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Registered Member #2838
Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333
Hello, I am getting ready to start a 0-30/32v regulated PSU using a single LM338T. I have been browsing/searching for a good circuit with this chip. It seems that most of the schematics that I have found use 24-25v input. I was hoping to use somewhere around a 34-36v input. Does anyone have a good schematic with 30-32v output? As of now I am leaning toward the one on the data-sheet.
Also, most of the circuits that I have found use a 4700-10,000uf cap for a filter. I have a vary large Mallory 8000mfd 40vdc (max surge 45vdc) aluminum electrolytic capacitor that I would like to use. It says "computer grade". Any suggestions would be greatly appreciated!
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
Use a circuit like Figure 3. on the datasheet. The part can tolerate 40v differential so do not exceed 40v input absolute max.
The to-220 version will not survive 5A at low output voltages. Multiple transformer taps are a Very good idea in a power supply like the on you intend to build. Your power dissipation will be (input (v) - output (v)) * output (A). For example, at 30v in, 5v out, 5A out, you dissipate 25 * 5 = 125W. 125W is a lot and will require a large CPU style heat sink and fan.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
If you haven't already bought the part, I'd suggest using the LM338K version, which has a TO-3 case making for better thermal contact with the big heat sink needed to turn all that wasted energy into hot air.
You'll be better off using the capacitor types and values from the manufacturer's data sheet application examples, than from sources of unknown reliability on the web.
Registered Member #2838
Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333
Thanks to everyone who has posted feedback on this issue. It seems that I am going to have to rethink this psu. I was trying to make a cheap supply based upon parts that I already have lying around. Since heat is going to be an issue perhaps I will take a different approach. As I see it I have two options with the supplies that I have available. Both of these options will reduce heat output from the 338.
First: I have 5 of the LM338T's and a center tapped MOT at 12/24/36/48v. Maybe I will try using 4 338's instead of one. I was thinking one at 0-12v with a 12v input, one at 12-24v with a 24v input, and one at 24-32 with a 36v input. I would assume that this would reduce excessive heat? I know that this will use a lot of excess parts but as I said "i am trying to use stuff that I already have lying around". I like this idea for a couple of reasons but it is a lot of extra work. Second: I was thinking maybe just putting a three way switch on the input voltage to the chip and switching from the output of the MOT. This would allow me to use one circuit by switching input voltage at 12/24/36v. The only problem that I have with this is that I need to find a switch that will hold up to 5-10A. Maybe I will check the local auto parts store...
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Oidium45 wrote ...
I have 5 of the LM338T's and a center tapped MOT at 12/24/36/48v.
I didn't know MOTs had all these windings on them - I assumed that there was just the low voltage magnetron heater winding, and the high voltage winding for the magnetron anode supply.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
was thinking maybe just putting a three way switch on the input voltage to the chip and switching from the output of the MOT.
Some supplies switch secondaries prior to regulator by using four windings, each 12volt, connected, parallel, then series parallel, then series. This way you can design the wattage to utilize the full transformer efficiency on the 3 ranges. You may need 3 times the amperage at 12 volts than you do at 48. The range switch is ganged to the regulator setup resistors.
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LM338 PSU 0-30v 5a (hunting for good circuit)
