Wasn't quite sure were to post this so apologies first if this is in the wrong section.
OK I have 248-9V mains, and I want to get that down to around 235V. I have a 50VA 240V - 2 x 12V transformer lying around idle.
I assume I can just use one of these 0-12V sections and create a 'buck' transformer to shave some volts of my high supply here?
My question is how to I wire this up please? I'd like to put it in a box with IEC connnectors so I can easily just plug my CD player (or whatever) into this bucking trannie, and then that into the mains.
Any diagrams for Mr mentally challenged here would be awsome 😉
Cheers,
- John
OK I have 248-9V mains, and I want to get that down to around 235V. I have a 50VA 240V - 2 x 12V transformer lying around idle.
I assume I can just use one of these 0-12V sections and create a 'buck' transformer to shave some volts of my high supply here?
My question is how to I wire this up please? I'd like to put it in a box with IEC connnectors so I can easily just plug my CD player (or whatever) into this bucking trannie, and then that into the mains.
Any diagrams for Mr mentally challenged here would be awsome 😉
Cheers,
- John
See attached.
The output you will get is closer to 236, but you get the idea.
If you use both 12V windings in parallel, you will end up with a VA capability of about 1000. That's the beauty of an autotransformer at the expense of lost isolation.
If there is any question about polarity of the windings, energize just the 240V winding with your 248V line voltage. Measure from the common point of the winding (bottom node) to the very top (dot end of the 12V winding). If you have greater than 250V, it is wired correctly.
edit: recommend a 4 or 5 amp fuse in both primary input legs
The output you will get is closer to 236, but you get the idea.
If you use both 12V windings in parallel, you will end up with a VA capability of about 1000. That's the beauty of an autotransformer at the expense of lost isolation.
If there is any question about polarity of the windings, energize just the 240V winding with your 248V line voltage. Measure from the common point of the winding (bottom node) to the very top (dot end of the 12V winding). If you have greater than 250V, it is wired correctly.
edit: recommend a 4 or 5 amp fuse in both primary input legs
Thanks for that that's great 🙂
Just to make sure I am doing this safely, the cd player consumes 60W, and at 240V I guess that's only 0.25amps? So this 50VA buck trannie is well up to the task.
Is it safe to only use one pair of the secondaries, and just leave the other two unsoldered? (they're terminals not wired).
Just to make sure I am doing this safely, the cd player consumes 60W, and at 240V I guess that's only 0.25amps? So this 50VA buck trannie is well up to the task.
Is it safe to only use one pair of the secondaries, and just leave the other two unsoldered? (they're terminals not wired).
Transformer is actually 100VA. If you only want to use one secondary, you will end up with a 50VA size. Connected as an auto, the size increases to 1000VA.
The black areas are to be soldered at the transformer lugs. Green wires are 248V input. Red wires are 236V output. You will have no issues running a CDP off this; the capability is 4 amps at 240V.
Leave the other 12V winding unconnected.
The black areas are to be soldered at the transformer lugs. Green wires are 248V input. Red wires are 236V output. You will have no issues running a CDP off this; the capability is 4 amps at 240V.
Leave the other 12V winding unconnected.
Attachments
Looks like I got it correct then - whew!
I take it I could safely run not only my CD player, but also amplifier (Stereo 70) and pre-amp (KingRex Pre) off the same transformer without having to use the second secondary? The Dynaco St70 consumes 190 watts which is around 0.8amps. Not sure of the current draw of the tiny KingRex pre but I think it's negligable...
Thanks for posting the pictures - very helpful to this layman!
I take it I could safely run not only my CD player, but also amplifier (Stereo 70) and pre-amp (KingRex Pre) off the same transformer without having to use the second secondary? The Dynaco St70 consumes 190 watts which is around 0.8amps. Not sure of the current draw of the tiny KingRex pre but I think it's negligable...
Thanks for posting the pictures - very helpful to this layman!
Just taken off the lid on the CD player (A shanling CD-T80, valve output stage job) and the mains transformer has written on it 75VA...
The buck transformer is only rated 50VA, so I assume I can't use it now?
Thanks,
- John
The buck transformer is only rated 50VA, so I assume I can't use it now?
Thanks,
- John
zigzagflux said:
You can run plenty of devices off a small transformer when it is connected as an auto. Your CDP has consumed 75VA of your total capability of 1000. Get it?
How appropriate that you should chime in on a discussion of autoformers. 😀
http://en.wikipedia.org/wiki/Zigzag_transformer
http://en.wikipedia.org/wiki/Zigzag_transformer
Good catch. Not often used but very handy winding configuration.
Most people think my name has something to do with rolling papers 😉 Back in the day.
Most people think my name has something to do with rolling papers 😉 Back in the day.
So, how does one going about computing the power handling capability of an autotransformer? I live in the US, and my line voltage is usually about 126 volts. I think if I make an autotransformer out of a 10:1 transformer I'll end up with about 114 VAC, which would suit my old gear fine.
I'm looking at this inexpensive part at Radio Shack:
http://www.radioshack.com/product/index.jsp?productId=2102702
It only has a single secondary winding. It says you can pull 3 amps off the secondary. How do I figure out what it can handle wired as an autotransformer?
What if I decide to take advantage of the center tap, and only use half of the secondary winding? It should get me close to 119 volts, which might be better, but I expect the power handling goes down.
I'm looking at this inexpensive part at Radio Shack:
http://www.radioshack.com/product/index.jsp?productId=2102702
It only has a single secondary winding. It says you can pull 3 amps off the secondary. How do I figure out what it can handle wired as an autotransformer?
What if I decide to take advantage of the center tap, and only use half of the secondary winding? It should get me close to 119 volts, which might be better, but I expect the power handling goes down.
Here's how I do it. Just take it step by step, and you can do this for any buck or boost configuration.
Wire up for buck by following the polarities as shown. Primary is then wound for (120+12.6) = 132.6V. Secondary is wound for 120V. Turns ratio is then 132.6/120 = 1.1
You place 126V on the input, therefore get 126/1.1 = 114V output. That much you have figured out.
The transformer you linked is rated 12.6V at 3A. Then force the 3A through this winding, as shown. So the 126V input is pushing 3A also.
For a transformer, power (VA) in equals power out. So 126*3 = 114*x. Therefore your output current is 3.3A.
These currents flow in both legs as shown.
Kirchoff's current law says 0.3A must be flowing in the 120V primary winding, as shown (3.3-3).
As a doublecheck, verify the transformer is "transforming" no greater than its rating. There is 12.6V@3A on low voltage winding, and 120V@0.3A on high voltage winding. The transformer is "transforming" only 36VA, which is its actual rating. Note the current directions through the windings, they are in opposition at the correct turns ratio (3/0.3 = 10), so ampere turns are satisfied.
Finally, note that you have 126*3 input, 114*3.3 output, so the total capability of this wiring configuration is 378 VA.
The approximate short cut is to recognize a 10:1 turns ratio will increase its capability by 10 times, but the long method helps you understand why and get a little more exact.
Wire up for buck by following the polarities as shown. Primary is then wound for (120+12.6) = 132.6V. Secondary is wound for 120V. Turns ratio is then 132.6/120 = 1.1
You place 126V on the input, therefore get 126/1.1 = 114V output. That much you have figured out.
The transformer you linked is rated 12.6V at 3A. Then force the 3A through this winding, as shown. So the 126V input is pushing 3A also.
For a transformer, power (VA) in equals power out. So 126*3 = 114*x. Therefore your output current is 3.3A.
These currents flow in both legs as shown.
Kirchoff's current law says 0.3A must be flowing in the 120V primary winding, as shown (3.3-3).
As a doublecheck, verify the transformer is "transforming" no greater than its rating. There is 12.6V@3A on low voltage winding, and 120V@0.3A on high voltage winding. The transformer is "transforming" only 36VA, which is its actual rating. Note the current directions through the windings, they are in opposition at the correct turns ratio (3/0.3 = 10), so ampere turns are satisfied.
Finally, note that you have 126*3 input, 114*3.3 output, so the total capability of this wiring configuration is 378 VA.
The approximate short cut is to recognize a 10:1 turns ratio will increase its capability by 10 times, but the long method helps you understand why and get a little more exact.
zigzagflux,
Here is what I did on my Darling amp last week. I'm using a Hammond 273X with these tubes : a 5V4G (2A), a 5965 (.45A x 2), and then the circuit draw of 60ma. Using a seperate trafo for the 1626 filaments.
I used the RadioShack 12.6V/3A and it brought my primary voltage down from 125-6VAC to around 111-12VAC. I've ran it using a 1A fuse (a dozen times or so on/off with no problem) and ran the amp playing music for about 2 hours or so with the bucking trafo just barely getting warm enough to be noticed, unlike the PT.
Do you see any problem with using this transformer at all?
FWIW, here is the diagram I used to hook it up.
Here is what I did on my Darling amp last week. I'm using a Hammond 273X with these tubes : a 5V4G (2A), a 5965 (.45A x 2), and then the circuit draw of 60ma. Using a seperate trafo for the 1626 filaments.
I used the RadioShack 12.6V/3A and it brought my primary voltage down from 125-6VAC to around 111-12VAC. I've ran it using a 1A fuse (a dozen times or so on/off with no problem) and ran the amp playing music for about 2 hours or so with the bucking trafo just barely getting warm enough to be noticed, unlike the PT.
Do you see any problem with using this transformer at all?
FWIW, here is the diagram I used to hook it up.
Well, that will work, as you have witnessed, and will probably provide years of reliable service. The connection is not my favorite, however. If you notice, you are running the 120V primary at 125V, and using the 12V winding to buck down. Perfectly valid, but I don't care to run transformers at high volts/turn. The 120V primary is being run at greater than its rating (not by much, I know) but you may find some potential for minor core saturation and increased stray fields. Won't hurt the transformer, but I don't like my audio equipment having more magnetic fields than is necessary.
With the other wiring config as I've shown, you are running the windings at less than their rated volts/turn. This is a good thing, IMO, especially if you don't need oodles of VA.
Your choice.
With the other wiring config as I've shown, you are running the windings at less than their rated volts/turn. This is a good thing, IMO, especially if you don't need oodles of VA.
Your choice.
So, if I have this correct, you are saying that either of the configurations shown in my attached image will work. But, you prefer the option shown at the top (L1/L2). Are there any hazards associated with running either winding significantly below its rated volts/turn or current specifications?
zigzagflux said:
So, if my 273X is rated at 120VA, do I need to be using a larger amperage "bucking" transformer in your preferred configuration?
It's still breadboarded so to speak so I can try different things still.
I know with the way I have mine wired now, it definitely brought the B+ into line. Definitely a difference when my mains runs at 126V and the power trannie is made for 115V. Nearly 35V drop in B+ using the bucking configuration I have now.
Ty:
Correct, either method would work, but I would prefer the top diagram for lower core flux.
The only problem with running a transformer at lower voltage is reduced VA capacity, which in your case is a nonissue. The benefit is lower magnetizing current, lower heat, and most importantly IMO, lower stray flux to couple into your other magnetics.
Correct, either method would work, but I would prefer the top diagram for lower core flux.
The only problem with running a transformer at lower voltage is reduced VA capacity, which in your case is a nonissue. The benefit is lower magnetizing current, lower heat, and most importantly IMO, lower stray flux to couple into your other magnetics.
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