piggyback transformers

milen007 · 8th March 2009, 01:18 PM

hi guys

i would like a power transformer of 300v secondary

can i make this from 2 off the shelf power transformer?
PT1: pri 220v sec 0v 20v 1A
PT2: pri 220v sec 0v 15v 1A

is it possible that i do it this way:
connect the secondary of PT1 20v to secondary of PT2 15v and take my required 293v (if my calculation is correct and there is no iron loss) from the primary of 2nd PT?

and how to calculate the VA rating from this kind of setup if above setup is possible

TIA

Erwin

wakibaki · 8th March 2009, 02:02 PM

Substantially, yes. You will get 220 * 20 / 15 = 293.3V

The consideration is that the 220V winding is rated 220V. It may withstand 293V, it may not. It's your call.

The VA rating will be that of the lower rated part.

w

AndrewT · 8th March 2009, 02:20 PM

Better still, buy a 1:1 isolating transformer.

Adapt any mains toroid transformer to make your own autotransformer to give any voltage you require.

Power the autotransformer from the isolated secondary and you are safe and completely flexible.

You can even use a Variac as your continuously variable autotransformer in complete safety.

milen007 · 8th March 2009, 02:27 PM

thanks wakibaki for the warning. its noted

how bout this:
piggyback the 15v 0v 15v with the 20v 0v 20v?

does the winding can withstand the 293v? does it become balance? as each end have half the voltage?

sorry if this question is ridiculous. i am learning. TIA

AndrewT · 8th March 2009, 02:38 PM

hi,
the turns ratio of the transformers is not what you might expect.
Let's take your first example using 15Vac and 20Vac transformers.
Both are small, 15VA and 20VA and likely to have regulation in the range 10 to 20%.

The open circuit voltage of these transformers will be ~15 * 1.15~=17.25Vac and ~20 * 1.12~=22.4Vac.
These can be used to determine the turns ratios.
220:17.25 and 220:22.4
Now use those ratios to determine the open circuit output voltage.
220*22.4/17.25~=286Vac.
The regulation of these cascaded transformers will be worse than the lower rated, i.e. worse than 15%. Let's assume 20%.
The rated output voltage is ~ 286 / 1.2 ~=238Vac not 293Vac.

Go back to post3 and build your own isolated autotransformer.

You can check the turns ratio by measuring the actual input and output voltages when the secondary has no load.

jcx · 8th March 2009, 04:04 PM

running the 2nd transfromer at higher V, same frequency also increases B in the core - power transformers are usually at the edge of acceptable loss with nominal line V for economic reasons

line power xmfr cores are operated at a very nonlinear part of their curve and Imag increases nonlinearly with V*t
the increased B in the core will draw lots more magnetizing I and increase losses more quickly than you might expect

milen007 · 9th March 2009, 05:18 AM

hi guys

thanks for all the help and pointers. will look into them

jcx, yep i tried piggyback 2 PTs to give out the same voltage as the main, trying to create isolation transformer from 2 off the shelf PTs, the output voltage is lower than mains.

at one times i try to draw only 1/2 of the rated current. the input volt is 220v but the output is only 190v. but they bring benefit to the circuit as long as lower voltage wont create problem.

Erwin

8th March 2009, 01:18 PM	#1
milen007 diyAudio Member Join Date: Dec 2007	piggyback transformers hi guys i would like a power transformer of 300v secondary can i make this from 2 off the shelf power transformer? PT1: pri 220v sec 0v 20v 1A PT2: pri 220v sec 0v 15v 1A is it possible that i do it this way: connect the secondary of PT1 20v to secondary of PT2 15v and take my required 293v (if my calculation is correct and there is no iron loss) from the primary of 2nd PT? and how to calculate the VA rating from this kind of setup if above setup is possible TIA Erwin

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Tried to piggyback two AK4395 dac chips	sendler	Digital Source	1	4th February 2009 06:59 PM
Help identifying a pair of output transformers and power transformers	mjk130	Parts	3	30th January 2009 10:44 AM
Little Piggyback Capacitors	CarlosT	Chip Amps	9	4th December 2006 03:16 PM
Back to Back transformers as Isolation transformers	mikee55	Chip Amps	2	17th March 2006 11:38 AM
AD1865 in piggyback	cmt42001	Digital Source	6	10th November 2005 09:30 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	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

8th March 2009, 02:02 PM	#2
wakibaki Banned Join Date: Jan 2008 Blog Entries: 2	Substantially, yes. You will get 220 * 20 / 15 = 293.3V The consideration is that the 220V winding is rated 220V. It may withstand 293V, it may not. It's your call. The VA rating will be that of the lower rated part. w

8th March 2009, 02:20 PM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Better still, buy a 1:1 isolating transformer. Adapt any mains toroid transformer to make your own autotransformer to give any voltage you require. Power the autotransformer from the isolated secondary and you are safe and completely flexible. You can even use a Variac as your continuously variable autotransformer in complete safety.

8th March 2009, 02:27 PM	#4
milen007 diyAudio Member Join Date: Dec 2007	thanks wakibaki for the warning. its noted how bout this: piggyback the 15v 0v 15v with the 20v 0v 20v? does the winding can withstand the 293v? does it become balance? as each end have half the voltage? sorry if this question is ridiculous. i am learning. TIA

8th March 2009, 02:38 PM	#5
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	hi, the turns ratio of the transformers is not what you might expect. Let's take your first example using 15Vac and 20Vac transformers. Both are small, 15VA and 20VA and likely to have regulation in the range 10 to 20%. The open circuit voltage of these transformers will be ~15 * 1.15~=17.25Vac and ~20 * 1.12~=22.4Vac. These can be used to determine the turns ratios. 220:17.25 and 220:22.4 Now use those ratios to determine the open circuit output voltage. 220*22.4/17.25~=286Vac. The regulation of these cascaded transformers will be worse than the lower rated, i.e. worse than 15%. Let's assume 20%. The rated output voltage is ~ 286 / 1.2 ~=238Vac not 293Vac. Go back to post3 and build your own isolated autotransformer. You can check the turns ratio by measuring the actual input and output voltages when the secondary has no load.

8th March 2009, 04:04 PM	#6
jcx diyAudio Member Join Date: Feb 2003 Location: ..	running the 2nd transfromer at higher V, same frequency also increases B in the core - power transformers are usually at the edge of acceptable loss with nominal line V for economic reasons line power xmfr cores are operated at a very nonlinear part of their curve and Imag increases nonlinearly with V*t the increased B in the core will draw lots more magnetizing I and increase losses more quickly than you might expect

9th March 2009, 05:18 AM	#7
milen007 diyAudio Member Join Date: Dec 2007	hi guys thanks for all the help and pointers. will look into them jcx, yep i tried piggyback 2 PTs to give out the same voltage as the main, trying to create isolation transformer from 2 off the shelf PTs, the output voltage is lower than mains. at one times i try to draw only 1/2 of the rated current. the input volt is 220v but the output is only 190v. but they bring benefit to the circuit as long as lower voltage wont create problem. Erwin