PC psu question

[Marc]

Hi guys, I have a few question to ask.
I finish the mechanicle part of a small version of the Phoenix,
a John C Kleinbauer design(thanks John) , cuttting area of just
7''x11'',I'll be using it to engrave and cut plastic. The stepper
motors are 60 oz in, 6 wires, 2.3v, 2.3a, 200 per rev, and I just
ordered the HobbyCNC Pro Chopper Driver Board Kits. I want to use a
450w atx 12v pc psu, the max output for the 12v is 20a.

First question. I saw a psu mod on the HobbyCNC page which consist of
connecting 2- 8 ohms 20w power resistor in parallel to the 5v lead which equals to 4 ohms, the problem is that I can't find any at that spec, would 1
25w 5ohm work ok? I tried it but seems to getting hot

Second question. So now I have the machine, pc, and soon the controlleur,psu, the sofware(TurboCNC) . What else would I need, hardware wise to get this thing going ?

I'm not looking for speed or power, just an introduction in the world
of hobby cnc

Thanks, Marc..

[pastera]

The difference in power is small (5W vs 6.25W) - what you need to know is if the 12v line stays in spec at full current draw. If it doesn't you will need to pull more power off the 5 volt line.

I have a 2.5ohm resistor on the 5 volt line of a test supply and this is plenty for the few amps I draw from the supply.

The resistor will get hot enough to burn you - this is not a problem but you can mount the resitor to an old CPU fan/heatsink to keep it cool.

Aaron

[Marc]

Thanks Aaron
So in the mod in question, what was the advantage of using two 8ohms in paralell to give 4ohms instead of using just one 8ohm ? Can you draw to much power from the 5 volt line? Sorry, I'm really a noob at this

Marc..

[pastera]

Most computer power supplies require a minimum load on the 5 volt line in order to properly regulate the voltage. If you place a significant load on the 12 volt line without loading the 5 volt, the power supply will shut down if you're lucky and smoke if not.

If you can find 5 volt fans, this is a good way of keeping things cool in addition to adding to the minimum load.

You can draw too much power from the 5 volt line and any more power than is needed to regulate the unit is wasted.

Two resistors in parallel or one equivalent resistor is a matter of what parts you can get. As long as the resistor is rated for the power dissipation, you should not havy any issues.

Aaron

[Marc]

So if I use a 5volt fan on the 5 volt line I won't need a resistor ?

Marc..

[pastera]

You will still need a resistor - unless you can find a fan that draws at least 5-10 watts (1-2 amps) - A 5 volt fan uses power that you were going to waste as heat anyhow. It also helps to get rid of the rest of the heat in your box.

Aaron

[Marc]

Thanks Aaron, it's all starting to make sense now

Thanks again for your time, I really appreciate
Marc..