jermpool

While I was looking around at radio shack I found an LED assembly with resistor for 12v, so I am going to hook it up to the bining posts and use it for a power indicator. Will post pic when I get finished, hopefully tonight. I started this project monday, it is now thursday. Actual work part not a whole lot, it's just waiting till after work the next day to get parts because I added something or changed plans.

Goldenduff

Just out of interest I need to make a voltmeter that tests under 250-500mah. Could I just attach a string of LEDs that draw this current to the end of my current voltmeter lead? Cheers and sorry for the random post!

bgosselin

I'm not sure what you want to do with the LEDs, but they by themselves will not limit current.

Voltmeter are one of two types: Hall-Effect or shunt driven.

The Hall-Effect devices are usually reserved for very high currents since they don't introduce appreciable resistance. Hooking them up is rather simple, but you need other circuitry to translate the output into something meaningful.

The shunt type is simply a very low value precision resistor wired in series with the load. When current passes through this resistor, a voltage drop is generated. You can then easily measure this v-drop with a voltmeter or whatever. The idea is to choose a shunt value that will provide the resolution you want at low currents, yet not introduce appreciable resistance to the circuit. Typically, a voltage drop of 0.05v or less is desired. So, if you want to measure a max of 500mA, a shunt value of 0.1ohms would be good.

jermpool

This is what I ended up with

Goldenduff

Thanks for the reply, I just want to load a receiver battery whilst testing the voltage (I know a non loaded voltage can look fine but not safe under load) cheers

bgosselin

Oh, in that case, just use a 12ohm resistor. 6v/12 ohm=500mA. Be aware that is going to dissipate 3W, so choose the appropriate resistor power value.

Goldenduff

Perfect thanks. Could I use a string of LEDs that draw 300mah?

bgosselin

That won't work. LEDs are rated for ~25mA, but you have to add a resistor to limit that current. If you hooked a resistor (or two) to a 6v supply, the current would theoretically be infinite: current = (v_supply - v_led)/resistance. Your supply is 6v, a typical white led is around 3v, and 0 means there is no resistance, so current=(6-3)/0.

A resistor to limit the current for around 500mA is the easiest way to do it.

gixer1k

I'd like to post my thanks to everyone participating in this thread. I just finished converting a 400 watt ATX power supply and it works great. Mine had two 12v rails, one at 14A and one at 15A, and I combined them into a single 12v post. So far I've only charged at 5A, but it's working great. Nothing gets hot and the resister barely gets warm.

I used my multimeter to monitor the voltage. While charging it stays at a constant 12.30v-12.32v, but jumps up to 12.5v briefly every 15-20 seconds. Not sure what's going on, but it works fine.

chuckk2

The voltage jump might just go away if you keep the rails separate instead of connecting them together.

ImpalaPolicePack

i just finished my second psu from a pc power supply.

the first one i did, was a 300w cheap power supply, worked good on my lipo charger but wanted something better so i sold it to buy a new one.
the new one is a thermaltake 500w power supply. i wired it just like the old one, but as soon as the charger starts charging the voltage drops to about 10v and the charger shuts off.

what can i do to make the 12v steady ?

thanks
Anthony

ImpalaPolicePack

anyone ?

Andrew

What value load resistor do you have on the 5v rail?

ImpalaPolicePack

10ohm 10watt ceramic resistor

bgosselin

You usually need to draw at least 1A or more on the 5v line to do any good. So, just parallel another one of those 10Ω resistors, which will bring total resistance down to 5Ω, which will draw 1A. The supply should be more stable, but if it's still not stable enough, add yet another one for 3.33Ω total (1.5A).

Andrew

Agreed.

While many of the newer supplies will latch on without a pre-load, the load resistors significantly improve the supply's performance and reduce voltage drop under load.

ImpalaPolicePack

allright, i will try this tonight and let you know how it does.
thanks a lot !

Anthony

bgosselin

Honestly, today's PCs put wayyy more stress on the 12v line than any of the others, so you'd think designers would put the feedback/regulation circuit on that rail instead of the less-used 5v line. I wonder when PC power supplies will be like server supplies where there is only one rail (12v) to worry about. That will make things much easier.

Andrew

With the ATX12v2 standard and later PCIx demands, the 12v rail is certainly carrying a lion's share of the load, but until there is an industry wide shift in power management architecture, I don't see change coming in the near future. The commercial server manufacturers target a small niche audience that is more willing to accept proprietary PSUs, likely due to extended warranty support and the need to maintain up-time on mission critical servers. I don't believe the consumer PC market is ready to be tied to more expensive brand specific components - COMPAQ learned that lesson. With everybody and their sister building PC MoBo's, it may take some time to get them to move away from the status quo.

On a different note, I had wondered how responsive the current standard ATX supply might be to perceived changes in voltage levels on the 3.3v rail, so I dropped a pot inline on the remote sense line running to the 3.3v connector pin. On the supply I used, a 350W SPI, it had essentially no effect on the 12v or 5v output. Unfortunate, since it would have provided an easy way to fine tune output when the PSU was under load.

bgosselin

I've had a couple supplies with a separate sense wire (old Antec true-power IIRC) for each rail. The pot trick does work if the 12v line has the pot, but seems to be hit or miss if only the other rails have the sense lead.

I've since given up on ATX supply conversion in favor of a server supply. The single 12v high current rail is nice, they are usually very robustly built, can usually be tweaked if they have some kind of sense terminal, and you can get them on eBay for ~$25 if you're lucky. I've been using a few of those HP 47A 12v supplies lately and they work great with VERY little voltage sag when drawing high currents. I've boosted the output to ~13.2v - any higher and the supply just shuts down. Soldered a couple banana jacks to the terminals and they're good to go!

ImpalaPolicePack

i tried puttin 3 resistor (10ohm 10watt) and they got pretty hot, but the voltage still drops and make the charger stop...
any other clues ?

is it possible that my power supply is not capable of keeping the voltage stable ?
its weird because its a good brand ps..

bgosselin

That is rather odd. You said this is a thermaltake 500w PS, but does it have a single 12v rail or multiple 12v rails? One of my converted PS units had two 12v rails, and when I paralleled them, they seemed to fight each other and got something like what you describe. I had to use each one individually.

ImpalaPolicePack

on the box, it says 2 12v outputs.
on the circuit board, i have 3 groups of yellow wire, labeled 12vA, 12vB, 12vC.
i use all of them together.

there is 2 big silver heatsinks.

bgosselin

What are the current ratings for each of the 12v rails? Odd that it says two rails, but there are 3 wire labels (A, B, C). Anyway, try hooking the charger to just one of the 12v wire sets by itself, preferably the highest current rated one (multiple 12v rails usually have slightly different current ratings).

Andrew

What is the model number of your PSU? Is there any way you could get us a good picture of the specification placard.

Thermaltake manufactures a pretty decent supply, and at 500W, you should be able to pull 20+ amps off a single rail. The 3.3 ohm load is more that sufficient to hold your voltage level, so there is likely something else in the mix. Separating the three rails would also be my suggestion at this time. Can you tell if your 12v lines come from separate pads on the circuit board or from the same location. Some manufacturers may claim two rails, but pull both from the same output.