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Dummy Load

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v1nd:
So I'm working on a project that requires me to put a 14A load on some batteries at 12V, I was just planning on throwing some resisters down with a large heat sink, but I neglected to take into account the voltage drop over time, changing the amount of amps the load is drawing.

I'm really not advanced enough to understand a lot of the circuit that was in the video that Dave put up, although more than some did make sense. Nevertheless, I still don't feel quite comfortable changing the setup on my own.

Instead, I was wondering if there's someplace I could buy a reliable dummy load at 14A. If it costs too much, I'll think about making my own...but that's once I check some prices.

Any replies would be helpful, Thanks!

batee:
I've seen some awesome discrete current sources come from the DIY audio crowd.

Nelson Pass uses these in his open amp projects:
http://passdiy.com/pdf/zen-ver2.pdf

The part of Fig 1 that's the current source is:
R1, R6, R7, R11, C3, Q3, and Q2.

To change the current, change the value of R1.

He also shows some mods to the current source, detailed in the papers.

http://passdiy.com/projects.htm

Bryan

batee:
Followup:

I noticed 14A as a requirement.  The FET shown in the design I linked to is rated for 11A.  Ideally the FETs should be run at <30% of their rated values, so it might be necessary to choose a different P channel FET or to use multiple FETs in parallel.

Here's the design, with the exception that it uses multiple N-channel FETs.  You should have no problems finding a single FET that'll handle 14A, and it shows how they can be connected in parallel to increase current-handling capacity.

Zero999:

--- Quote from: batee on September 09, 2010, 03:41:56 am ---I noticed 14A as a requirement.  The FET shown in the design I linked to is rated for 11A.  Ideally the FETs should be run at <30% of their rated values, so it might be necessary to choose a different P channel FET or to use multiple FETs in parallel.

Here's the design, with the exception that it uses multiple N-channel FETs.  You should have no problems finding a single FET that'll handle 14A, and it shows how they can be connected in parallel to increase current-handling capacity.

--- End quote ---
I think you'll need to parallel the MOSFETS.

The problem is not the current rating but the power dissipation, the circuit dissipates 168W, more if the battery voltage is on the high side. You need at least four MOSFETS, five would be ideal.

mikeselectricstuff:
Multiple fets can help spread the heat out over a heatsink, but 168W in a single  part is probably doable with good thermal coupling (i.e. no insulating washer!).
However a  cheaper solution may be to dump the majority of the power in a resistor and the remainder in the fet.
For a battery discharge application, you're not going to care about maintaining constant curent down to a low input voltage, so connecting a power resistor in series with the FET calculated to draw a bit more than 14A at whatever is your minimum input voltage of interest  will divide the power between the R and the Fet, as well as protecting the fet from transients. You could also use the resistor for current sensing.

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