LM317 like - Page 1
Forum section: electronics / general

author: J4e8a16n
author_rank: Regular Contributor
timestamp: May 01, 2013, 08:23:31 pm
content: Hi,How can I get a 'kind of' variable LM317 voltage regulator without the 1.25 volts lost on the adjustement pin.JP

author: IanB
author_rank: Super Contributor
timestamp: May 01, 2013, 08:28:48 pm
content: Provide a -1.25 V supply rail.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: May 01, 2013, 09:54:55 pm
content: How do I do that?Thanks from the newbe.

author: mariush
author_rank: Super Contributor
timestamp: May 02, 2013, 12:25:31 am
content: 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 -1.25vSeehttp://www.ti.com/lit/ds/symlink/mc33063a.pdf, page 9.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: May 02, 2013, 01:20:54 am
content: Thanks to you.JP

author: peter.mitchell
author_rank: Super Contributor
timestamp: May 02, 2013, 02:23:46 pm
content: 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.

author: J4e8a16n
author_rank: Regular Contributor
timestamp: May 03, 2013, 01:12:36 pm
content: You put it between the output of lm317 and 10uf capacitor?JP

author: Paul Price
author_rank: Super Contributor
timestamp: May 04, 2013, 03:19:44 am
content: Create a negative supply with a LM555C timer. See full circuit below:

author: Psi
author_rank: Super Contributor
timestamp: May 04, 2013, 03:22:47 am
content: 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 MAX871They're only $2

author: c4757p
author_rank: Super Contributor
timestamp: May 04, 2013, 03:24:05 am
content: Quote from: Paul Price on May 04, 2013, 03:19:44 amIf you've got a transformer just pick a negative rail off that...

author: codeboy2k
author_rank: Super Contributor
timestamp: May 04, 2013, 04:23:35 am
content: There's at least 3 threads on this specific topic currently active ...

author: c4757p
author_rank: Super Contributor
timestamp: May 04, 2013, 04:42:22 am
content: Quote from: codeboy2k on May 04, 2013, 04:23:35 amWelcome to the EEVblog forum, enjoy your stay!

author: digsys
author_rank: Unknown
timestamp: May 04, 2013, 06:52:00 am
content: Yeshttps://www.eevblog.com/forum/beginners/zero-volts-from-a-lm-317/msg227118/#msg227118

author: codeboy2k
author_rank: Super Contributor
timestamp: May 04, 2013, 07:39:20 am
content: Quote from: digsys on May 04, 2013, 06:52:00 amYep. 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.

author: Psi
author_rank: Super Contributor
timestamp: May 04, 2013, 09:09:43 am
content: Assuming you already have a negative supply then yes, a 337 is the better way of doing it.

author: digsys
author_rank: Unknown
timestamp: May 04, 2013, 09:40:14 am
content: 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.