Need help for a 12v-18A /5V power supply

[Niggo]

Hi! I need to build a power supply for my stepper motors and drivers with 12V-18A and 5V outputs. It should be cheap and possibily compact for some space problems I have, do you know where should I look into? Do you think ATX psu of about 450-500W can be used for this? Thanks for any help and plans

[NC Cams]

Daisychaining ATX power supplies:

http://www.cnczone.com/forums/showth...015#post142015

http://forums.bit-tech.net/showthread.php?t=108208
Linear P/S design/construction


Linear supply:
http://www.campbelldesigns.com/files...ply-part-1.pdf

The AT or ATX supply should work fine if it has the amperage you need - check the specs before you start to modify it and DON'T try to run it long at 100% current output capacity - shoot for 50% or less.

[Niggo]

Thanks for your help! My problem now is how to figue out what resistor to use across the 5v to give as many amp as possible to 12v rail, probably is better to use 2 PS in parallel.

[dertsap]

dual at supplies work well ,ive done it in the past , definitely a cheap source of power

[Niggo]

Yeah, I red. I am searching for some info about this damn load resistor (value mainly) to use as much amp as possibile from the 12V.

[JRoque]

Hello. The value of the resistor so it gives a certain amperage is: Ohms = Volts/Amps. So if you need 1A from your 5V power supply, the resistor would be 5/1 = 5 ohms. The resistor wattage rating is V*A so 5V * 1A = 5W.

Keep in mind, though, that your 5V circuit will use only the amperage it needs and nothing more so there's really no need to current limit anything.

Also, switching power supplies don't like the inductance load and voltage generated by motors. Look into TVS to help with the latter or even a large series diode might do.

If the 12V power supply is for the stepper motors, consider that steppers like to run at about 15 times their voltage rating. If your steppers say they are 1.5V, a 24V power supply will run them much better than a 12V one. Two 12V power supplies that have their secondaries isolated will do the trick when connected in series. However, 24V PS are common and plentiful.

JR

[kreutz]

From your specs you are looking for a > 350 W power supply. Most AT and ATX power supplies have the 12 volt and 5 volt current ratings written on the case. You did not specify the current for the +5 volt and is the +5 volt circuit voltage what is really regulated in the switching power supply.

In order to make them work you will need to put a load on the +5volt , this has to be a power resistor between +5 volt and ground, 1 ohm / 10 Watt is OK (will draw about 5 Amps) or you can use two 2 ohms 5 watt in parallel. You will want to attach it to the case or housing so the case acts as as heatsink, or to a suitable heatsink, it will get hot...

I don't know if you are going to get the 18Amps out of the 12 volt circuit, it depends on the power supply you get, aim for 25 Amps minimum, you need a margin. In the first place the 12 volt circuit is generally non regulated. Then, if you are trying to use it as a power supply for steppers or servos you will need to beef-up the output capacitors in the +12 volt output ( because of the high ripple current at about 20 KHz) by adding low impedance (low ESR) capacitors, at least 22,000 uF, 5 x 4,700 uF (around $1.36 each + shipping/handling e.g. Digikey part# 493-1308-ND) in parallel is better than 1 of 22,000 uf because you are paralleling the internal impedance. Voltage rating 25 volts minimum. The more capacitors you add the better, but too many will trigger the power supply protection circuit at startup because of the inrush current.

How did you come to the 18 Amps figure?

[Niggo]

Thanks to all! I think I need to find the highest resistor that can give me the less amp to power up the PSU so I waste less wattage (it depends on PSU), isn't it? The 18 amp comes from the specs of my motors: about 2.1A per phase, 4 of them (18A is a good round up ). The idea of the capacitors is new to me, I thought the 12v of an atx has no ripples...investigating on this, thanks!

p.s. the 5v rail is only for the driver supply and interface board so few Amp are required

[kreutz]

I think I need to find the highest resistor that can give me the less amp to power up the PSU so I waste less wattage

Remember that your power supply is made for a PC , a 350 Watt power supply is designed to be under at least a minimum load from the CPU and chip-set, 5 watt is a very low and conservative load I don't think you are going to get it working with less than that (1.4% of the power supply capability)

All switched mode power supplies have to work with a minimum load. When no load (or under-load) is applied they won't switch anymore, so no voltage will be at the output.

The idea of the capacitors is new to me, I thought the 12v of an atx has no ripples...investigating on this

The 12 volt has minimum ripple from the power supply when is connected to a PC. It is your load the one that is going to stress it beyond the limits of the design. Your motors consume current in pulses, those pulses create a ripple current around the chopping frequency, that ripple is in the order of the peak current going through the load, so the PSU capacitors, not designed to support that ripple at at frequency, are going to heat-up and explode.

I think you better go with a commercial +12 volt switched power supply, then use a 78S05 to regulate your +5 volt output. Approx. 300 watt power supply, 12 volt 25 Amp, you can find them new around $65.00 to $80.00 or even bellow when used. You will have to beef-up the capacitance at the output anyways. Most of them are designed for continuous current load.

What kind of drivers are you going to use?, 12 volt is a little low for steppers if you are going to have a fast step-rate you won't have much torque available.

[Niggo]

Thanks for your informations! My motor is unipolar 2A/phase 1.1 ohm, not so much but I need to turn a small cnc .

[kreutz]

1 ohm / 10 Watt is OK (will draw about 5 Amps) or you can use two 2 ohms 5 watt in parallel. You will want to attach it to the case or housing so the case acts as as heatsink, or to a suitable heatsink, it will get hot...

Sorry, I should have said 4.7 Ohm /10 watt will draw about 1 Ampere or you can use two 2.2 ohm 5 watt in series. I shoud have been sleeping writing when I wrote that.

[Niggo]

No problem, you gave the idea