An electrical question

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R

rzw0wr

Newbie Switchman
I have a 300 watt power supply from an old computer.
Using a watt to amp calculator I find that a 12 volts I will have 25 amps.
I want to use it to power 17 Tortoise machines and 34 leds.
Adding all that up I came up with about 1 amp.
34X.02 + 17X.016 =.68+.30 = 1 amp.


Would I be correct in thinking that I could use this for other things if I calculated the watts I was using and subtracted that
from 300 and used that wattage at say 5 volt to figure out the amperage I would have left to use?
 
rzw0wr said:
I have a 300 watt power supply from an old computer.
Using a watt to amp calculator I find that a 12 volts I will have 25 amps.
I want to use it to power 17 Tortoise machines and 34 leds.
Adding all that up I came up with about 1 amp.
34X.02 + 17X.016 =.68+.30 = 1 amp.


Would I be correct in thinking that I could use this for other things if I calculated the watts I was using and subtracted that
from 300 and used that wattage at say 5 volt to figure out the amperage I would have left to use?
Click to expand...


First a word of caution; computer power supplies can be very dangerous. They have high voltage capacitors and can hold a charge indefinitely after being unplugged. They can easily discharge through YOU even after sitting for long periods of time.

Keep in mind that the total power output of the supply is 300 watts, but that is spread across several outputs. 12volt, 5volt, etc. What you need to know is the maximum current for each output. Also the total power rating is for the power it consumes, not the power available. You could lose as much as 20% or more through the natural inefficiency of the power supply. Much of the wasted power is consumed by making heat and running the fan to keep the system cool.

Another thing to keep in mind is the power supplies are one of the most fragile parts in a computer. From my experiences (several hundred systems) power supplies are the cause of breakdowns about 20% of the time or more depending on the quality and age of the computer itself. The older the power supply, the more likely it is to fail.

I looked into using a PC power supply, but considering the work and parts involved it does not seem practical for me. The power supply should be new or nearly new. You will also need a complete schematic for the particular power supply to be used. The electrical setup should be made so that the power supply itself can be easily changed when it fails and it will eventually. Ideally you would want gauges to monitor the outputs. You will need to devise some sort of power protection for possible overloads.

These and other factors drove up my estimated cost and it just seemed to me there were other less expensive alternatives.

It can be done and has been done. Using used parts it can cost very little, but I would question the reliability of a used power supply.

Let us know how you make out and be safe!
 
Last edited by a moderator:


Hi Dale -

Pay close attention to Louis' cautions about PC power supplies. There are most definitely lethal voltages inside them, so if you are not experienced in working safely inside high-voltage electronic assemblies, you probably should not undertake the conversion of a PC power supply into a general-purpose supply for hobby use. And some sort of conversion/modification WILL be required because PC power supplies are not designed to operate outside of a computer.

Having said that, I converted a PC power supply to hobby use about 12 years ago, and it is still performing well. My first use for it was to power battery chargers for R/C aircraft batteries. Now I use it as a general-purpose power supply on my workbench. I have decades of experience working with power supplies providing many thousands of volts, so I am confident in my ability to work on them safely.

On most PC power supplies, there is a label stating the maximum current that can be provided by each of the several voltage outputs. As Louis explained, you cannot simply divide the power rating by the voltage of one output to get the current capability of that output.

Another quirk of many PC power supplies is that they often require a load on the 5V output in order to provide the nominal voltage on any output. For example, without a load on the 5V output, you may not see full voltage on the 12V output. Some PC power supplies won't even turn on without a load.

I summary, converting a PC power supply for general use is less straightforward than it seems. It is seldom as simple as plugging it in and selecting the desired output wires.

- Jeff
 
I can only echo Louis' words of caution - PC Power supplies are nasty critters than can bite you (hard) without warning. Considering the cost of a lot of power supplies now a days, I honestly can't see how the dangers involved with converting a PC power supply make it worth while.

If you do take this on, be very very careful my friend.
 
An even better approach is to go on ebay and look for laptop power supplies. These are self-contained supplies with built-in power cords. They come in a large variety of voltages and currents (some quite high) and can be had for under $10. We've used many of them to light architectural models for clients.

Mark.
 
The power supply I have has been converted and works well.
I never open the case even though I have built several power supplies.

Thanks for all of the tips and the cautions.
 


Can't say I've heard of anyone using computer power supplies except on the low 5 volt side of it for liting and small stuff with resistors of course but Louis certainly is making it a lot clearer. Will be interesting.
 
OK, you guys win.
I can not use the computer power supply like I thought.

I will have 2 control panels that are on different sides of the layout.
I don't want to run wire from one to the other.

I ordered 2 wall warts last night.

There are you happy now? :p
 
fcwilt, the 2 panel are my turnout control panels.
I could run wires to both however, I thought 2 wall warts would be better.
Also, the computer power supply is pretty old. From some where in the mid to late 90s.
 
rzw0wr:

The other members of the forum provided some good words of caution about using computer power supplies. I have 20+ Tortoise machines on my layout and I use a power pack from Radio Shack that has adjustable power supplies of 3-6-9-12 volts and works great for powering the Tortoise. The adjustable power allows you to select the right operating speed for your machines. The packs also have a built in circuit breaker.

The cost several years ago was in the range of $10-12.00.

Thanks.

Greg
 
I have 2 things to say about this thread.

1. If you do use a computer power supply make sure you fuse it to the size of the wire you are using. If you were to have a short with the amperages available without a fuse for the size of wire you could cause a fire. Not a good thing.

2. Not all wall warts are unregulated. Look at the label, usually if they are regulated they will say so. If unregulated they will have a higher voltage with no or very light load and the voltage will drop as the load goes up.
 
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