Current Limiting on Power Supply - Page 1
Forum section: electronics / projects

author: Anson
author_rank: Regular Contributor
timestamp: March 18, 2013, 03:56:31 pm
content: How important is current limiting on a power supply? I am in the process of building myself a bench top power supply. I have so far a LM317T and an LM337T a 48v transformer model 187D48 Hammond 48VCT @630ma 30VA. Which bring up another question how many actual amps will I get with this thing say when running at 12 volts? Or is it limited to only 630ma? I also have a rather large bridge rectifier a G3SBA20. And I have caps and resistors and a pair of 5k pots for adjusting from 1.25v up to wherever it goes. Haven't bread boarded what i have yet not sure on what size caps and resistors to use yet I think for the resistors I will try 120 ohm with the 5k pots. I have a pair of 50v 2200uf caps that should work good for the first set after the bridge. But how important is current limiting and what would be the simplest way on a simple dual polarity linear supply? I have seen so many examples from op amps to transistors from before the regulators to after. I just don't know which route is easier or better or if I even need it. Also if I do use it do I need it for both the positive and negative and if so should they be set up separate or together?

author: mariush
author_rank: Super Contributor
timestamp: March 18, 2013, 07:00:58 pm
content: It depends on what you'll use the power supply for. It can be useful.Your transformer has 48v AC on secondary, with a center tap.If you ignore the centertap, when you rectify the voltage to DC, you have about 68v DC (about 66v if you consider the 2 diodes that are each conducting at a time) at probably about 0.45A - you can't get more than 48V x 630mA = ~30VAIf you use linear regulators, you can't get more current than what the transformer can do, if you'll want to get 12v on the output, you'll still only be able to max out at around 0.45-0.5A, maybe less.Be careful because LM317 and LM337 may not be able to handle more than 50v, it may depend on who makes it. Your 48v rectified to DC is be more than that.A split power supply is done quite easily. Since your transformer has a center tap, you can take advantage of that to make it the ground of your power supply:You'll have about 35-40v on each side, on each capacitor, so at least capacitors rated for 50v are required. You then mirror the power supply on the positive and negative side, so you'll need a pair of each LM*** with the adjustment pots and so on.2200uF on each side is probably enough considering the maximum current (since each side has half the transformer, it will be able to do a maximum of 250-300mA)More capacity only helps to prevent the rectified DC voltage from dropping too low - your LM* chips need about 2v over the voltage you want at output, so the rectified DC voltage has to stay above that voltage.In your case, with center tap you have 2 x 24v AC, rectified each side will be 24x1.41 = about 34v , minus the losses in the rectifier diodes of about 1-1.5v ... that's about 32v.Depending on the current you pull, with a 2200uF the voltage may hover after it between 28 and 32v, so your LM* regulators would only be able to output a maximum of about 26-28v or so.You can play with this online circuit simulator :http://www.falstad.com/circuit/e-fullrectf.html(java required)Adjust the capacitor capacitance, change the AC source to 24v ac, change the load to something that would use more current v = i x r , you want to aim for about 0.5A so r = 32v / 0.5 = ~60 ohm

author: smashedProton
author_rank: Frequent Contributor
timestamp: March 20, 2013, 05:23:33 am
content: you could get away with a simple polyfuse. it may fry the ass out of your circuit before the fuse has time to go open though XD

author: Anson
author_rank: Regular Contributor
timestamp: March 20, 2013, 11:01:46 pm
content: I got a few 5k pots out of an old tv and was able to set up a bread boarded test of my supply circuit it works really well. I actually got 36.02 volts dc on each side so that is perfect for me i doubt I will ever need more than that. I don't know if I have anything to put a good load on it with though to see how many amps it can output. I don't see myself really needing current limiting on it I guess. That seems to be more for diagnostic work which I won't be doing much of. I just need this for powering my experiments. I wanted the dual polarity because it seems most synth machines and other more complicated circuits use negative voltages so I figure it would come in handy and I found the transformer at a yard sale for a dollar anyway seen the center tap and said sold. It was brand new in a box even. strangely that was the only thing electronic the man had other than a box full of old wall warts.