Current Inrush Question - low power xfmr - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

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
Thread Tools Search this Thread
Old 16th March 2004, 03:53 PM   #1
lgreen is online now lgreen  United States
diyAudio Member
lgreen's Avatar
Join Date: May 2003
Location: San Diego, USA
Default Current Inrush Question - low power xfmr

Here is the situation, a 12 VCT transformer rated at 750 mA. The power supply connected to this is using the only caps I have, which are 4,400 uF for each rail. The circuit itself only uses about 100 mA, so this is all the power that is drawn in normal operation.

I am worried about a current surge to charge the caps when this thing is turned on, should I be worried about damaging the 750 mA rated transformer when powering on where the 4,400 uF caps are initially charging?

If so, what's the easiest way to do a little inrush suppression, connect one of these thermisters

in series with the primary? or secondary?
other ideas?
  Reply With Quote
Old 16th March 2004, 04:11 PM   #2
HDTVman is offline HDTVman  United States
diyAudio Member
Join Date: Aug 2001
Location: Chicago area
I don't think you need to worry about it. The resistance of the secondary will current limit to some degree and the charge time is short for that value cap. The transformer will not heat up at turn on because the charging time is so short.

Later BZ
What ever makes the tunes flow
  Reply With Quote
Old 16th March 2004, 04:35 PM   #3
Eva is offline Eva  Spain
diyAudio Member
Eva's Avatar
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
50-60Hz transformers are usually very forgiving of load surges

This is due to the thermal inertia of all the copper mass, that allows to operate them even in short-circuit conditions for a fraction of a minute without enough temperature increase to damage anything. 120șC to 180șC are usually required to damage winding insulation

In the other hand, capacitors and diodes or other solid state devices may suffer damage due to these current surges, but this is hardly seen in 10VA applications
  Reply With Quote
Old 16th March 2004, 04:55 PM   #4
diyAudio Member
Join Date: Dec 2003
Location: Orange County, CA
Default Surges

Your transformer will be fine. Its your rectifier diodes you have to worry about. WIth caps that big, you may blow 1 Amp diodes. I suggest at least 6 amp diodes be used to handle the turn on surge.
Dan Fraser
  Reply With Quote
Old 16th March 2004, 05:04 PM   #5
lgreen is online now lgreen  United States
diyAudio Member
lgreen's Avatar
Join Date: May 2003
Location: San Diego, USA
Default Diodes

FYI, should have mentioned this but the diodes are MUR820, rated at 8 Amps 200V, and are not heatsunk.

While we are at it, what about the on/off switch? This is rated for 120 VAC and 6 Amps.
  Reply With Quote
Old 16th March 2004, 05:31 PM   #6
subwo1 is offline subwo1  United States
diyAudio Member
subwo1's Avatar
Join Date: Jan 2002
Location: North American Continent
All should be fine.
  Reply With Quote
Old 16th March 2004, 05:46 PM   #7
diyAudio Member
promitheus's Avatar
Join Date: Jan 2001
Location: Germany
If it is a toroid it could trip your automatic fuse. You could use two thermistors in series with the primary. Check the power supplies from the Aleph amps from passlabs.
  Reply With Quote


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
inrush current doktor Solid State 124 16th June 2012 09:22 PM
Inrush current on AV800 stereo power supply emuman100 Solid State 35 20th April 2009 05:51 AM
Question on Inrush Current Limiter pwan Pass Labs 4 16th April 2006 03:06 AM
Elektor amp inrush current limiter question jgwinner Parts 14 12th December 2003 04:26 AM
About Inrush Current JDeV Parts 0 2nd December 2002 06:40 AM

New To Site? Need Help?

All times are GMT. The time now is 03:40 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2