Author Topic: LM317 like  (Read 4692 times)

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Offline J4e8a16n

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    • Jean Pierre Daviau
LM317 like
« on: May 01, 2013, 08:23:31 pm »
Hi,

How can I get a 'kind of' variable LM317 voltage regulator without the 1.25 volts lost on the adjustement pin.

JP
Equipment Fluke, PSup..5-30V 3.4A, Owon SDS7102, Victor SGenerator,
Isn't this suppose to be a technical and exact science?
 

Offline IanB

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Re: LM317 like
« Reply #1 on: May 01, 2013, 08:28:48 pm »
Provide a -1.25 V supply rail.
I'm not an EE--what am I doing here?
 

Offline J4e8a16n

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Re: LM317 like
« Reply #2 on: May 01, 2013, 09:54:55 pm »
How do I do that?

Thanks from the newbe.
Equipment Fluke, PSup..5-30V 3.4A, Owon SDS7102, Victor SGenerator,
Isn't this suppose to be a technical and exact science?
 

Online mariush

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Re: LM317 like
« Reply #3 on: May 02, 2013, 12:25:31 am »
Use a transformer with a center tap :



or use something like MC34063 in inverting mode configuration to get something like -5 to -3.5v with some ripple, followed by a linear regulator like LM337 to get a smooth -1.25v

See http://www.ti.com/lit/ds/symlink/mc33063a.pdf , page 9. 
 

Offline J4e8a16n

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Re: LM317 like
« Reply #4 on: May 02, 2013, 01:20:54 am »
Thanks to you.

JP
Equipment Fluke, PSup..5-30V 3.4A, Owon SDS7102, Victor SGenerator,
Isn't this suppose to be a technical and exact science?
 

Offline peter.mitchell

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Re: LM317 like
« Reply #5 on: May 02, 2013, 02:23:46 pm »
Put a diode on the output to drop off the first however much. I mean, you're going to be pissing away energy as heat with the linear regulator, why not piss away a bit more with a diode.
 

Offline J4e8a16n

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Re: LM317 like
« Reply #6 on: May 03, 2013, 01:12:36 pm »
You put it between the output of lm317 and 10uf capacitor?

JP
Equipment Fluke, PSup..5-30V 3.4A, Owon SDS7102, Victor SGenerator,
Isn't this suppose to be a technical and exact science?
 

Offline Paul Price

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Re: LM317 like
« Reply #7 on: May 04, 2013, 03:19:44 am »
Create a negative supply with a LM555C timer. See full circuit below:
 

Offline Psi

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Re: LM317 like
« Reply #8 on: May 04, 2013, 03:22:47 am »
Or use a simple 5V or 3.3V DC/DC isolated module to convert your +V to -V.

The Adj pin current is tiny so any cheap low power one should work.
I'm not sure if switchmode noise would be an issue though.

Could probably even use a simple switched capacitor charge pump like a MAX871
They're only $2
« Last Edit: May 04, 2013, 03:33:50 am by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline c4757p

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Re: LM317 like
« Reply #9 on: May 04, 2013, 03:24:05 am »
Create a negative supply with a LM555C timer. See full circuit below:

If you've got a transformer just pick a negative rail off that...
No longer active here - try the IRC channel if you just can't be without me :)
 

Offline codeboy2k

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Re: LM317 like
« Reply #10 on: May 04, 2013, 04:23:35 am »
There's at least 3 threads on this specific topic currently active ...

 

Offline c4757p

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Re: LM317 like
« Reply #11 on: May 04, 2013, 04:42:22 am »
There's at least 3 threads on this specific topic currently active ...

Welcome to the EEVblog forum, enjoy your stay!  :)
No longer active here - try the IRC channel if you just can't be without me :)
 

Offline digsys

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Hello <tap> <tap> .. is this thing on?
 

Offline codeboy2k

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Re: LM317 like
« Reply #13 on: May 04, 2013, 07:39:20 am »
Yes
https://www.eevblog.com/forum/beginners/zero-volts-from-a-lm-317/msg227118/#msg227118


Yep. As digsys posted in the other thread, use a LM337L.  This is the way to do it.  The LM337L is  $0.50 to $0.80, it regulates well, and there is no additional ripple from any charge pumps.  The -1.25 rail is clean.  Any ripple on the LM317 adj pin will transfer to the output, so you want it as clean as can be.

 

Offline Psi

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Re: LM317 like
« Reply #14 on: May 04, 2013, 09:09:43 am »
Assuming you already have a negative supply then yes, a 337 is the better way of doing it.
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline digsys

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Re: LM317 like
« Reply #15 on: May 04, 2013, 09:40:14 am »
The circuit shows a simple -ve generator. Using a DC-DC generator is fine as long as the Filter cap voltage is low enough,
otherwise it gets messy.
Hello <tap> <tap> .. is this thing on?
 


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