Function and Value for Resistor after Rectifier - diyAudio
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 28th January 2013, 12:12 AM   #1
ron944 is offline ron944  United States
diyAudio Member
 
Join Date: Sep 2010
Location: Florida
Default Function and Value for Resistor after Rectifier

Hi, in a tube preamps high voltage power supply there is a resistor going to ground after the full wave rectifier before the first power supply cap. I imagine this is to discharge the cap. Is this correct and what value resistor would you recommend. There is 445 VDC going to this resistor. Also will it have any effect on the voltage going to the first cap.

Thanks,

Ron
  Reply With Quote
Old 28th January 2013, 12:50 AM   #2
DUG is offline DUG  Canada
diyAudio Member
 
Join Date: Jan 2003
Location: mississauga ontario canada
All loading will affect the supply to the filter caps. The rectifier(s) and transformer have a source impedance. Any loading will affect this.

How much? It depends.

As to what value the discharge should be...I have seen suggested 10% of max load.

Someone else with more experience will be able to give you a better answer on that.

Sometimes this resistor discharge is disguised as a voltage divider for other purposes.

Two thing to consider when selecting the value are power dissipation and time to discharge.

Well, I guess voltage rating of the resistor(s) is important as well.

5 time constants (ohms * farads) will discharge the cap to less than 1% of initial charge.

Hope that helps a little.
__________________
Doug We are all learning...we can all help
"You can't stop the signal, Mal. Everything goes somewhere..."
  Reply With Quote
Old 28th January 2013, 01:02 AM   #3
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
I usually use a 470K 0w5 on mains equipment.
I usually use 2, one on the input to discharge the capacitor on the plug side (to stop shocks off the plug) and one on the smoothing capacitor (post bridge) to discharge the equipment.

It will have minimal effect on voltage going to first capacitor.

Bear in mind it will still take a while to discharge the smoothing capacitors after power off. I usually give it a few minutes then short out the smoothing capo wit ha resistor to make sure before touching circuit.
__________________
PCBCAD50 software. http://www.murtonpikesystems.co.uk

Last edited by nigelwright7557; 28th January 2013 at 01:06 AM.
  Reply With Quote
Old 28th January 2013, 01:37 AM   #4
ron944 is offline ron944  United States
diyAudio Member
 
Join Date: Sep 2010
Location: Florida
Thanks for the info. Other than for safety is this resistor really necessary at all.

Ron
  Reply With Quote
Old 28th January 2013, 01:43 AM   #5
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Its for safety only.

I heard of a famous company who made vacuum cleaners and they missed out the resistor on the plug side and customers who touched the plugs got a shock !

They had to take all the vacuum cleaners back and fix them !
__________________
PCBCAD50 software. http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 28th January 2013, 01:51 AM   #6
ron944 is offline ron944  United States
diyAudio Member
 
Join Date: Sep 2010
Location: Florida
Thanks again
  Reply With Quote
Old 28th January 2013, 10:13 AM   #7
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Because it is for safety this resistor is really necessary. You could argue that it is the most necessary resistor in the whole circut. Omitting other resistors may stop the amp from working. Omitting this resistor may kill you. Which is the most serious outcome?
  Reply With Quote
Old 28th January 2013, 10:49 AM   #8
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
IEC power input sockets can be bought with a filter built in.
This filter always has this resistor added in, to protect user from shock across the plug pins.
__________________
regards Andrew T.
  Reply With Quote
Old 28th January 2013, 11:43 AM   #9
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Remember resistors have a maximum voltage rating and that high voltage resistors must be specified for reliability in these kind of applications despite the power dissipation being well within spec.

(To anyone thats ever worked on TV/VCR and Satellite gear you'll know just how many SMPS start up problems were caused by open circuit start up resistors and these faults were simply because they were not high voltage rated)
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  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
Resistor before tube rectifier habsrock93 Tubes / Valves 4 14th November 2008 06:55 AM
Anybody heard about Tarzian Silicon Rectifier for Tube Rectifier Replacement? zxx123 Tubes / Valves 4 21st February 2005 04:02 AM


New To Site? Need Help?

All times are GMT. The time now is 03:33 AM.


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