Chargers My new charging setup

[callsign4223]

Hello guys, as some of you may already know, a lightning storm blew up my trusty Thunder AC6. Considering as I now have 6 3s batteries, I decided I needed something a little beefier to replace it. Being a computer geek, I have all sorts of old hardware laying around. I decided to re-purpose an old computer power supply for my needs. The power supply is a 300w with a single 12v rail rated at 15A. If I did my math right that means I should be able to get 180W out of the 12v rail. I guess the other 120w is designated for the 5v and 3.3v rails. Anyway, 180w should be plenty for my needs to parallel 6 3s batteries at 1c. If my math is wrong, please enlighten me, I am a computer tech, not an electrical engineer.

Here is a pic of my new setup. It's the 300w computer PSU and my new iCharger 206.



Yes, I am going to clean this up ALOT. This is just my testing phase. I intend to build a case for the PSU with plugs for the 12v and 5v rails to charge and to test RX and servos.

Here is my question for Xokia and Gaba, our resident electrical engineers. Do I need resistors on the 5v rail to keep everything happy. Most everything I have read online has said to use them. Here are some stats I have compiled from testing this setup.

12v reading No resistors
Without Load 12.23v
With load 11.8v

12v reading 1 1 ohm 10 watt resistor
Without load 12.58v
With load Didn't test, resistor started smoking after 10 seconds without load

12v reading 2 1 ohm 10 watt resistor
Without load 12.4 v
With load 12.2 v Resistors got VERY HOT. One steadied out at 90c, the other climbed to 130c after 4 minutes and I aborted the test. The one that got to 130c is also the one that I tested by itself that started to smoke so it might be internally damaged already.

I thought about getting another package of 1 ohm 10 watt resistors and just doing 3 in series and see what happens. I also wanted to try a 10 ohm 10 watt resistor, but Radioshack didn't have any in stock last week.

Also, if I was looking for an analog 12v power meter, what would I search for on google. Something that looks like the old analog readouts on car battery chargers. I can't seem to come up with the right terms on google.

 

[Derek]

I really wish that I understood this "electrical" stuff of RC'ing a bit more. I'd like to have a portable charging station but I'd probably blow myself up if I attempted a project like this, lol.

 

[callsign4223]

This was many hours of google before I felt comfortable putting this Frankenstein setup together. It's gonna get a lot better too though, wait till you see the final product.

Ohh, by the way, I found the gauges I was looking for. The proper keywords were DC 0-20V gauge.

 

[Lee]

Looks Good Matt.
I had almost the same charger before. Waiting to upgrade to a bigger charger.

 

[Tony]

I can't remember what I had in my PSU that I built, but it was one of the large ceramic resistors. As I said in the text, I always use a resistor on the 5v rail, it helps to produce more even voltage when you start drawing power on the 12v side.

I think I have the 10ohm 10w resistor in mine. If you are using small resistors like would be used on an LED, then those are going to smoke every time. I have never had luck with them but I'm no engineer. I just use the big ones, and shoo-goo them to the head sink in there. You can JB Weld them as well.

 

[callsign4223]

These are large ceramic resistors. I was gonna use a 10 ohm 10 watt resistor, but RadioShack was sold out of them.

 

[Tony]

interesting, what voltage are they rated for?

 

[Stambo]

I use 2x 21w car light bulbs in paralell on the 5v rail to bring the 12v up and stabilize it. Same as a resistor but cheap.
I just grabbed a couple of old bulb holders out of some tail lights that were broken and gonna be thrown out.
The thing about light bulbs is you have a visual indicator that PSU is turned on.
These "smart" power supplies don't like seeing load on the 12v rail when there is none on the 5v side.

 

[callsign4223]

I don't think resistors are rated on voltage. They are rated on wattage they can handle. The formula I dug up on the internet is W=(V*V)/R so at 5 volts I have W=(5*5)/R and with 1 ohm that gives me 25 watts on a 10 watt resistor, no wonder it smoked. Even with 2 ohms it's still 12.5 which is over the rating of the resistors. I am going to stop by radio shack again tonight, hopefully they will have some 10 ohm resistors in tonight.

 

[pvolcko]

Yeah, 1 ohm is not much resistance. You can series resistors together to add their resistance. So if they don't have 10ohm get a bunch of 1 ohm and solder them together. You can go with a lower watt rating in this configuration (ten 1 watt 1ohm resistors would do the trick, or five 2W 2Ohm resistors).

And, actually, you could go with lower Watt rating still. If you have 10ohms of resistance on 5V rail, that's .5A of current through the circuit. P = I^2R so .5*.5*10 = .25*10 = 2.5W If you're using multiple lower ohm resistors in series, say five 2 ohm resistors, that's .25*2 on each or .5W each. Maybe go double W rating to be safe, so for a single resistor you can go 10ohm 5W, or five 2ohm 1W in series.