Author Topic: lm78xx series question and some more stuff  (Read 934 times)

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Offline KilroywashereTopic starter

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lm78xx series question and some more stuff
« on: January 23, 2020, 12:47:23 am »
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?



 
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Offline Vovk_Z

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Re: lm78xx series question and some more stuff
« Reply #1 on: January 23, 2020, 07:27:11 am »
I guess to bring the output to 0 v?)
- yes.
 
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Online MarkF

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Re: lm78xx series question and some more stuff
« Reply #2 on: January 23, 2020, 01:05:36 pm »
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.)

913312-0


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?

You only need a negative if you want to pull the minimum output voltage down to 0V.
« Last Edit: January 23, 2020, 01:10:13 pm by MarkF »
 
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Online MarkF

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Re: lm78xx series question and some more stuff
« Reply #3 on: January 23, 2020, 01:13:43 pm »
A LM317 circuit that goes down to 0V.

 
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