# DC-DC conversion

Discussion in 'General Electronics Chat' started by RogueRose, Dec 27, 2015.

1. ### RogueRose Thread Starter Member

Oct 10, 2014
198
4
I would like to make a DC to DC converter that will output something around 12.5-14.4v (think car battery output) and roughly 5-5.2v (USB voltage). Input voltage will range from about 18 - 24v (maybe up to 36). I doubt that I will need more than 10A @ 12v and 5A @ 5.2v. I was thinking that I might be able to parallel 2 LM338's for the 12v rail and a single 338 for the 5v rail. If I remember correctly all I need to do to get the correct output voltage is place the proper resistance/resistor on one of the terminals of the LM338 (appropriate for each voltage needed). I guess I will also have to have some diodes in place to make sure there is no feedback from the 12v to the 5v rail.

The problem is with the varying input voltage. Is there a way to do this so the unit can be used with a
switch to select voltage (or better sense voltage and select proper path)? It seems the most expensive parts
of this design will be the LM338's so if there is a way to use the same ones on for each voltage path that would
be ideal.

2. ### #12 Expert

Nov 30, 2010
16,705
7,358
I think a switching (buck) regulator is the way to handle the wild input voltage without needing a fan to keep it cool. Somebody bought Texas Instruments, but the calculator page is still available for a DIY project. And, no, one LM338 can not do both jobs at the same time.

Summary:
18V to 36V becomes 12.5V @ 10 amps and a second output which is 5.2V @ 5 amps.
Do you want somebody to design the whole thing for you and post the schematic?

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3. ### RogueRose Thread Starter Member

Oct 10, 2014
198
4
Thanks for the reply. What I meant by using the same 338 for each voltage, meaning using 2 for the 12.5v and 1 for the 5v for 3 total. Now as the inputs vary, doesn't that mean that the resistors are going to vary for each each input voltage, ex - for 18v a 120 ohm resistor needed, 160 ohm for 24v and 240 ohm for 36v - to get the proper output voltage from the 338 (those R values are made up so I know they aren't correct). So I was asking if I could use a switch to select the proper R value in this setup.

Thanks for the offer to design, I'm looking over some schematics ATM and am not totally sure what all I want this to do.

4. ### sailorjoe Member

Jun 4, 2013
361
63
RogueRose, you may not know that the whole point of a voltage regulator is to take an input voltage that varies widely and stabilize it so it remains constant. Most people think its just stabilizing the output voltage with varying load, and it does that too. So don't worry about the variable input voltage, as long as you stay inside the maximum ratings for the regulator chip.

If you look at the data sheet for the LM338, you'll see that the voltage adjusting resistors are on the output side of the regulator, so aren't affected by the input voltage. Cool, huh!

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5. ### #12 Expert

Nov 30, 2010
16,705
7,358
The resistors which set the output voltage are regulated by the LM338. You will use 120 ohms from the output to the adjust pin, and another resistor from the adjust pin to ground to set the final voltage of the output. The input voltage has nothing to do with this because the LM338 regulates the adjustment resistors.

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6. ### dl324 Distinguished Member

Mar 30, 2015
3,391
653
At the currents and voltage differentials you're talking about, you need to use a switching regulator; as #12 mentioned in the first sentence of the first response.

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7. ### ian field Distinguished Member

Oct 27, 2012
4,460
792
An easy way to do the 5V USB voltage is a discount store car adaptor - shop around for one with 2A output.

If you're lucky, it might just happen to contain an MC34063 - there's loads of examples online of how to wire an external boost transistor and beef up the inductor.

So far - I've not found a chip type that I couldn't get at least the datasheet, but few are as well resourced as the 34063.

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