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lm78xx series question and some more stuff
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Kilroywashere:
Hello world,

I'm building a lab power supply and I only have 1 Lm317, I want to have a current limit of up to 1 amp . I know that you can use a 78xx series to provide a constant output current, I was wondering can I just use a 7805 as my current regulator or will 1 amp be too much for this thing to handle ? (get super hot and such) I wanted to save the Lm317 for the voltage adjustment, or is it better to use it for current if you only have one?


Also some designs I look at have two 1n4001 diodes and a transistor leading into -10volts  (I guess to bring the output to 0 v?) is it better to have  some sort of duel  + - power rails powering the thing?



 
Vovk_Z:

--- Quote from: Kilroywashere on January 23, 2020, 12:47:23 am ---I guess to bring the output to 0 v?)

--- End quote ---
- yes.
MarkF:
Do you want to just limit the current to 1A or do you want an adjustable current limit?

If you want an adjustable current and are okay with a minimum of 1.2V of the LM317,
you can do something like this:




If you just want a fixed 1A current limit,
take a look at this battery charger circuit:
The sense resistor value can set a fixed current limit with the minimum LM317 voltage output.
You will probably want a full-wave bridge and a large cap for the input.  But you get the idea.
(You can use any NPN transistor, 2N2222, 2N3904, etc.  But will need to select a different sense resistor value.)





--- Quote from: Kilroywashere on January 23, 2020, 12:47:23 am ---Also some designs I look at have two 1n4001 diodes and a transistor leading into -10volts  (I guess to bring the output to 0 v?) is it better to have  some sort of duel  + - power rails powering the thing?

--- End quote ---

You only need a negative if you want to pull the minimum output voltage down to 0V.
MarkF:
A LM317 circuit that goes down to 0V.

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