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Model Forum / Radio Controlled / Air Models / April 2005Utilising a PC PSU as a 12V supply for battery chargers
If anyone is interested I have complied a short set of instructions on using an old PC PSU for a 12V supply for most battery chargers. See below. http://www.mmaddock.com/rc/psu_mod.html Comments welcome. Matt. > If anyone is interested I have complied a short > set of instructions on using an old PC PSU for [quoted text clipped - 7 lines] > > Matt. Interesting Matt Could you not complete the circuit of the green wire out side the box and just canablise one of the HDD plugs for use, save going inside the case? -- Vass Already been done! check here http://www.rcbatteryclinic.com/ >> If anyone is interested I have complied a short >> set of instructions on using an old PC PSU for [quoted text clipped - 13 lines] > -- > Vass Vass I did that to test if it worked initially, but I just wanted a neater solution - a lot less wires hanging around! Matt. >>If anyone is interested I have complied a short >>set of instructions on using an old PC PSU for [quoted text clipped - 13 lines] > -- > Vass Good job! I would only make three recommendations: 1. Do not do any unsoldering on the board. You may unintentionally cause a solder bridge or loosen another component. Just cut off any unused wires. 2. Use the green wire and a black wire with a small toggle switch to turn on and off the PS. Better to switch low voltage than mains all the time. Plus, you can mount it anywhere you like. 3. Add a 5ohm, 10watt resistor between one red lead and one black lead. Mount this resistor near the fan for cooling. This will help the PSU maintain its voltage better under load.  SignaturePaul McIntosh http://www.rc-bearings.com > If anyone is interested I have complied a short > set of instructions on using an old PC PSU for [quoted text clipped - 7 lines] > > Matt. >Good job! I would only make three recommendations: > [quoted text clipped - 8 lines] >Mount this resistor near the fan for cooling. This will help the PSU >maintain its voltage better under load. I agree with all Paul has to say, but I'd also stick a load (resistor) on the other lines too (+12V, +5V, +5V Standby, +3.3V, -5V, -12V). Switching PSUs don't like running without a load, if you look at the specs for a computer PSU (a decent one where they publish proper specs) it generally has a minimum as well as a maximum load specified for each line. As Paul suggested, not having a load could affect regulation, but it could also cause it to go pop prematurely. Doing this is a bit "belt and braces"; a load may not be necessary for every line of every PSU, in particular you probably don't need to load -5V and -12V, but it's almost zero extra cost as you'll be buying some components and have the soldering iron out anyway. Pick resistors to get the desired current using a couple of equations I'm sure we all learnt at school: R = V/I and P = VI. A higher power rating does no harm, but don't go lower. I second the idea of putting them in the airflow from the fan. A couple of LEDs - one for mains power (only for ATX PSUs, use the +5V standby line to power it - not the mains itself!) and one for power-on would be a nice addition too, but hardly necessary. Tim SignatureComplex in, complex process, complex out. Since you will be using the 12V to power the charger, you don't need any resistors on that line. The minus voltages are not directly driven by the switched portion on most PSUs so I wouldn't bother with them either. Look to get about 1A draw on the 5V line. You only need one resistor on one line as all the various colors are tied together at the board. SignaturePaul McIntosh http://www.rc-bearings.com > >>Good job! I would only make three recommendations: [quoted text clipped - 34 lines] > > Tim >Since you will be using the 12V to power the charger, you don't need any >resistors on that line. The charger is a sufficient load when it's on, but many 12V lines are rated as 1-2A minimum, which is less than you'd use at the end of a charge when it switches to trickle-charging. Plus of course there's no load when you're switching batteries. I am being a bit anal about all this though, no-load for a few minutes won't do any harm. Tim SignatureComplex in, complex process, complex out. Tim (et al) Thanks for the comments re. loading resistor on the other lines. Probably a good idea, and if I was starting over I'd do as you suggest, but I'm gonna stick with mine as it is - I have about 3 other PC PSU's lying around should it go pop!! LOL If it does, I'll make the next one with the mods you have suggested to help prevent future failure. Good point regarding the switch, I was just trying to keep loose wires to a minimum - if I didn't have a switch on the back of the PSU, I'd probably have gone for that option to start with. Regards all, Matt. >>Since you will be using the 12V to power the charger, you don't need any >>resistors on that line. [quoted text clipped - 6 lines] > > Tim >>Since you will be using the 12V to power the charger, you don't need any >>resistors on that line. [quoted text clipped - 6 lines] > >Tim Another option to consider for those electronically inclined is to dispense with the load resistor, and move the 5v feedback to the 12v line. This involves removing the internal feedback resistor from 5v, and replacing it with a higher value resistor connected to the 12v line. The value needed has to be determined empirically as it depends on the PSU design. This gives you a regulated 12-13v supply depending on the resistor value, and naturally will now maintain a constant voltage under load. In some cases chargers will malfunction without this as the 12v supply is not normally very well regulated. Anybody not familiar with electronics and the danger of mains voltages or indeed having any doubts about their ability should NOT attempt this! SignatureSteve I missed most of this thread so if I repeat, sorry. Here is my experience.. I just modified a PC supply. Basically, I removed all the output wiring and then re-wired the + and - 12V to a pair of banana jacks mounted on the side. I also wired in a panel light on the 12V. At first I could not figure out why the 12V would drop to around 5 or 6 V when a load was put on. My brother who is a PC guy pointed out to me that I had to load the +5V as that is the circuit that provides the regulation. I put a 5ohm 20W resistor across the +5V and voila, perfect regulation on the 12V when pulling 3 or 4 amps. The supply is rated at 9Amps so I assume it would regulate reasonably well up to that point. You need to be careful working in here as you all know. 9A can guarantee you a place downtown with a tag around yr big toe. I actually saw a supply recently that had been modified and the chap had simply brought the 12V out to a couple of naked posts so he could clip his charger on. Not a good idea. Here is what I did. I bought a small plastic project box, and mounted a single throw/double pole switch on it. I then double sided taped this to the left end of my Triton charger. I cut off the battery input line (leave a couple of inches) and wired it to the output side of the switch. One of the input sides of the switch goes to a set of leads with plugs that plug in to the PC supply. I wired the other input side to the piece of wire that I cut off the original Triton input. Now I can use either the PC supply or a battery simply by hooking one up and throwing the switch to that side. I will post a picture on the binary news grp. My disclaimer: Don't do this if you do not know what you are doing. You could kill yourself. Gord Schindler MAAC6694 Sorry gang. This should read a "double pole, double throw" (centre off) switch on the project box. Jeeez, talk about knowing what yr doing. My face is red. Gord > I missed most of this thread so if I repeat, sorry. Here is my > experience.. [quoted text clipped - 27 lines] > Gord Schindler > MAAC6694 |
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