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Old 22nd February 2013, 04:27 AM   #1
slicey is offline slicey  United States
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Default Need help designing a rectified circuit to AC-power my DC power supply

Hello, I've always powered my variable power supply builds with either a fixed, 24v, 6 amp, power supply or a 12v lead on a PC PSU.

Now, I need to power one of these circuits, employing a single LM338K, using 120v AC power. I have struck out trying to learn how to do this via a Google search.

I don't know if all I need is something simple, like this rectifier: 25 A 200 PIV BRIDGE RECTIFIER | AllElectronics.com

or if I need to include a transformer, and perhaps some other components.

Basically, I plan to change out resister 1(R1) on my existing variable power supply to be able to output the 32v max the LM338K is capable of producing.

I need to find out how to power this circuit with 120V AC.

any help is much appreciated

Last edited by slicey; 22nd February 2013 at 04:31 AM.
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Old 22nd February 2013, 06:03 AM   #2
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Quote:
Now, I need to power one of these circuits, employing a single LM338K, using 120v AC power
I'm not quite understanding what you need. "One of these circuits"?? You want to build a 120VAC to +1.25-32V @ 5A variable DC supply?
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Old 22nd February 2013, 07:37 AM   #3
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Originally Posted by sofaspud View Post
I'm not quite understanding what you need. "One of these circuits"?? You want to build a 120VAC to +1.25-32V @ 5A variable DC supply?
I've already built the power supply years ago, but it was always powered by a plug-in(120vac} PS that outputted a fixed 24vdc.

Now, I want to eliminate the 24v PS, and add a circuit to my +1.25-32V @ 5A variable DC supply that takes in 120vac, then outputs >32vdc to power my variable power supply.

I need a schematic and parts list to build the AC side of my variable power supply.

thx for reading
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Old 22nd February 2013, 08:19 AM   #4
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That's going to be a pretty big chunk of transformer for a linear supply. All Electronics doesn't have anything that big, but you could parallel a couple of their #TX-245 units. That would provide plenty of current, but you'd have to settle for a max voltage of ~30VDC from it.
Have you considered using something like this 36 Volt 4.2A Power Supply?
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Old 22nd February 2013, 08:47 AM   #5
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His regulator is LM338.
It is 50W.
He therefore needs max.50W transformer...
Max input voltage 33V.
Otherwise he will blow his regulator.
Wont he?
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Old 22nd February 2013, 09:01 AM   #6
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If the maximum output is 30VDC @ 5A, that's 150 watts. The transformer secondary must meet that rating or more (to account for losses). The power dissipated by the regulator is the dropout voltage across the regulator times the current through the regulator, eg with a 3V drop at 5A the dissipation is 15W (it will need a decent-sized heatsink!). The LM338 has a floating ground so it is only the input/output differential that is a concern. For the LM338 the max differential is 40V; that shouldn't be a problem for this application.
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Old 22nd February 2013, 09:08 AM   #7
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Hi Slicey, whatever you do, do not rectify the mains directly! That is a very dangerous thing to do. You need a transformer.

I'm assuming your variable supply will need at least 3V headroom to be able to deliver 32V so you need to target around 35V after rectification.

5A at 35V is roughly 175VA so I would say you really need to go for 200VA or higher. So you are probably looking at a 200VA 25-26V transformer. a bridge rectifier like the one you linked to. Some biggish capacitors I'd say at least a couple of 4700uF in parallel and that should do you. The transformer is going to be the expensive bit!

Tony.
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Old 22nd February 2013, 09:39 AM   #8
slicey is offline slicey  United States
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Quote:
Originally Posted by sofaspud View Post
If the maximum output is 30VDC @ 5A, that's 150 watts. The transformer secondary must meet that rating or more (to account for losses). The power dissipated by the regulator is the dropout voltage across the regulator times the current through the regulator, eg with a 3V drop at 5A the dissipation is 15W (it will need a decent-sized heatsink!). The LM338 has a floating ground so it is only the input/output differential that is a concern. For the LM338 the max differential is 40V; that shouldn't be a problem for this application.
If you mean a heatsink for the LM338, I got that covered. It's about 4x4x1 inch tall.

Trying to power this thing with AC is harder than I thoughtn that's why I was hoping for a parts list and schematic for the AC part of my power supply

My neighbor is an EEn but he's out of town, and I have no idea when he's coming back. Just my luck
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Old 22nd February 2013, 09:45 AM   #9
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Hi Slicey, please read this it may seem a lot but it should cover everything you need to know with respect to converting your 110V AC to ~ 35V DC to feed into your variable supply. Elliott Sound Products - Linear Power Supply Design

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Old 22nd February 2013, 09:46 AM   #10
slicey is offline slicey  United States
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Quote:
Originally Posted by sofaspud View Post
That's going to be a pretty big chunk of transformer for a linear supply. All Electronics doesn't have anything that big, but you could parallel a couple of their #TX-245 units. That would provide plenty of current, but you'd have to settle for a max voltage of ~30VDC from it.
Have you considered using something like this 36 Volt 4.2A Power Supply?
I may just go with that unit in your link. It looks just like the PS I have now but with a higher output voltage(like I need) yet with lower current capacity by about 1.8A. I think it might work. I going to measure the max current draw of the 2 fans this whole contraption is going to be powering.
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