Set me straight on the circuit math.

[mreff555]

I'm pretty new to circuits and I am trying to build sortof a bench supply.
I would like to have as many outputs and I can have while still giving
me the variability of power which I want.
I'm starting off with a supply I got out of a printer which has outputs of
31.5 VDCp and 31.5 VDCm, each at 1.4A

Using this as a starting point I believe all I need to do is regulate and clean up the power.
I would like to use multiple 317 IC's so I have a little freedom to make some adjustments
I know the 317 max's out at 1.5A, I shouldn't need more than that anyway.
I know I will need 5V at 1A. My idea was to use the p leg to get circuits in the range of 5v and the m leg to get circuits in the 12V range.

I'm a little rusty on how I would calculate the resistor sizes for something like that.
Also, what size electrolytic caps would you recommend? I was thinking 3300uf

Thanks

 

[Harald Kapp]

The connotation "P" and "M" is probably there to indicate that the "P" leg has +31V with respect to GND (0V) and the "M" leg has -31V with respect to GND. Therefore you cannot use a linear regulator to create +5V from the "M" leg.

Another consideration is that a linear regulator has a voltage drop from input to output that is the difference between the two voltages. In your case that's 31V-12V or 31V-5V, respectively. This voltage drop in conjunction with the current flowing creates a proportional power loss:
19V*1A = 19W or 26V*1A=26W. You will need a really big heat sink to dissipate all that energy and save the LM317 from a premature death from heat.

I suggest you use a switch mode regulator to overcome that hurdle. You can buy complete modules or kits that can regulate fixed or adjustable output voltages from a high input voltage. The advantage is that a switch mode regulator has a high efficiency (80% or more), A 12V*1A load (12W) will create a loss of only 2.4W (12W*20%) in the regulator. Therefore the total power drawn at the input of the switch mode regulator is 12W+2.4W=14.4W (compare this to 31W using a linear regulator!). You can power 2*12V*1A or 12V*1A+5V*1A from the same "P" leg of the printer's supply.

The advantage of the switch mode regulator is low loss and higher number of output voltages possible from your single supply.

The disadvantage of a switch mode regulator is the higher noise floor of the output voltage. You may be inclined to build a 2-stage design:
Stage 1 is a switch mode regulator down from 31V to Vout+3V to create the right" input voltage for
Stage 2: a linear regulator to finely control the output voltage with little noise.

How to calculate the required resistors is in the LM317's datasheet.

For the electrlolytic capacitors you can do some more or less sophisticated calculations, or follow a rule of thumb: 1000µF per 1A out output current. The operating voltage of the electrolytic capcitors should be at least 20% (or more) higher than the max. voltage actually present across the capacitor. This will increase reliability and lifetime of the capacitor.
You probably don't need that much capacitance if the printer's supply alreaady has built-in capacitors.

Note that it doesn't hurt to add some smaller capacitors (e.g. 100nF) type ceramic of film in parallel to the output capacitors to improve high frequency behavior.

 

[mreff555]

Hey thanks for the useful information