Question Regarding Soft-Start - 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 16th September 2010, 05:32 PM   #1
diyAudio Member
 
Join Date: Aug 2008
Default Question Regarding Soft-Start

For a relatively large transformer, must the soft-start circuitry be on the primary side? Is the current in-rush required to charge the magnetic field of the transformer the primary issue, or is it the load seen on the secondary side, as presented by a bank of uncharged capacitors? Or is it that both are of equal concern?

Jim
  Reply With Quote
Old 16th September 2010, 05:34 PM   #2
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
As I understand, the soft start should be on the primary of the transformer.
That way, it will handle both the primary surge and the charging of the capacitors on the secondary.
__________________
Frank
  Reply With Quote
Old 16th September 2010, 05:39 PM   #3
diyAudio Member
 
bobodioulasso's Avatar
 
Join Date: May 2008
Inrush current limiters are firstly put on the primary side.
  Reply With Quote
Old 16th September 2010, 05:40 PM   #4
diyAudio Member
 
Join Date: Aug 2008
Hi Frank,

Thank you for your reply. Yeah, I think you are right. I've been considering a few soft-start methodologies for an upcoming project. It would be nice to have it on the secondary side - that way I could have the secondaries go straight from the transformer into a PCB containing the soft-start, and there's no messing with mains voltages. But I suppose that would be neglecting the in-rush current of the transformer.

I've been toying with the idea of making a time-delay relay, using a thermistor. I'll post up a schematic or two when I have something that might work :P

Jim
  Reply With Quote
Old 16th September 2010, 08:09 PM   #5
DF96 is online now DF96  England
diyAudio Member
 
Join Date: May 2007
When you connect a transformer primary to the mains it may draw a DC current of up to the value of the initial AC current - it depends on where in the AC cycle the switch closes. This DC current then decays, according to the L/R time constant of the transformer. It can cause transformer saturation, which can often be heard.

Except for small items, I now routinely include an NTC inrush limiter in the mains circuit. This handles both the capacitor inrush and the DC, by temporarily increasing the resistance. Inrush limiting on the secondary side will help by limiting the initial AC current, but it won't quicken the decay of the primary DC.
  Reply With Quote
Old 16th September 2010, 08:38 PM   #6
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Have you check out my solution?
__________________
/Per-Anders (my first name) or P-A as my friends call me
  Reply With Quote
Old 18th September 2010, 08:51 AM   #7
diyAudio Member
 
Join Date: Sep 2010
Location: Melbourne
I always put the soft start on the primary side as it both covers inrush and filter capacitor charging. For a system with filter capacitors use a relay or contactor with a coil voltage equal to any convenient winding on the transformer usually the primary. wire the coil straight across that winding and use the contacts to short out a heavy duty resistor in series with the primary, Avoid the white ceramic block type wirewound resistors they blow operating into a heavy load. I have used this technique on power supplies up to 40 Kw NTC devices are fine for small power supplies I have found they give trouble once you get in the Kw range and restart after momentary power loss can be quite a problem.
  Reply With Quote
Old 18th September 2010, 03:18 PM   #8
DF96 is online now DF96  England
diyAudio Member
 
Join Date: May 2007
Yes, I should have said that NTC is OK for 'domestic' type operation provided that you don't keep switching - the thermistor needs a few (tens of?) seconds to cool down before it will limit inrush current again.
  Reply With Quote
Old 18th September 2010, 04:46 PM   #9
diyAudio Member
 
Join Date: Jul 2005
Location: Timisoara
NTC is ok for powers up to hundreds of watts (the input current that passes through it is low enough - a couple of amps - to prevent the ntc from overheating). After that you need a softstart circuit.
  Reply With Quote
Old 18th September 2010, 05:23 PM   #10
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
NTCs are suitable for transformer soft start upto thousands of Watts (or VA).
Just choose the appropriate NTC or ask the manufacturer's sales rep.

40kW on the other hand is quite something else.
__________________
regards Andrew T.
  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
soft start help pmchoong Power Supplies 18 14th January 2011 07:50 PM
Soft Start again marsheng Power Supplies 0 29th June 2010 11:09 AM
Soft start question a.wayne Solid State 39 8th November 2009 06:34 AM
Need a soft start? maxpou Solid State 19 18th April 2007 10:23 PM
Soft start & Soft Switch circuit: can anybody help? m.parigi Solid State 95 22nd August 2005 04:32 PM


New To Site? Need Help?

All times are GMT. The time now is 10:44 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