Gallery, Projects and General > How to's
How to Make a Power Supply
Darren:
Eric,
For your supply I would build this
It's simple, it's safe and it will be very reliable.
If you need more help just shout up
spuddevans:
--- Quote from: Darren on May 15, 2009, 12:11:50 PM ---Eric,
For your supply I would build this
It's simple, it's safe and it will be very reliable.
If you need more help just shout up
--- End quote ---
Just a little point that occured to me when looking at your circuit diagram above, I cant see how that will deliver 3amps if those 2 resistors are (as I read it) 1K each, surely you will only get a few 10's of milli-amps if that? Even if you had just 2x 1ohm resistors in series, at 3amp the voltage drop would be 6v giving an output well below 24V.
I assume ( very dangerous thing to do, it tends to make an ass out of me :lol: ) that this circuit is in responce to Erics post
--- Quote from: Brass_Machine on May 14, 2009, 02:24:58 PM ---Ok... so Darren... I need a PS that can do up to 42v and 3.0 Amps Maximum per Phase. What do I have to do different to pull this off? Looking at the pictures of other PS for this they have a BIG cap.
--- End quote ---
If I'm completely wrong, and lets face it, most of the time I am, please disregard my ramblings.
If you are wanting a PSU circuit for 24V @3amps, That circuit diagram is great, I would maybe just change the value's of the 2 1k resistors to 0.5 Ohm each and change the 1st capacitor to 6800uF and the 2nd to 10,000uF, that should give you a nicely smoothed DC output.
Just my thoughts,
Tim
Darren:
Tim you could be perfectly right, I copied this circuit off the net as I have never used resistors in a smoothing circuit.
I always use chokes in my supplies.
I guess what I should really do is make that supply above and see what really happens, I took it from a respected sited and just took it that is was a working schematic as portrayed.
I've been very busy today but when I get change I'll power up PSUD and pop those values in and see what it comes up with.
Tim, those caps seem rather large capacitance to me? I'm not doubting you, they just seem large. I remember once I built a PS with only 10uf and it was prob the smoothest I've ever seen. Your talking about a combined 16,800uf :bugeye:
Darren:
Silly me, I was looking at a low pass filter circuit not a smoothing circuit, modified it and never even noticed :doh:
Yep 1R resistors at 24V would indeed drop 3V each so a 6V drop.
You're spot on Tim... :thumbup:
18VAC rectified would be 18V x 1.41 = 25.38VDC - then 1.4V dropped by the rectifier = 23.98VDC
Then minus the volt drop over one 0.5 Ohm resistor at 3A = 1.5V, times that by two = 3V
so the result would be 23.98VDC at no load and 20.98V at a 3A load.
I doubt the circuit pulls 3A so the real voltage would be somewhere in between. Call it 50% so the result would be around 23V
Perfect.
Thanks for pointing out that huge error on my part Tim Total brain fade on my part there..... :doh:
Now then, those huge caps, could you explain this to me Tim cos I'm lost there? I would prob in reality have tried two 330uf caps cos I have hundreds of them.
( my power supplies are usually in the order of 300 - 1,500VDC at around 60 -200 ma for valve HT supplies with about 20uf max and two chokes around 10-20H each)
spuddevans:
--- Quote from: Darren on May 15, 2009, 07:10:05 PM ---Now then, those huge caps, could you explain this to me Tim cos I'm lost there? I would prob in reality have tried two 330uf caps cos I have hundreds of them.
( my power supplies are usually in the order of 300 - 1,500VDC at around 60 -200 ma for valve HT supplies with about 20uf max and two chokes around 10-20H each)
--- End quote ---
The reason for the bigger caps is because of the greater current draw, from what I read, Eric was wanting a PSU for a stepper motor driver, perhaps CNC. If his steppers are rated for 3amp and he has at least 2 of the ( x & y axis) there will be times when both are operating at the same time. Now even if you have a (say for arguments sake) 10A transformer, with a instant current demand there could be a slight delay (1/100th sec if using a bridge rectifier, 1/50th sec if just normal rectifier) in the supply of said current demand, this could lead to lost steps and innaccuracies in the CNC. Hence the need for such big caps, they just hold more current that is instantly available for use. In fact I would be tempted to put 2 10,000uF caps instead of the 2nd single cap in case of greater current demand in the future (ie a z axis and even a 4th stepper for a rotab), and I would get 63volt rated caps in case he wanted to up the transformer voltage to gain a higher top end speed. ( I know steppers rotate in fixed steps, but the speed that they can "Step" from one position to the next is limited by the voltage they are driven at)
The reason you will have used lower values of Cap on your valve amps is that they draw much less current, although at heart-stoppingly high voltages. While the wattage may work out similar for Eric's psu needs, when dealing with lower voltage and higher amperage the uf of the caps need to be much higher for them to work effectively as buffers/smoothers of the DC.
In low voltage, high amperage you can not have too much uF, the only thing you have to be careful of is to limit the inrush of current to charge the caps at switch-on, and that is achieved by the 2 0.5ohm resistors, if you dont have those you will find yourself going through a lot of fuses and in extreme cases possibly damaging the transformer. (but that would be in a very extreme case)
But take all that with a healthy dose of salt,(or beer :beer:) I am not qualified by any secular organisation, all this comes from just reading a lot, experimenting a lot, and also a miss-spent youth :zap: messing around building various electronic projects like power mosfet amps, psu's, my dad's LGB railway layout controllers and other stuff.
Hope this explains my thinking,
Tim
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