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Old 11-07-2011, 10:51 PM   #1
rooster92
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Default DIY Discharger for 4s and 6s packs

Attached is a simple discharger I'm planning on building. Anyone see problems with using this for both 4s and 6s packs?

6s packs should discharge at ~6.8 amps
4s packs should discharge at ~5.4 amps

The bulbs I plan on using are 12v (20w) halogen. Because they produce a lot of heat and are VERY bright, I'm toying with the idea of enclosing them, hence the fan.

I'm hoping someone with more experience with electronics can give me some advice before I build.
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Old 11-09-2011, 02:21 PM   #2
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You'll need to monitor the voltage to cut out your bulbs at certain voltage level, otherwise you risk overdischarching and ruining your packs. Redbird did it some time ago, the post is a couple of posts below:

http://www.helifreak.com/showthread.php?t=277056
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Old 11-09-2011, 02:57 PM   #3
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I had planned on just using one of these (http://hobbyking.com/hobbyking/store...idproduct=7367) and staying nearby to monitor. Redbirds solution is so nice and much safer, I should probably invest a bit more money into this project.
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Old 11-09-2011, 06:40 PM   #4
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If you're using 7812 for the fan supply anyway you can use it as a reference voltage and feed it into the comparator. Then you can use 2-resistor voltage divider to measure a voltage of the Lipo that will give you 12V when the Lipo is 22.2V (for 6s) and feed this into the second input of the comparator. The output of the comparator would drive a power transistor switching the light bulbs on and off. You can even make a second voltage divider to give 12V when the battery voltage is 14.8V and use a toggle switch to feed the voltage to the comparator from either divider for easy selection between 6s and 4s. Simple enough?
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Old 11-10-2011, 09:14 AM   #5
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Quote:
Originally Posted by jperkosk View Post
If you're using 7812 for the fan supply anyway you can use it as a reference voltage and feed it into the comparator. Then you can use 2-resistor voltage divider to measure a voltage of the Lipo that will give you 12V when the Lipo is 22.2V (for 6s) and feed this into the second input of the comparator. The output of the comparator would drive a power transistor switching the light bulbs on and off. You can even make a second voltage divider to give 12V when the battery voltage is 14.8V and use a toggle switch to feed the voltage to the comparator from either divider for easy selection between 6s and 4s. Simple enough?
With my very limited knowledge of electronics, this doesn't sound simple at all. I'll have to do some homework!
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Old 11-10-2011, 10:07 AM   #6
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Quote:
Originally Posted by jperkosk View Post
If you're using 7812 for the fan supply anyway you can use it as a reference voltage and feed it into the comparator. Then you can use 2-resistor voltage divider to measure a voltage of the Lipo that will give you 12V when the Lipo is 22.2V (for 6s) and feed this into the second input of the comparator. The output of the comparator would drive a power transistor switching the light bulbs on and off. You can even make a second voltage divider to give 12V when the battery voltage is 14.8V and use a toggle switch to feed the voltage to the comparator from either divider for easy selection between 6s and 4s. Simple enough?
Jerry, got ya, great idea !
Also remember that not everyone speaks "Electronish" as fluent as you do, my friend.
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Old 11-11-2011, 01:12 PM   #7
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Sorry, got too geeky, I guess
Will try to come up with a diagram this weekend to explain this concept...
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Old 11-14-2011, 07:03 PM   #8
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OK, here is the promised schematic. The comparator can be anything, even the standard operating amplifier will work. The proposed resistor values will discharge the 6S battery to 22.4V, or 3.73V per cell.

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Old 11-14-2011, 07:53 PM   #9
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And this is the same discharger modified to be selectable from 6S to 4S. In 4S position it will discharge the pack to 15V, or 3.75V per cell.

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Old 11-15-2011, 02:09 PM   #10
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Wow, thanks. It will be a challenge for me to build one of these but I look forward to it.

I'm very new to electronics and am using a mac program called iCircuit to "prototype". I like it because it allows me to run the circuit, see voltages, current, etc. It even has a oscilloscope built in.

It has an op-amp which I understand does the same thing as a comparator? However, the component looks different than what you show. See attached.

Side question, what program are you using to draw these up?
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Old 11-15-2011, 07:54 PM   #11
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You're good to go with op-amp, they're practically the same thing as comparator. The difference is in the gain, the comparator's gain is much higher and therefore closer to the ideal theoretical infinite gain. In the real world you'll find op-amps dirt cheap and there may be a bit of jitter while switching from on-to-off and back, of no consequence here.

I'm using AutoCAD Electrical, which is an AutoCAD with some extra tools for drawing up the electrical circuits, but it's very expensive, over complicated and pain in the rear end to use, don't even know myself how to use it efficiently. Picked up few second hand copies on the cheap a couple of versions ago from the company that hasn't survived the last economic downturn, so that's what we're using in our company.

Couldn't find a symbol for op-amp, so made it up myself, that could explain why it looks weird
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Old 11-16-2011, 09:36 AM   #12
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Quote:
Originally Posted by jperkosk View Post
Couldn't find a symbol for op-amp, so made it up myself, that could explain why it looks weird
So, does this look about right as far as combining to hook it up? I'm going to harvest some components this weekend and see what I can find. I'll be pulling apart an old Tivo, DVD player, and power supply that have been sitting around forever! Otherwise, I'll have to put an order together from digikey or something.

A friend of mine lent me a heat gun (paint stripper) which I found works much better for removing components than trying to use a soldering iron!
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Old 11-16-2011, 09:52 PM   #13
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No, you can't do that, my diagram is correct even if the op-amp symbol is makeshift.
The comparators and op-amps have differential input to compare two voltages, hence + and - inputs on the left, + sensing the voltage from the battery, - seeing a fixed reference voltage. The whole chip will need a power supply which typically is 10-18VDC, that's what the top and bottom pins are, taking 12V supply from the output of 7812.
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Old 11-19-2011, 01:18 PM   #14
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Quote:
Originally Posted by jperkosk View Post
And this is the same discharger modified to be selectable from 6S to 4S. In 4S position it will discharge the pack to 15V, or 3.75V per cell.
Couple quick questions:

1. What kind of switch should I be using for the 4s or 6s? The diagram is a bit crowded and hard to see.
2. What kind of Transistor would you recommend? Here are some that I have on hand, salvaged from a UPS and an Amplifier.

- B817E 7E3 (http://www.alldatasheet.com/datashee...N/2SB817E.html)
- 2SD1047 (http://www.alldatasheet.com/datashee...S/2SD1047.html)
- IRFZ 46N (http://www.irf.com/product-info/data...ta/irfz46n.pdf)

On a side note, I was able to figure out the op-amp in the program I'm using (iCircuit). I was confused that your symbol for it had 5 connections whereas the symbol in iCircuit had only three. In iCircuit, you specify the "Vout" and "Vin" on the properties page of the element. Kind of odd if you ask me. But hey, I've never worked with anything but LEDs and Resistors.

Thanks again for your help with this!
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Old 11-22-2011, 06:24 AM   #15
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  1. Single pole double throw switch. Common goes to the "-" voltage sensing input of the op-amp. NC and NO go to reference voltages that we created from 12V for 6S and 4S batteries.
  2. Transistors:
    1. First is 12A but it's PNP, so you'll have to mount it between "+" of the battery and the light bulbs and modify the voltage sensing circuit accordingly.
    2. Second is NPN, but only 10A. You must mount it on the heat sink and not exceed 8-9A of discharge current, maybe only 1 bank of light bulbs? You could mount 2 or even 3 of them on the same heat sink, and connect them in parallel, just remember that each has to have its own 2k2 resistor going into the base.
    3. Last is FET and only 20V, will blow with 6S.
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Old 11-22-2011, 09:51 AM   #16
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Looks like I may be getting closer. Thank you for all the help. Sounds like the D1047's would be a good option for me; I happen to have two of them too. I also added another bank of bulbs. If the calculations are correct, each bank will draw 4 amps at 4S and 5.1 amps at 6S. I added a switch to the 2nd bank which will allow me to choose whether to discharge with one bank or two.

So, am I there yet?
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Old 11-22-2011, 10:42 AM   #17
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Looks perfect to me

Don't forget the heat sink, the bigger, the better.

EDIT:
Looked up the specs on your transistors again, noticed very low current gain of 50. It means to get 10A discharge rate you'll need close to 200mA EB current, that means 80-100 Ohms resistors in the base instead of 2k2. The rest should be fine. Make sure your op-amp can do 400 mA on the output, otherwise you'll need an intermediate transistor driving the big ones
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Old 11-22-2011, 03:17 PM   #18
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looks like I need to order some parts. I was planning on using a LM741 Op Amp from Radioshack and it appears it is only good for 25mA output.

Would you recommend getting more appropriate transistors or a different Op Amp? I bet I already know the answer to that. Rather than complicate this any further, best to get correct transistors. I'm in no hurry and am having a lot of fun learning. Heck, before this, I didn't even know how a transistor worked.

Edit: Just stumbled across this (
) video and wonder if I should be looking at MOSFETs??! Going to give my brain a rest now.
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Old 11-22-2011, 05:26 PM   #19
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FETs have very low gate current, the problem is finding one for 50V and 10A; been almost 25 years since I built a decent circuit so I'm a bit rusty on newest components out there...

Sticking a small intermediate NPN transitor to drive your 10A power transistors is an option too, it's easy to find one with 350-500 current gain and 500 mA output (emitter-collector) current.
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