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Old Dec 09, 2010, 12:55 PM
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Cheap high power discharge load

This is not a recommended discharge load due to possible instability. An unstable regulator won't last long and can explode when it fails. A light bulb load is safer.

Warning:
Do not build this load unless you have an oscilloscope and some electronics experience. It could be dangerous, and you may not be able to eliminate the instability.
.


I built a discharge load that can accept any voltage from 3 to 37 volts, and is self regulating to prevent overcurrent. I like this better than bulbs.
Simply bolt LM317T regulators to a heatsink (use thermal compound) and basically short them out to run at current limit. You can just keep adding lm317's until you reach the current required( approx 2.2 to 2.5 amps each at 12VDC). No isolation is required for the lm317's to the heatsink. The only other part required is a .1uF capacitor.
Here is mine built using a cpu heatsink and fan, 15 amps at 12 VDC.

Notes:
- The LM317's operate at current limit. The current limit varies depending on the input voltage since the device limits it's power dissipation to about 25-35 watts. The current limit graph is shown in the LM117 data sheet. The current limit for Fairchild LM317's is a little different, see figure 6 in the Fairchild data sheet.
- The LM317's over temperature protection activates at a higher temperature than the device's maximum allowable temperature. The protection will prevent destruction of the device, but operating at this temperature for extended periods will reduce the life span. It is recommended to keep the heatsink temperature at a reasonable level, around 80-85C maximum.
- The fan on my load is connected to the battery. This is ok if using a 12 volt fan with 3s LIPO.
- A .1uf capacitor should be installed near the lm317's between input and common. This will prevent failures if long wiring is used.
-In the pictures of my load, the negative battery wire is connected to the heatsink using one of the LM317 mounting screws. This has the potential of having all of the load current through that mounting bolt, which is not good practice. The negative wire should be in contact with the heatsink using a separate mounting bolt.
-There has been two incidents where bad/fake/reject LM317T regulators were purchased. In one case a regulator exploded. You can avoid this by buying from a reputable electronics distributor.
- This load should operate correctly if regulators from the four major manufacturers are used. They are ST Microelectronics, Fairchild Semiconductor, ON Semiconductor, and Texas Instruments.

Warning:
This schematic is only being provided as a service to the RC Groups forums. Any time a high energy power source is connected to an electronic device, there is the risk of bad things happening, including component explosion and fire. It is strongly recommended to protect the circuit with a fuse and enclose all components in a suitable enclosure. As with any project, you build it at your own risk and I am not responsible for any damages or personal injury the construction and/or use of this circuit might cause. Do not attempt to build this circuit unless you have experience building similar circuits and take all safety precautions during its construction and use. Do not exceed the recommended temperature, low voltage cutoff and discharge current ratings for any cell or battery pack discharged by this load.
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Last edited by spog; Jun 23, 2014 at 03:21 AM. Reason: added notes
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Old Dec 09, 2010, 03:34 PM
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North East England UK
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Spog,

For the technicaly challenged, could you post up a schematic please?

At what voltage does the load stop discharging?

Thnx

Chip
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Old Dec 09, 2010, 04:14 PM
"Simplify, then add lightness"
Raleigh,NC
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It would not stop discharging until below 1.25 volts. The only control on the discharge current would be the number of LM317's wired in. I would think you would want more controllability in a load.
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Old Dec 09, 2010, 04:48 PM
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He might as well just use power resistors, the 50W Dale gold kind with fins that
bolt to heat sinks. Funny use of a regulator!
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Old Dec 09, 2010, 06:37 PM
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Quote:
At what voltage does the load stop discharging
Dropout voltage is prob around 3 volts at current limit, so this would be suitable for for discharging a 1s lipo.
Quote:
The only control on the discharge current would be the number of LM317's wired in. I would think you would want more controllability in a load
Absolutely, this was my test to make sure the heatsink could dissipate the wattage, I have added two more lm317's and can switch for 10, 15 or 20 amp load. any load can be switched , but only in 2.5 amp increments which is fine for me.
Quote:
He might as well just use power resistors, the 50W Dale gold kind with fins that
bolt to heat sinks.
Assuming you have scrap heatsinks or anything aluminum to bolt these to, resistors are far more expensive and a different resistor is required for different cell counts.
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Last edited by spog; May 24, 2013 at 11:30 AM. Reason: mistake with suitability for 1S
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Old Dec 09, 2010, 07:34 PM
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Ohio
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Look at a chopper with fets, I am using OmniFets which have current limit and
temp protection. That way you can control the current. And only have one
or two TO-220 devices. For 50A.
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Old Dec 09, 2010, 09:34 PM
Way to many airplanes!
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Joined Oct 2009
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Agreed, FETs are the way to go...

And if you have both LM317s and power resistors, and want to know which one is best, then the answer is simple... Use both, and make an adjustable current regulator. You could then adjust the current to your liking with a simple potentiometer.
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Old Dec 09, 2010, 11:40 PM
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Quote:
Look at a chopper with fets, I am using OmniFets which have current limit and
temp protection. That way you can control the current. And only have one
or two TO-220 devices. For 50A.
The idea of this is cheap and easy. The reason fewer fets can handle 50 amps is that they are not dissipating all of the heat. A TO-220 case cannot dissipate 200 watts, but several can. Several TO-220 devices is not a disadvantage when they cost 50 cents each, and can dissipate up to 25 watts. The LM317 also has excellent temperature protection and current limit.
Quote:
And if you have both LM317s and power resistors, and want to know which one is best, then the answer is simple... Use both, and make an adjustable current regulator. You could then adjust the current to your liking with a simple potentiometer.
Resistors are expensive since they usually come wth their own heatsink. Hard to beat a 50 cent device that can dissipate 25 watts, and several can be installed on one makeshift heatsink. Fine adjustability is not required, I don't care if it's 10 or 10.5 amps, as long as it's relatively constant and always the same when I select that load, it does the job for me.
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Last edited by spog; Dec 10, 2010 at 01:36 AM.
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Old Dec 10, 2010, 11:09 AM
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Using either Lmxxx or FET's is, IMHO, overkill and too expensive. KISS. Use a 12 volt car tail light bulb and just put it across the terminals and leave it for a couple of hours. the battery will be discharged to the point where you can now just tie the two leads together and pitch it.
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Old Dec 10, 2010, 11:47 AM
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Rodney, a high current discharge load is used to calculate battery capacity and to check performance near the battery's C rating.
My discharge load cost me less than $5 since I had a scrap heatsink and fan. A car bulb costs about the same and won't survive a 6S LIPO discharge.
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Old Dec 11, 2010, 08:42 AM
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Please accept my apologies spog, I was assuming that you were only interested in discharging batteries to be trashed. Yes, if you need to control the value of the current being used and make valid measurements of capacity, you need the control of current to make your measurements.
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Old Dec 11, 2010, 11:16 AM
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No need to apologize Rodney, I don't mind my ideas being picked apart. There's a lot to learn from this large community.
I posted this design since I think there are a lot of young people who value inexpensive but functional DIY projects, especially when they are fairly easy.
I agree with some of the posters that there are better discharge loads, but at a cost. I posted this because it is cheap, easy, and safe.
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Old Dec 16, 2010, 08:33 AM
Proud to eat Kraut ;-)
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Germany
Joined Dec 2003
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Hi spog,

great idea. Are you located in Canada by chance? The screws look like the have a Robertson drive...

Anyhow, I plan your idea for a battery heater. I have an electric bike which I also ride during winter. But the battery in it's case loses performance when cold. Therefore I plan to built a small heater unit in it, consisting of 4 pieces 15x15mm aluminium tubes in parallel sqare configuration.
Cross section:
[][]
[][]

A 12V fan blows air lenghtwise through these. The only question is how to heat them. Your idea comes handy. How can I control the current? I do not know what will work best by now...
I plan to start with 12W, this would mean 1A load total, 250mA per regulator if I use 4 of them, one on every tube.

Cheers,

Julez
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Old Dec 16, 2010, 10:25 AM
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Julez,
These regulators do come in handy as cheap heaters. I have used them on a heated transmitter box.
http://www.rcgroups.com/forums/showthread.php?t=1340114

What makes this design cheap is that the regulator is shorted to run at current limit, so no other components are required. Since you want lower power per device, you need a device with a lower current limit. The national LM317L would work with a current limit of about 200ma at 12v. See the current limit graph - http://www.national.com/ds/LM/LM317L.pdf
The problem with lower current devices is heatsinking. These come in TO-92 cases so cannot easily be bolted down. It may work fine by gluing them down with epoxy.(only about 2.5 watts each.
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Old Dec 16, 2010, 10:58 AM
Proud to eat Kraut ;-)
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Thanks for your suggestion, but I think that I prefer the To 220.
Shouldn't it be possible to put a resistor between output and ground, that produces just the right voltage drop at the current one wants?
If I purchased 3.3V regulators, and connected a 15ohm 5W resistor as described, shouldn't it work as desired?
This way, both the resistors and the regulators work below spec, which would make a long-lasting design.
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