Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 7th September 2006, 07:56 PM   #1
Roushon is offline Roushon  India
diyAudio Member
 
Join Date: Sep 2004
Location: Mumbai (Bombay)
Default LM338 regulator InputMax voltage..

I just purchased toroidal trnasformer for a LM338 regulated power supply for LM3886. I ordered 2 x 30v sec. (300VA). But the transformer giving 2 x 32v. So after the bridge rectifier I will get around 45v approx. Is this supply safe for LM338? The datasheet of the regulator says max input voltage is 40v.
I want 35volt output from the regulator.

Thanks in advance for any suggestion...
__________________
S. Roushon
  Reply With Quote
Old 8th September 2006, 10:44 AM   #2
Roushon is offline Roushon  India
diyAudio Member
 
Join Date: Sep 2004
Location: Mumbai (Bombay)
i got my answer. sorry for starting a new thread. just read, several people are using with even 60v input for LM338. so i guess, i can too with 45v.
__________________
S. Roushon
  Reply With Quote
Old 8th September 2006, 09:56 PM   #3
Tweeker is offline Tweeker  United States
diyAudio Member
 
Join Date: Dec 2004
Location: Cincinnati, Ohio
Vin-Vout <35V.

The regulator can be used at hundreds of volts provided its protected and doesnt see greater than 35V across it.
__________________
Be sure your foil hat has a good low impedance ground.
  Reply With Quote
Old 9th September 2006, 05:18 AM   #4
Roushon is offline Roushon  India
diyAudio Member
 
Join Date: Sep 2004
Location: Mumbai (Bombay)
Default thanks

thanks Tweeker. i read about it in the datasheet, but never saw an example with large input voltage till i searched the net like mad.

i have another concern. the regulator can supply around 5 amp current. is this independent of the input voltage as along as Vin-Vout<35v?
__________________
S. Roushon
  Reply With Quote
Old 12th September 2006, 12:12 AM   #5
diyAudio Member
 
FastEddy's Avatar
 
Join Date: Jun 2006
Location: North Californie
Default ... and another thing ...

Just because the spec sheet says "up to 35 DVC" does not mean success in all cases. The Absolute Maximum is just that = absolute. If there is any ripple or surges coming from upstream then = no joy

http://www.national.com/ds/LM/LM138.pdf ...

Here is a neat trick however ... you can use a daisy chain of diodes to "knock down" the incoming voltage = each diode = 0.6 VDC throw away = 10 diodes = 6 VDC down.

You can also do the same daisy chain on the ground leg 'tween the adjustable resistor and ground ...

  Reply With Quote
Old 12th September 2006, 12:14 AM   #6
diyAudio Moderator
 
pinkmouse's Avatar
 
Join Date: Apr 2002
Location: Chatham, England
Default Re: thanks

Quote:
Originally posted by Roushon
The regulator can supply around 5 amp current. is this independent of the input voltage as along as Vin-Vout<35v?
You need to check the load/SOA lines in the datasheet for your application.
__________________
Al
I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort
  Reply With Quote
Old 12th September 2006, 05:44 AM   #7
Roushon is offline Roushon  India
diyAudio Member
 
Join Date: Sep 2004
Location: Mumbai (Bombay)
Default thanks

thanks for the responses. using series of diodes to drop the voltage is really a nice idea. to extend this idea, i think it will be even better to use more diodes in series in the rectifier stage itself. although it will be more expensive and cumbersome.
__________________
S. Roushon
  Reply With Quote
Old 12th September 2006, 03:29 PM   #8
diyAudio Member
 
FastEddy's Avatar
 
Join Date: Jun 2006
Location: North Californie
Default That won't do it !!

Roushon: " ... using series of diodes to drop the voltage is really a nice idea. i think it will be even better to use more diodes in series in the rectifier stage itself. ..."

Warning: the use of diodes in a daisy chain (stacked / chained in series) from a pure AC source (like right outta the transformer to the rectifier diode bridge) is NOT advised. This is how higher voltages can be generated, not lower voltages !!! (the AC peaks get "pumped up" in the diode series ... This daisy chain of diodes is sometimes used in old radio circuits to increase peak voltage (!!!!) = a bad idea for AC circuits, but fine for well filtered DC only.

Try something else !

(The use of a daisy chain of diodes to "drop" DC voltage is an old solar power supply trick as published in Popular Electronics circa 1989 ... I know 'cause I wrote it up)

  Reply With Quote
Old 12th September 2006, 05:38 PM   #9
diyAudio Member
 
Join Date: May 2006
>This is how higher voltages can be generated, not lower
>voltages !!! (the AC peaks get "pumped up" in the diode series

There's no "pumping" to a higher voltage unless you specifically connect up diodes and capacitors in a multiplier (cascaDe) circuit, like those used in CRT HV supplies.

Diode drops are fine when you need a small voltage drop. In Roushon's case, an extra transistor is much safer. See http://www.national.com/an/LB/LB-47.pdf for a circuit.
__________________
--WD
  Reply With Quote
Old 12th September 2006, 06:11 PM   #10
diyAudio Member
 
FastEddy's Avatar
 
Join Date: Jun 2006
Location: North Californie
Default Pumped?

" ... There's no "pumping" to a higher voltage unless you specifically connect up diodes and capacitors in a multiplier (cascaDe) circuit, like those used in CRT HV supplies. ..."

If poor wiring practices are used, the stray capacitance could conceivably cause the voltage to pump up, possibly much more than the 0.6 volts "lost" to the diode barrier. ... I would not recommend diodes in series passing AC = too unpredictable ... (Diodes in parallel are another matter.)

  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
LM338 current Regulator allowing modulation Chris Daly Power Supplies 3 20th April 2007 06:58 AM
15 A Voltage Regulator with lm338 seftali Chip Amps 18 1st February 2007 09:00 PM
12v+ Voltage Regulator? silentblackhat Power Supplies 7 11th June 2006 11:49 PM
2 separate TX in Regulator LM338 JhonDoe Chip Amps 2 21st July 2004 01:57 AM
Voltage Regulator zoro72 Everything Else 0 26th April 2004 05:30 PM


New To Site? Need Help?

All times are GMT. The time now is 07:08 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2