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Current Boost LM317 Adj. Power Supply

243 ratings | 27572 views
http://www.bristolwatch.com/ccs/LM317boost.htm Use an LM317 adjustable voltage regulator at a higher current.
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Text Comments (60)
Mrityunjay Sharma (1 day ago)
Can I convert 24V to 5V, with 4A output with a good heatsink? Or Will it get too hot to use? Or maybe it's a bad idea bc it's wasting some 90W, right?
Mrityunjay Sharma (12 hours ago)
+Lewis Loflin I'd be glad if you can tell me about the name of 48v to 12v, output at 36Watts buck converter
Lewis Loflin (1 day ago)
I would step the voltage down with a fixed switching regulator circuit then use this. A level of 7-8 volts will do fine.
Karapet Botzikian (9 days ago)
Can I put somewhere a potentiometer to adjust the output current. If it's possible can you tell where i should connect it and what kind? thank you for this upload, you help me a lot to understand.
Lewis Loflin (9 days ago)
Not without a major redesign of the circuit. Sounds like you want a current limiter.
Belete Shebabaw (10 days ago)
i like the explanation you use but what is the purpose of the transistor if we get 1A out fro 1A in.
Belete Shebabaw (8 days ago)
Lewis Loflin (10 days ago)
If you use more than an amp the excess current is carried by the larger transistor.
Capi Berra (23 days ago)
I don't have any silicon pnp power transistors at hand but I do have a few npn ones. Could I mirror your transistor / Rb circuit on the output side of the lm317 with a pnp transistor? Thanks.
Lewis Loflin (23 days ago)
No you can't if you want a positive output.
Mohamed Magdy (23 days ago)
great video
Learn BOX (2 months ago)
Lewis I made this circuit but 10 ohms resistor was burned. Why?
Learn BOX (2 months ago)
+Lewis Loflin Can I connect second PNP 2955 in parallel with 1st one. Emitter to emitter, base to base, collector to collector.
Learn BOX (2 months ago)
+Lewis Loflin Thank you Sir lewis, it is working and deliver 3.28Amps I attached large heatsink but now Transistor 2955 is still heat up but less than before.
Learn BOX (2 months ago)
+Lewis Loflin Can I charge car battery of 12V at 80AH.
Learn BOX (2 months ago)
+Lewis Loflin It works well but I think there is a need of base resistor?
Learn BOX (2 months ago)
+Lewis Loflin 7812 delivery fix 12V but I need 13.8v.
Popeye (3 months ago)
LM317 adjustable voltage regulators are not efficient and they can get very hot. A variable Step Down DC to DC Buck Converter are very efficient with little heat dissipation. Only draw back is they cause interference to radio recievers
Lewis Loflin (3 months ago)
Yes I mentioned that in other videos. This is about circuit theory.
Rizwan Hashmi (4 months ago)
i can use 19v 3amp
Lewis Loflin (4 months ago)
Nice keep on
Robert s (6 months ago)
Can we use bd249c?
Lewis Loflin (6 months ago)
No that's an NPN transistor.
ibles bosuok (6 months ago)
I prefer 6.8 ohms for Rb, so the current threshold would be about 100mA. Choose 390 ohms for feedback resistor, the maximum output voltage would be about 15V which convenient for many applications. You can supply with 18VAC secondary voltage. Greetings from Indonesia
Hamad Zeb (8 months ago)
can we use mje 2955
Lewis Loflin (8 months ago)
Yes it will work. If drawing a lot of current heat sink the device.
Learnings... (9 months ago)
Can you post a circuit with 2N 3055 transistors please...
Lewis Loflin (4 months ago)
I'm doing a circuit on this and will post it soon.
Learnings... (9 months ago)
can you provide me the ckt where we can use 2n3055 pleaseee
Lewis Loflin (9 months ago)
You can't use a 2N3055 in this circuit. The circuit would be a total redo.
Learnings... (9 months ago)
can I use 2n3055 instead of MJ2955
ibles bosuok (6 months ago)
Learnings... , you can use 2N3055 with an LM337 Greetings from Indonesia
Joey Mac (10 months ago)
That was a good practical video, thanks. Once the current is such that the resistor develops 0.7 volts or so it turns on the transistor and now you can supply more voltage. I like these analog lessons.
Lewis Loflin (10 months ago)
The BE voltage stays around 0.7V once turned on because addition current flows from BC.
Joey Mac (10 months ago)
Doesn't the transistor turn on when the voltage drop across Rb is about 0.7 volts, enough the forward bias the emitter resistor? I thought that was what Rb was used for.
Lewis Loflin (10 months ago)
This about current not voltage. The voltage drop depends on the load resistance.
njfulwider5 (10 months ago)
Great Video, Very Detailed. I am just messing around with a few boost, and buck converters I have purchased overtime. I have seen a few video's, explaining how to bypass the LM317/LM388 for higher current than what the chip allows. Couldn't you also bypass the limitation of the converter like the LM317 circuit in your video. Looks like you could do the same thing without fail. What do you think. For Example, I would be using a Buck converter with XL4005.
Michael Schwager (11 months ago)
Cool little circuit... Thanks!
Axel Werner (11 months ago)
where is the current limiting resistor for the base? if the lm pulls enough current base current will overdrive and might destroy the tran.
dand8282 (10 months ago)
the LM is a magic auto-adjusting resistor, forming a voltage divider with Rb that always balances to 0.7v, given a reasonable input voltage and load resistance.
Lewis Loflin (11 months ago)
The LM317 doesn't pull current through the base the voltage drop across R does. That type transistor can take a lot of current.
Axel Werner (11 months ago)
How can you have 19.7 volts on a node, when there are only 19.0v input.
Lewis Loflin (11 months ago)
It's an illustration. the input was 19.7. Thanks.
Kerry Wong (11 months ago)
Great explanation! Should point out that one issue of using a PNP transistor to boost the output of a three terminal regulator is that there is no short circuit protection and in the event when the bypass transistor fails the input voltage is directly dumped onto the load.
Ton Bovee (9 months ago)
Yest you are right . For information , I have tried both solutions, the LM317 has temp and internal current protection of 1.5A. I just got 10pcs($1) from TK Electronics 99.5% on Ebay , tested those (short output at 39V input)and believe it or not, not one failed! I use a lm431 to drive 6pcs lm317 resulting in a very very stable output between 0 and 9A
Lewis Loflin (11 months ago)
Good point I'll do a vid on circuit protection which I did in other vids. The same problem exists with the LM317 alone. Thanks.

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