smaller holes are 1mm with a 2mm pad, for the gates and lower corner inputs, (i consolidated after making the images)

there is still the 4th op amp to use, and with some shifting someone could easily break out some pins for whatever they wish, this is just the nicest design that came to mind for 5x5cm

red is copper side, and green is component side, though its not that critical, as worst case it just involves flipping the op amp 180 degrees,

hope this helps, the last post just covered some of the bacground theory, with 4V in, means 0.4V on the + input of the op amp, and being how op amps love keeping there 2 inputs at the same voltage, it will then regulate the mosfet until it passes enough current to produce 0.4V across the sense resistor, (mosfets linear range from datasheet for 0-4A is ~1.8 to 3.2V)

while most of this is to help you i am dumping in bits of info so any luckers or later readers can actually understand how it works :)

edit: R6 and C1 and there repeated ones are just some optional footprints to try and stabalise the circuit a little if your mosfet is oscillating, though i may have got it wrong and i may need to be added between the inverting and non inverting instead, also the pin numbers on the schematic are meaningless, they where just from me throwing together a quick diagram, though the indicator mark on the pcb is correct,
« Last Edit: June 26, 2012, 07:52:29 am by Rerouter »
 

Offline Rerouter

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Re: Another Dummy Load Project!
« Reply #22 on: June 25, 2012, 08:55:00 pm »
my copper layer has an error with R31, i will post the revised board this afternoon,
 

Offline codeboy2k

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Re: Another Dummy Load Project!
« Reply #23 on: June 26, 2012, 12:52:03 am »
Your schematic.... it has the old style pen plotter lettering.  I miss that in today's tools :)

what schematic capture are you using?
 

Offline Rerouter

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Re: Another Dummy Load Project!
« Reply #24 on: June 26, 2012, 04:22:01 am »
express pcb's schematic editor, (i don't use their services, just the fasted thing i can bash together designs in)
 


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