HE-104 AC p/s

The spec says 50W at "up to" 95%.

I would allow 100W - so that is basically a 12v 8A supply.

As it takes such a wide range of input voltages, a regulated supply isn't needed. It is evidently a switch mode power supply - so will have its own local energy storage capacitors.

I would just get a continuously rated 8A 12v battery charger and hook it up to that.

Thanks Sue, I appreciate the help.

-Kevin

Duh, have you looked at the nameplates? It' susually there.

I took a trip to radio shack and the closest thing I could find was a

13.8v/15A unit for $80. Are there cheaper alternatives? I know that a PC power supply has a +12vdc but I'm not sure what modifications I'll need to do to get it to work off the motherboard.

-Kevin

Good Idea!

A PC power supply *should* work. Look at the rating on the unit and see what its 12v output current rating is - it needs to be rated at 8 A or more.

The problems you may encounter are:

1) You may need a load on the 5v output as well - particularly if it is a cheap power supply. Something like a one ohm, 25W resistor should do. This is because some of these power supplies use the current in the 5v rail, as load is applied, to provide feedback to tell the power supply to work harder. Without the load on the 5v rail, the power supply thinks it has nothing to do - and the output to all the rails, including the 12 v rail, can be low or very unstable.

2)An AT supply may be easier for you to use than an ATX. The former just uses a mains on/off switch - whereas the latter has to connect a switch to the right pins of the output connector.

3)The supply was intended to be put in another case. The output wires are short and there isn't much strain relief to stop them being pulled too hard. Make sure they are secured properly.

4)The power supply needs the fan to work to keep the electronics cool. So it will be noisy. You can get fanless models but they are very expensive.

Basically, you could buy a new, or second hand, AT power supply for very little. Just tie-wrap up all the cables except one 4 wire cable that normally goes to something like a disk or CD drive. Stick a 50 watt car headlamp between the 12v (yellow) wire and a return (black) wire. If it looks to be on full power (or you can measure that it has 12v across it with a meter) - then you don't need a 5 v load. If it is dim, much below

12v or flashing, get the 1 ohm resistor and stick that between the red and black wires. That should get the lamp nice and bright and stable.

Then you can replace the lamp with your unit. Mount the resistor, if you need one, with care - you may need to buy a heatsink to mount it on or stick it in the airstream coming out of the power supply.

Here are instructions for modifying a PC supply to get 13.8V/ 15A

This modification re-works the supply to obtain a higher voltage and current than the original design.

This assumes an old AT PC supply. The ATX supplies have a "soft start" circuit that requires pin #8 on the main 20 pin connector to be grounded to turn it on after the input 120/240 AC is applied. You might find an ATX supply whose 12V output is sufficient without modification.

I have done such conversions but wouldn't recommend them to someone with little experience of such things.

Not only is there a good chance of electrocution and of burning the house down, there is a good chance that the modified unit will not work. I had to depart from the circuit mods shown in published designs in several areas. Easy for an electronics engineer but less so for someone not familiar with switch mode power supply design.

This particular design uses the original 12v winding of T1 - at a current well in excess of what it was designed for. Even if the wire thickness is the same as for the 5 v output (and I have yet to strip a supply where this is the case), it has far more turns and may still be unsuitable. There are alternate designs which require re-winding of T1 which are better and safer.

This should be pin 14, (not 8). This is usually yellow, and can be grounded to any black wire to start the PS.

I thought there might be some interest, but I agree using the +12 V output of a standard ATX supply would be easier. A load on the +12 V alone is sufficient for most units. I use your suggestion of a 12 V automobile bulb. I still have a few 6 V lamps from my old auto days. They work well on the 5 V output for testing.

Note the first four, (red), lines in the reference.

It's also probably hard to find an AT PS anymore. :-( I've seen some for about $10, but the un-modified 12 V is only 6 A. It's a 225 W unit, so rewinding the output transformer would work.

One would think a battery charger would provide the easiest answer, but those having an output above a "trickle" charge are expensive.

LOL. What it doesn't mention is the chance the thing won't work - even after risking life and limb...

Second-hand AT supplies can be found for next to nothing, if not nothing. I have some "Intel" 500W ones that would do the job nicely, with more than enough amp rating on the 12v rail.

I had hesitated to suggest two or three identical low cost chargers with their inputs and outputs paralleled. I can't think, at 01:15 in the morning, why that shouldn't be fine. I have done that to charge a car battery, more than once. In fact more often than I care to remember - as I tend to try to keep resuscitating old car batteries when most people would have pulled the plug and let them go to that big battery heaven in the sky. But a PC supply will probably be cheaper and better.

MPJA has a 24v CT 10 amp transformer for $17.99 stock# 7846 TR

Connect stock # 4248 ($2.70) - a 25 amp bridge rectifier - to one

12V leg of the transformer lead and the center tap. Feed the output of the bridge to your he-104

Ed

Actually, the he-104 will accept 6-40v so maybe i don't need the bridge. Thanks Ed. In the mean time, I canabalized an atx power supply I had on hand. It's only 6amps but I'm hoping it works anyways. I just need to get my hands on a car lamp.

-Kevin

I thought the he-104 needed DC input? If so, you need the bridge.

Ed

OK, I went ahead with my 6am/200watt atx p/s. I connected a 12v/50watt auto bulb to the yellow/black on one of the 4 pin connectors. Then I connected the yellow and black wires on the 20 pin harness. This did nothing so I did some further reading which said it should be green. Connecting these two did the job...sort of. The light got really bright and the p/s fan spun up but then everything turned off. Then I went over to radio shack and found the largest watt-rated 1ohm resistor they had which was 10 watts. It came with two. So I wired up one of the resistors across the black and red of the same 4 pin connector and tried again. This time, the fan and the bulb stayed lit. Soon, I smelled a strange odor :). I touched the resistor and it was extremely hot so I turned everything off. I wired up 2 resistors in series, thinking I would distribute the load and the resistors seemed to be hotter, faster. At this point, I figured I should wait for what the experts think.

For shock value, here is a picture of my setup. I'm sure it's not the safest aperatus at this point.

Sounds like it did need the 5 volt load then..

It is only rated at 10W and you are shoving 25W into it - yes it will get extremely hot. The resistor will start to "cook" and probably will smell not nice at all.

I wired up 2 resistors in series,

This should only be 12.5W total - about 6W per resistor. They will still get pretty hot getting rid of that much power, even though they are rated for 10W. It shouldn't get hotter, faster. But maybe it was still pretty warm from last time..

If, however, you put the two in parallel, not series, that would be 25W each. Yes they would then each get extremely hot.

Looks like you are almost finished.

You just need to mount the two resistors (wired in series) where they can get lots of airflow and get rid of their heat safely. I would suggest a couple of bits of thick aluminium sheet, bent into L shapes. Drill suitable holes in the sheets and bolt the two sheets together, with the resistors sandwiched betweeen them. You can use the "feet" of the assembly to screw it to somewhere suitable. The aluminium sheets will help the resistors get rid of the heat - but if you can mount it somewhere that it gets moving air from the/a fan - even better.

Crimping connecting wires onto the resistor leads is better than soldering.

The power on is a green wire on pin #14, which when grounded, should turn on the PS.

Some supplies require a minimum load on the +5 V rail to stay on.

A one ohm resistor will draw 5 Amperes at 5 V. This is 25 Watts. The series resistors equal 2 ohms and draw 2.5 A. This is still 12.5 W, or

6.25 W/resistor. They will get hot, even though they're operating within their power rating.

You might find that the +5 V needs less than the 2.5 A you're providing. If you have a 6V electric torch lamp, you might try that. A low power

6V automobile lamp would be worth trying, but they're almost extinct! Even a low power 12 V auto bulb might work. If you can find a lamp that works you can avoid the need for a heat sink for the resistor(s). A lamp also makes a nice power on indicator.

Looks like you can handle the hardware part OK. Your bench looks a lot neater than mine!

Thanks, I'll try and go the 5v light route since I like your "on lamp" idea. Maybe there's an LED out there at 6v.

An LED won't hack it! The PS requires a minimum load on the 5 V output for the regulator circuits to work. A LED draws only about 20/1000 ampere, and their voltage is usually 2-3 V, requiring a series resistor to limit the current. You've shown that two 1 ohm resistors in series, (2 ohms drawing 2.5 A @ 5 V), will work OK. It probably doesn't need

2.5 A, and the goal is to find a smaller load that will work. A lamp has the advantage that it may be easier to find, doesn't need a heatsink, and provides a "pilot lamp". It won't hurt to try various low power lamps rated at least 5 V.

Good luck!

I mostly agree, FWIW.

I suggested a 25W load as a starting point - a little less than what could be assumed to be 10% of the rated output. I was pretty certain that would be enough load to satisfy whatever unit the OP used. Below about 5% then things start becoming less certain.

I, personally, would not rely on a single lamp as the minimum load. If the lamp does fail, then the resultant instability can cause the other rails to exceed nominal values. These rails seldom have crowbar protection. In the OP's case - it would be unlikely that the rails could go to voltages that could damage his particular load, however.

A standard suitably rated resistor will last the lifetime of the system.

On Fri, 27 Jan 2006 18:23:05 +0000, Palindr?me Gave us:

It only needs to be a few watts of loading to correct the regulation circuits.