Transformer current question - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 14th March 2008, 11:40 PM   #1
mikecj is offline mikecj  United States
diyAudio Member
 
Join Date: Dec 2007
Default tranny current question

General question here regarding "old" used and unmarked NOS transformers.

I have no problem figuring out if some of these trannies I have are "shorted" or not.

HAve even set up an iso and lamp board for checking voltages.

But is there a simple way to determin or estimate the current capabilities of a given unmarked power transformer????

I have several that range in bulk all apparently in good order and would really like to use them rather than buy new or have them gather more dust.

Any helpful suggestions to some one who is somewhere between the apprentice and journeyman stage of this addiction.

Thanks

Mikecj
  Reply With Quote
Old 14th March 2008, 11:58 PM   #2
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
I don't think there is any "easy" way of determining current rating. You can make a guesstimate based on physical size and previous use if known. Sometimes removing the shells and getting a peek at the wire used on the coils will help make a fair estimation compaired to other known transformers. And you could load them down for suspected currents and monitor heat build up and voltage drop. But many times it's a c-r-a-p shoot. (stupid sensor )

Victor
__________________
"It is hard to imagine a more stupid or more dangerous way of making decisions than by putting those decisions in the hands of people who pay no price for being wrong." ~Thomas Sowell
  Reply With Quote
Old 15th March 2008, 01:19 AM   #3
diyAudio Member
 
tubewade's Avatar
 
Join Date: Jun 2007
There are some generalisations you can use. For one, you can guess approximately 35VA per pound of iron in the core. This is rough, of course, but will give you a general idea based on mass. This is based on 60 cps use in the USA. On 50 cps the output capability would be less.

Also, you can load the transformer until the loaded output falls to about 90 percent of the unloaded output. Operate under these conditions for some time whilst monitoring the temperature. If everything goes well you can assume this amount of current is safe for the transformer. If it gets too hot to touch comfortably then the transformer is overloaded.

Yes, these are crude methods, but will get you in the right neighbourhood with transformers you don't have specs for.

Best, Wade
  Reply With Quote
Old 15th March 2008, 03:25 PM   #4
mikecj is offline mikecj  United States
diyAudio Member
 
Join Date: Dec 2007
Thanks for your replies.

Even though the issue is not "easily" resolved I do feel somewhat better that I hadn't missed some obvious and well known rule during the journey.

Mikecj
  Reply With Quote
Old 27th March 2008, 02:06 AM   #5
diyAudio Member
 
Cycline3's Avatar
 
Join Date: Jul 2007
When in doubt, go for it. You shouldn't be building anything nice or critical with unspeced parts, but fun projects or research/fun amps, just guess based on weight and size. What's the worst that could happen? It dies? It's not a new part anyway... just something you had laying around. The learning and fun of building should make the risk of failure worth it. Again, do not put unspeced parts in anything nice, unless you KNOW you are running them under spec - like say a 10lbs transformer at 20VA or something ridiculous. And always be safe.
__________________
My site:
http://www.seanrose.com
  Reply With Quote
Old 27th March 2008, 01:42 PM   #6
diyAudio Member
 
Tom Bavis's Avatar
 
Join Date: Jun 2002
Location: Macedon NY
A quick estimate of capacity can be made from winding resistance. Allowing about 2 watts loss per winding - the corresponding current is 2/sqrt(R). Start with primary resistance to estimate overall capacity. Of course with a large transformer, the total loss can be more, and a small transformer with multiple windings can't take as much per winding.

The "10% drop" rule is good for small transformers, but larger ones may be at their temperature limit at 5% loss (A 200VA transformer will be dissipating 10W at 5% drop) Use SPICE or PSU designer to get an estimate of RMS current - usually considerably more than DC current..
  Reply With Quote
Old 27th March 2008, 03:11 PM   #7
diyAudio Member
 
Ty_Bower's Avatar
 
Join Date: Mar 2007
Location: Newark, DE
I've found this article to be very helpful:

RATING UNKNOWN POWER TRANSFORMERS
How to find the current rating and operating characteristics
of standard power power transformers by using two simple graphs.

http://www.diytube.com/phpBB2/viewto...t=1465&start=7
  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
calculating transformer current draw question gary h Tubes / Valves 5 20th June 2008 01:14 PM
Transformer current question whitelabrat Tubes / Valves 5 18th April 2008 08:39 PM
Transformer Current output ocool_15 Parts 4 3rd December 2004 07:19 AM
Transformer surge current lawbadman Solid State 20 7th September 2003 07:47 PM


New To Site? Need Help?

All times are GMT. The time now is 08:38 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