Power supply question

klem kedidelhopper · 2013-02-18T17:49:18+00:00

I'm in the process of building a dual purpose power supply. The supply will have two outputs. One will be 24VAC rated at 4.0 A. The load on this supply will probably never exceed 2.0 A. The second supply will consist of a bridge rectifier off the 24V tap connected to a suitable filter cap of say 1000UF. This filtered DC will then be connected to a small surplus 12V regulator board and heat sink assembly which by the looks of it can handle 5A or better. I plan on mounting this regulator in the cabinet and providing a terminal strip for the 12V output. The DC load will probably never exceed .500 - .750 A. Now here is the dilemma. I haven't tried to put all this together yet, but I know that once I rectify and filter the 24VAC I'll probably end up with something like 30VDC out. The regulator uses a ua723, a TO3 and a smaller TO transistor as well a whole bunch of discrete components. I have no specs on this regulator, but it is well built, appears to be commercial grade, and although it might handle it fine I'd feel a lot better hitting it with something like 18VDC instead of 30. I would hate to blow it up trying to see if it would work on the higher input. I could add some series resistance either before or after the bridge, but the voltage drop across this resistance would vary depending on the total load and I'm not sure how well such a scheme would work. So I was thinking about employing a voltage divider at the output of the filter. The resistance ratio would be easy to figure out, however I'm just not sure how to determine the optimum resistor values. Does this seem like a viable plan, or perhaps someone my have other thoughts as to how to address this? If someone could please help me with this I would be very grateful. Thanks, Lenny

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John-Del 13 years ago

Snort.... No one is more stubborn than Phallus'n. No one. Phil stubbornl y hangs on to an argument when he's been long proven wrong. His stubbornne ss is so well known in fact that he was given a guest cameo on a sitcom. H e's the one on the right. Enjoy:

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John Robertson 13 years ago

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t explain this properly. I have a 24VCT transformer. The transformer need s to power 24VAC equipment as well as 12VDC equipment. For the DC circuit I first tried using a bridge directly off the 24V winding. As soon as I connected up the filter cap the DC output went to 35V, which is probably too high to feed my 12V regulator.

I then realized that I had a center tap that was not being used. So I co nnected my bridge across the center tap and one side of the secondary. Th is time the 13VAC when FW rectified using the bridge went to about 19VDC, which is a safe input to the regulator. Although this worked, I wasn? t happy about unbalancing the transformer this way so I posted my results .

e off the full secondary output with two diodes, cathodes tied together, (typical FW rectifier), and use the center tap as my negative return. I d idn't think that would make any difference in the output voltage from usi ng a bridge without the center tap however I tried it anyway. As I suspec ted it would, the DC output again went to 35V when I connected the filter up.

e is catching up with me. I guess I just don't understand the explanation of how to get a lower voltage to my DC regulator by what I thought sound ed like just configuring the transformer properly. So at the risk of soun ding like a complete idiot here can someone please explain this further? Thanks, Lenny

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Until one of your zener diodes short out (a zener will eventually short out due to inrush current issues is my guess) and you then punch around

19V to the circuit that wants 12....not a good design as a result, more of a hack.

John :-#(#

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Gareth Magennis 13 years ago

Until one of your zener diodes short out (a zener will eventually short out due to inrush current issues is my guess) and you then punch around

19V to the circuit that wants 12....not a good design as a result, more of a hack.

John :-#(#

Well then you want another protection Zener shunted on the 12v line.

Gareth.

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josephkk 13 years ago

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Two questions:

Do you have LTSpice?

Can you read alt.binaries.schematics.electronic?

If the answer in no in both cases, get LTSpice.

The .asc files are very handy for this kind of discussion.

?-)

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Power supply question

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