Need help designing a rectified circuit to AC-power my DC power supply

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slicey

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Joined 2005
#1
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
 
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#3
sofaspud said:
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?
Click to expand...

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
 
#4
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?
 
#6
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.
 
#7
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.
 
#8
sofaspud said:
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.
Click to expand...

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
 
#10
sofaspud said:
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?
Click to expand...

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.
 
#12
MPJA had a higher amperage 36V supply but they are out of stock. A decent surplus-type DC switcher could probably be had for less than what the necessary transformer alone would cost. What type of fans are we talking about here? 150W of power should move some air!
 
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#13
sofaspud said:
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.
Click to expand...

Then what happens if he adjusts the regulator output to 1.24V with the 150W transformer giving lets say 36V to the regulator input?
If input current limit (the transformer itself) is out of the question, i suggest output current limit...
 
#14
You're right, it'll need a much BIGGER heat sink if the load requires anything like 5V @ 5A. There will be >100W that will need to be dissipated. Do I hear another fan blowing?🙂
I'd like to know a little more about the load and its requirements.
 
#15
sofaspud said:
MPJA had a higher amperage 36V supply but they are out of stock. A decent surplus-type DC switcher could probably be had for less than what the necessary transformer alone would cost. What type of fans are we talking about here? 150W of power should move some air!
Click to expand...


They are 6.75" emb-papst fans(2 of them)

they operate in the 12-28v range and draw 1.1a @ 24v each

I agree with you about just buying a ready to go PS. My eyes bulged when I saw what transformers cost. I'm buying at this point, instead of blowing components trying to build the voltage supply side of my variable PS.

thx for everyone's help!
 
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