Anyone wants a schematic for a high current load?

[simingx]

Anyone? I built one out of a circuit modified from a Maxim application note, variable from 0-30 amps. Good for testing batteries and power supplies. It's in the background in this picture...

[Andy W]

Yes please!!

[simingx]

Here you go... Sorry for the poorly drawn schematic... :o
Again I take no credit for the original circuit, just the modifications for load shutdown, discrete current steps and the parallel FETs.

You can find the original schematic (http://pdfserv.maxim-ic.com/en/an/A0912.pdf) at Maxim's website, type in MAX480 for part number search and it'll turn up as an application note. Or you could just click on the link. :D

Grab the schematic here (http://evangelion.natriumtech.com/discharger.pdf)

Op-amp is a MAX480 but I think any type with rail to rail inputs will do. (hint, you can get samples from Maxim :D )

MOSFET can be any N-channel type with 40A or more drain current rating. I used 2 On Semi NTY100N10 (100V, 123A) devices.
Make sure they are very well heatsinked as they will dissipate nearly all the power.

SW2A and SW2B are 2 poles of a rotary switch. I think the maximum available is 6 positions, so one has a maximum of 6 discrete current selections. If you can afford a multiturn pot, you can do away with this switch but I like the ability to set the current easily.

If going above 15 amps, build one more of the circuit on the right hand side of the dotted line and connect it in "parallel". This is to share the load equally between the 2 FETs and to avoid having one FET try to throw away tons of heat.

Calibration is done by setting the 2K and 100K potentiometers such that the voltage on pin 3 of the opamp is 1V and 0.1V respectively. This will give a maximum current of (I = V / R -> I = 1 / 0.05) 20 amps and 2 amps respectively.

This is a picture of the inside of mine.. Note the huge (fan cooled) heatsink.

[Neil Stainton]

Hi simingx,

The unit looks excellent, but I can't load your schematic. Can you please check the URL?

TIA,

Neil.

[simingx]

Originally posted by Neil Stainton
Hi simingx,

The unit looks excellent, but I can't load your schematic. Can you please check the URL?

TIA,

Neil. Hi,

Link works for me... drop me an email and I'll send it to you. :D

[MAVA]

Schematic loads fine

Right click on the link and save to your desktop or any where else on your PC.

Martin

[Neil Stainton]

It worked OK for me to just now - must have had an Internet glitch before, as I got a file not found error. How critical is the ICL8069 in the circuit? Can any voltage reference be used? Are there any really big n-channel mosfets I could use at 30A without having to duplicate the right side of the circuit?

TIA, Neil.

[simingx]

I guess you could use other voltage references, just be sure to check the voltage going into pin 3 of the opamp (because I = V / R where V is the input voltage and R is the resistance of the sense resistor). For high powers you will have to use TO3 (large metal can) FETs which do get expensive... and you will still need a massive heatsink. Remember if you have 10 cells at 30 amps you are going to dissipate around 300 watts in the transistor, to get an idea of how much this is, think about a halogen floodlight...:eek:

[HELModels]

I think I might be able to build one of those. I need one to test the 386 laptop power supply I converted for Lipo charging. This power supply puts out 13.5V @1.5A. I added 2 diodes in series along the + output to get the voltage down around 12.5 and then put a 15k resistor across the output +/- terminals. I dont know what this does to the current output. I could build this and test it. I do know that I tried other resistors and the power supply blinked on and off. This would test it, or maybe dummy resistor would be enough for my purposes?

[meteor]

Hi!

Great circuit, but I have a few questions:



1) Could you draw a quick diagram to show how two (or more) parallel FETs/circuits would be wired?

I can't visualize the wiring layout for the additional 1K resistor from pin 2 of the op-amp. (would all extra the circuit's pin 2 terminate at the source of the FET(s)?)



2) Also, on your circuit diagram, I'm a bit confused what the inputs "V+" refer to?

Is "V+" the same as "Load+"?

I assume that the far left-hand +/- are something like a 9V battery, but the rest of the positive inputs are a little vague!



3) Does closing the "shutdown" switch turn off the circuit?



4) Lastly, will this circuit maintain a constant-current as the resistor heats up? (does it heat up much? Does the FET dissipate most of the heat, or the resistor?)



Thanks!

[AndyOne]

The current stability of this circuit is only as good as the shunt resistor so if you use a good one like Welwyn W series with bags of dissipation to spare the current will be about as stable as you can get.

Andy.

[simingx]

OK, to answer your questions...

1) Could you draw a quick diagram to show how two (or more) parallel FETs/circuits would be wired?

I can't visualize the wiring layout for the additional 1K resistor from pin 2 of the op-amp. (would all extra the circuit's pin 2 terminate at the source of the FET(s)?)

Each individual circuit has it's own FET, 1N4148 and 1K resistor(s).
In essence, build as many of the right hand side circuit as you like, then just connect them in parallel.
It's easier if you build one first, test it then start paralleling them together. That way you'll know at least that it works before you start introducing other unknowns into the equation.. :D

2) Also, on your circuit diagram, I'm a bit confused what the inputs "V+" refer to?

Is "V+" the same as "Load+"?

I assume that the far left-hand +/- are something like a 9V battery, but the rest of the positive inputs are a little vague!

V+ is just the supply voltage from something like a 9V battery to the opamp... in the interests of neatness :rolleyes: I didn't draw one line over... :P
V+ is NOT the same as Batt+...

3) Does closing the "shutdown" switch turn off the circuit?

Yes it pulls the opamp pin HIGH which will disable the output.

4) Lastly, will this circuit maintain a constant-current as the resistor heats up? (does it heat up much? Does the FET dissipate most of the heat, or the resistor?)

As AndyOne has stated, yes the current will drift somewhat as the sense resistors heat up. If you're going to use a stopwatch to determine mAh drain, then it would be best to get low tempco resistors.
However if you use something like a whattmeter which will actually calculate mAh based on the actual current flowing through, then it's no problem at all.
E.g. on the 30A setting, the current will drop to about 28A after a while, so it's not so bad.

Again, you HAVE to make sure the MOSFETs (and to a lesser extent, the sense resistors) are very well heatsinked, otherwise you'll just get blobs of molten silicon... Remember that the FETs are continuously dissipating power here (linear mode), so you can pretty much ignore all the manufacturer's figures of Id(max), Rds(on) etc etc. Look at the power dissipation capability instead.

Get the biggest, baddest FETs you can lay your hands on (larger physical package = more heat dissipation capability) and it wouldn't be a bad idea to heatsink the resistors too. The FETs I use come in TO247 packages, which are 4 times the size of the puny TO220 parts.

[simingx]

Ah.. just remembered another point.
I would suggest a small capacitor (few nF) from the gate of the MOSFET to ground... that will dampen any oscillations. Right now I'm getting a whistling sound at the higher currents... :o

[meteor]

OK, the parallel thing is still a little fuzzy with respect to the actual connections, but I'll try and draw what I think it needs, and you can correct it.

I guess for clarities sake, the circuit diagram should be broken into three parts:

1) the "tuning" resistors, etc.

2) the op-amp/FET stage(s)

3) the load stage (essentially the resistance...)




V+ is just the supply voltage from something like a 9V battery to the opamp... in the interests of neatness :rolleyes: I didn't draw one line over... :P
V+ is NOT the same as Batt+...



OK, it was the line connecting "+12V in" and "V+" at the top-left of the diagram that threw me off! ;) (the line to the right of the 49.9k resistor is irrelevant, right? I guess "V+" was just a note, in this case?)





As AndyOne has stated, yes the current will drift somewhat as the sense resistors heat up. If you're going to use a stopwatch to determine mAh drain, then it would be best to get low tempco resistors.
However if you use something like a whattmeter which will actually calculate mAh based on the actual current flowing through, then it's no problem at all.
E.g. on the 30A setting, the current will drop to about 28A after a while, so it's not so bad.




That's a bummer, as for use as a battery test unit, you wouldn't want that kind of variability.

Also, since the FET controls how much power gets dissipated, can you simply not have a near dead-short in the "load" circuit, thereby giving you unlimited load capability, and negating any temperature variability?



Thanks.

[chevymn1964]

Hello, I cant get the link to work to view youre schematic...... Would you mind sending it to me???

[xtal]

Hello, I cant get the link to work to view youre schematic...... Would you mind sending it to me???

msw101144@charter.net

[Chippie]

ditto.....link is still u/s

Any chance you can fix it please ? :)

[Neil Stainton]

I uploaded and saved the documents. Hope it is OK to post the PDFs here.

Neil.

[mongater]

I actually build one from the Elektor, it allow upto 20AMP but I only got a 10amp range meter, it's fully adjustable, and it comes with warning leds which is great for testing switchmode PSU,and i parallel upto 4 Fets,the HS is undersize and get's really hot,added 2 fan until i find a suitable HS. Works really good and one of my favourite project! before I forgot it's 02-2002 issue.

[simingx]

Sure it's ok to post my PDF... I lost the original a while ago.
Any improvements are welcome... I think this one will oscillate (there's a high pitched sound coming from mine) so if anyone can fix that it'll be great.

[z-matrix]

hi,
i have designed a somewhat similar testing equipment a few years ago, yet simple, and its cool.

you can stop the oscillation using a small capacitor between the gate drive output and inverting input of the opamp. 100pf-10nf
note this will increase transient response time too, so don't turn up the current and put on a li-po.
power off, connect, set current.

cheers

Z