Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Help identifying the transformer
Help identifying the transformer
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 23rd October 2017, 01:18 PM   #1
bancika is offline bancika  Serbia
diyAudio Member
 
Join Date: May 2010
Location: Nis, Serbia
Default Help identifying the transformer

I can get one of these cheap and I need a transformer for my next project (6W stereo amp based on TDA1521A). It's a Holden&Fisher toroidal that seem to be well regarded. It's 88mm x 34mm big with 115/230V primary and secondaries that measure 2x10v and 1x22.5v. Based on the size, I'd assume that it's at least 50VA, so it should be fine for my project, but how do I determine which of the secondaries to use? My guess is that 2x10V was used for symmetrical power supply used by power amplifier of sorts. That would work for me because I need +-12V (up to +-20V) to power the TDA1521A. Can I measure anything else to determine current capabilities of the two secondaries?

Maybe someone can recognize it from a device. I know they were used in Chrome bumper NAPs and few other good quality devices.

The pics are below

Thanks!

Last edited by bancika; 23rd October 2017 at 01:21 PM.
  Reply With Quote
Old 23rd October 2017, 01:48 PM   #2
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Help identifying the transformer
Measure the DC resistance of the windings. The two 10 volt windings should be similar and if the combined resistance of these two is lower then that 22 volt winding then it would appear they would have greater current capacity.

You can also load each winding with a suitable resistor (value chosen to draw the same current) and the winding with least droop should be the higher current ones.
  Reply With Quote
Old 23rd October 2017, 02:49 PM   #3
bancika is offline bancika  Serbia
diyAudio Member
 
Join Date: May 2010
Location: Nis, Serbia
thanks, that makes sense.
  Reply With Quote
Old 25th October 2017, 07:26 AM   #4
bancika is offline bancika  Serbia
diyAudio Member
 
Join Date: May 2010
Location: Nis, Serbia
Hi,

I measured what you suggested, but got somewhat inconclusive results. The two 10V windings measure around 1.2ohms combined, although it's hard to tell exactly, as my multi-meter isn't super accurate with such low resistances. The 22V secondary measures around 0.7V which would indicate that it's higher current. Also tried to measure the drop using a 100ohm resistors with 15V connected to the 115V primary, to be safe with lower voltages. The two 10V secondaries (now reading around 2.6V) show voltage drop around 0.07V across the 100ohm resistor and 22V secondary shows around 0.095V of drop. So it looks like the two secondaries have similar current capabilities. Voltage drop would indicate that the 10-0-10V is slightly more capable.

Would you agree?

Judging by the size of the transformer, I'd guess that it's rated 60-80VA. If my estimate is correct, I'll have 30-45VA for the 10-0-10V secondary I need for the symmetrical power supply for my project. It should be fine for a 2x6W amp.

Cheers,
Bane
  Reply With Quote
Old 25th October 2017, 08:54 AM   #5
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Help identifying the transformer
So 0.6 ohms for each 10 volt secondary and 0.7 ohms for the single 22 volt one. That seems to indicate the 22v one is the higher rated.

I would try this on full mains with more suitable resistors tbh. Mains voltage halogen lamps have a low resistance when cold/cool and so should draw respectable current.

Nothing bad is going to happen, even if you momentarily overload it.
  Reply With Quote
Old 25th October 2017, 09:28 AM   #6
bancika is offline bancika  Serbia
diyAudio Member
 
Join Date: May 2010
Location: Nis, Serbia
hmm, good idea re: halogen lamps, I'll try that. It's the same idea, measure voltage drop across the lamp, right?

Thanks
  Reply With Quote
Old 25th October 2017, 11:11 AM   #7
KatieandDad is offline KatieandDad  United Kingdom
diyAudio Member
 
KatieandDad's Avatar
 
Join Date: Nov 2011
Location: UK
You can also measure the regulation by loading each secondary. If you assume that the voltage drops by approx 10% at full load.

Slowly load up each secondary and measure the ACV.

The full load may not be an ideal rating of that secondary but it will indicate which windings are capable of what currents.

Last edited by KatieandDad; 25th October 2017 at 11:13 AM.
  Reply With Quote
Old 25th October 2017, 11:30 AM   #8
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Multimeters can not be trusted measuring very low ohm resistors, say 5 ohms and below, because you are also measuring *all* small resistances in the path (switch contacts, banana plug contact resistance, test probe resistance, test wires, surface contact between probes and what you are measuing, etc.) which add up to somethying between 0.3 and up to 1.8 ohms.

It wouldnīt be that bad if it were *constant* so you can reliably substract it from displayed value but itīs random and depends on many things , so ....

What I do is to separate "current supply" and "voltage measurement" functions which otherwise are both (unreliably) done by your meter.

Get a PC type supply (everybody has an old one somewhere collecting dust) and a 5 ohm resistor, 5W or higher.

This is a crude but workable 1A "constant current" supply; in any case we are not after 1% precision values but itīs a *comparison* between 2 low resistance values, so....

Apply said "1A DC" to winding under test and measure DC voltage across it ... your scale is now "1 Volt DC per Ampere" , meaning, say, 50 mV DC across winding means 50 milli Ohms (0.050 Ohms) and you have about 10% precision or better
Try that with a regular multimeter !!!

I use that setup to measure contact resistance in switches, jack/plug contacts, relay contacts, *Fuse* resistance, etc.
Eye opening and thatīs an understatement
__________________
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
  Reply With Quote
Old 25th October 2017, 01:01 PM   #9
bancika is offline bancika  Serbia
diyAudio Member
 
Join Date: May 2010
Location: Nis, Serbia
Thanks guys, great ideas.

@KatieandDad, that's a good point. But I'd need a 30W resistor if I were to load it to the assumed full load, right?

@JMFahey, great stuff as always. I don't have a PC supply, but I'll try to get one. What does the 5 ohm resistor do in that arrangement? I put it in parallel with the 5V DC output from the power supply to load it to 1A?

Thanks again!
  Reply With Quote
Old 25th October 2017, 10:17 PM   #10
bancika is offline bancika  Serbia
diyAudio Member
 
Join Date: May 2010
Location: Nis, Serbia
Update: I tried loading the secondaries down with halogen bulbs because that's what I had available at the moment. I wired two, then four 12V 35W halogen bulbs in series and powered them with each of the two secondaries.

The secondaries measure 20.3V and 22.7V without load.

With two bulbs in series, the secondaries measure 14.8V and 17.6V. That's 27% drop for the first one and 22.5% for the second one.

With four bulbs in series, the secondaries measure 16.4V and 19.3V. That's 19.2% for the first one and 15% for the second one.

Judging by that, it seems like they are relatively close, but the second secondary seems to be a bit more capable. Now, I'm trying to interpolate what the load would be to achieve the 10% drop (don't have anymore bulbs )

If my math is correct (and it probably isn't), by powering two bulbs in series with 20V instead of 2*12=24V, they draw ~20% less than the original 70W which comes down to 56W (should be 56VA with resistive load?). When we put four bulbs in series, the series resistance is doubled, so the current is halved to around 30VA. If my center-tapped secondary drops 19.2% at 30VA it's probably not even close to 30VA rating, right? The drop should be around 10% at the target "max" load. Although it's quite possible that I got the math wrong because bulb resistance isn't constant and rises with the current.

Now I'm thinking about a different approach of getting the +-12V rail voltage out of this transformer while maximizing the current capability. What if I rectify the two secondaries independently, ignoring the center tap on one of them, that would get me 25-30VDC on each of them. Then I could put one buck converter on each of them, drop down to 12V and then connect them back to back, like shown on this schematic. That should work, in theory. I'd be able to squeeze out almost every single milliamp, as buck converters should be pretty efficient. Another alternative are regulators, but I'd need to dump a ton of heat.

Click the image to open in full size.
  Reply With Quote

Reply


Help identifying the transformerHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Identifying this RCA Transformer 1303 Tubes / Valves 3 26th May 2012 06:22 AM
Identifying an output transformer. HELP! Phildog Power Supplies 0 25th November 2011 01:59 AM
Help identifying transformer wires. Skullface Tubes / Valves 6 28th January 2011 10:58 PM
Help Identifying Transformer SLT5M329 Redshift187 Parts 1 18th August 2008 12:04 PM
Help identifying transformer? dclapp Parts 2 2nd May 2007 01:28 AM


New To Site? Need Help?

All times are GMT. The time now is 07:11 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki