Author Topic: Power transformer VA, max current for transformer  (Read 575 times)

blankrazor

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Power transformer VA, max current for transformer
« on: December 31, 2013, 11:26:47 AM »
I have a power transformer that has an unused tap rated for 5v and at least 6 amps to power to 5u4 rectifiers. I am not planning on using this tap and am wondering if that means that the 6.3v tap can draw more current without burning up? The 6.3 tap is rated for 6 amps but I was hoping to pull about a max of 9 amps since the 5v tap won't be in use.
« Last Edit: December 31, 2013, 11:53:52 AM by blankrazor »


JohnRoberts

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Re: Power transformer VA, max current for transformer
« Reply #1 on: December 31, 2013, 12:10:40 PM »
Perhaps but there will be more voltage drop from winding IxR losses.

The primary is sized for total power so the primary can handle it, the secondary winding will be sized for expected current draw so pushing the current 50% will increase the forward voltage drop a similar amount. The extra winding heat dissipation will be somewhat mitigated by the other winding not making heat, but your actual results may depend on how conservative the winding is sized. If you can live with a little extra voltage sag you might get away with it. I sure wouldn't try to get much more out of it.

JR
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blankrazor

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Re: Power transformer VA, max current for transformer
« Reply #2 on: December 31, 2013, 12:24:44 PM »
Thanks for the clear response. I had worried as much. Does this mean that all transformer secondaries have a certain degree of voltage drop with any given current draw or only after a certain amount of current is drawn is there a voltage drop?

JohnRoberts

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Re: Power transformer VA, max current for transformer
« Reply #3 on: December 31, 2013, 02:05:17 PM »
Thanks for the clear response. I had worried as much. Does this mean that all transformer secondaries have a certain degree of voltage drop with any given current draw or only after a certain amount of current is drawn is there a voltage drop?

All windings have resistance so express a voltage drop. It isn't as simple as first seems since the current draw is not evenly spread over the entire waveform, but concentrated near the top (and bottom) of the sine wave with typical rectifier input power supplies.

If the average current is a few amps, the current for the mSec or so at the peak of the waveform when it is recharging the reservoir cap is 10x that or more. For back of the envelope math if the reservoir caps are being recharged every 10 or 16 mSec, and diode only conducts for say 1 mSec, the transformer must put out 10x or 16x to make up for the average draw. (note this is not exact numbers but describes the general concept).

Secondary winding resistance is small but real... It also increases (copper wire has positive temp co) as transformer heats up.

JR
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CJ

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Re: Power transformer VA, max current for transformer
« Reply #4 on: December 31, 2013, 03:28:00 PM »
unused tap is 5 volts x 6 amps = 30 VA

you want to add 6.3 volts x 3 amps = 18.9 VA, so the power capability of the core will not be an issue,

so what John pointed out about the DCR is the only issue,

usually there is a safety margin built into the wire size, but how much?

differs depending on who designed the xfmr,

so what you do is set the xfmr on the bench for a few hours while drawing 9 amps from the 6.3 winding, monitor the heat buildup, if you smell varnish cooking, then you will probably want to invest in an inexpensive 6.3 volt xfmr,
If I can't fix it, I can fix it so nobody else can!
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JohnRoberts

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Re: Power transformer VA, max current for transformer
« Reply #5 on: December 31, 2013, 03:49:07 PM »
OK, I'm thinking out loud and this is a wild a__ idea but bear with me. 

What would happen if we used two reservoir capacitors in series. The bottom of lower cap connects to ground and the diode bridge output for +5V, the top cap connects to the 6.3V winding.  If the capacitors are the same value the ripple voltage at the mid point will be roughly 1/2 the ripple voltage seen at the top cap.

In an ideal world the 5V winding will supply 1/2 the total current.....   or not.

Watch out for exploding caps, but what else could possibly go wrong.  ;D ;D ;D

Maybe time your test for midnight tonight in the spirit of the new years fireworks.

JR

PS: I do not approve this circuit for production,,,, but at least no one will accuse you of copying.  8)
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PRR

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Re: Power transformer VA, max current for transformer
« Reply #6 on: January 01, 2014, 03:23:18 PM »
6.3V at 9A sure sounds like a HEATER load to me. And one that can surely eat AC (unless he's building a 30-tube preamp).

Not sure if you propose mixing DC with AC or doing the whole load at DC, but it leads to massive rectifiers and caps and nasty waveform, whereas a hot winding or second PT is clean and simple.

blankrazor

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Re: Power transformer VA, max current for transformer
« Reply #7 on: January 02, 2014, 10:25:09 AM »
I am not sure I understand JR's last post but from the previous posts can I make the assumption that by knowing the resistance of the secondary winding and knowing the the current being drawn you can approximate the voltage drop? I assume that for high current draw either the secondary has to put out a higher voltage at low current draw.
Also is the max current decided by core size solely or does the secondary wire size have to be taken into account as well?
I have built single preamps before but this is the first power supply I am trying out for multiple preamps and thus many tubes so a much higher current demand. If I utilize good layout design should the high current AC heaters be a problem or would separate lower current heater supplies be better or even dc heaters?

ps.- I have been a long time reader of this forum and have learned so much from you guys. This is my first real post and am really thankful for the responses you guys have already given.
« Last Edit: January 02, 2014, 10:31:59 AM by blankrazor »

JohnRoberts

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Re: Power transformer VA, max current for transformer
« Reply #8 on: January 02, 2014, 10:53:53 AM »
I am not sure I understand JR's last post but from the previous posts can I make the assumption that by knowing the resistance of the secondary winding and knowing the the current being drawn you can approximate the voltage drop?
Using the winding to directly drive a load with AC the voltage drip will be straight forward like a resistance divider. For a diode capacitor PS, the current is concentrated into a small fraction of the waveform so peak current is higher (for a shorter time) and the IxR loss is higher. 
Quote
I assume that for high current draw either the secondary has to put out a higher voltage at low current draw.
Yes, this is called regulation and voltage output sags with increasing current.
Quote

Also is the max current decided by core size solely or does the secondary wire size have to be taken into account as well?
The core size and magnetic circuit determines the maximum power transfer between primary and secondary. The secondary wire size will determine how much of that output power is wasted in wire losses as heat. 
Quote

I have built single preamps before but this is the first power supply I am trying out for multiple preamps and thus many tubes so a much higher current demand. If I utilize good layout design should the high current AC heaters be a problem or would separate lower current heater supplies be better or even dc heaters?

ps.- I have been a long time reader of this forum and have learned so much from you guys. This is my first real post and am really thankful for the responses you guys have already given.
Please disregard my experimental suggestion... PRR will be better for practical tube gear advice.

JR
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AErige

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Re: Power transformer VA, max current for transformer
« Reply #9 on: January 02, 2014, 01:22:45 PM »
I am not sure I understand JR's last post but from the previous posts can I make the assumption that by knowing the resistance of the secondary winding and knowing the the current being drawn you can approximate the voltage drop? I assume that for high current draw either the secondary has to put out a higher voltage at low current draw.
Worst case scenario you got a tranny w/ 20% regulation rate...it means you get 6VAC unload and 5.5 VAC @ 3A....won't be enough if you're looking for 6.3VAC but depending how you split heaters supplies it could do it for 6.3VDC. Can you measure your 5V secondary without load ? btw how is your 110V ? here I always get higher secondaries due to my 240V instead of 220V
Quote
Also is the max current decided by core size solely or does the secondary wire size have to be taken into account as well?
As john stated when winding a tranny you first calculate your nominal current need and your power need with that current then you got a size core (in practice you'll often end up with something around P= (S/1.1)² , P in VA, S tranny section in cm²). And if you're a cheap bastard like my stepbro you can even choose a wire size with the smaller margin so you can wind more turn in the same window and use a smaller core (which lead to more wire losses and poor regulation rate). So unless you have access to the 6.3V tap wire size I wouldn't try to put 50% more consumption to it or I'll do a quick test like CJ described ( "if you smell varnish cooking"  ;D too late)


PRR

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Re: Power transformer VA, max current for transformer
« Reply #10 on: January 02, 2014, 05:37:14 PM »
> power supply ....for multiple preamps and thus many tubes so a much higher current demand. ....high current AC heaters be a problem or would separate lower current heater supplies be better or even dc heaters?

While good microphone(??) preamps have been built on AC heat, in a BIG project it is probably much better to go ahead and do it DC. At as high a voltage as possible (to reduce bus-wire size and rectifier losses).

Given a 5V and a 6V winding, I'd strap them for 5+6= 11.3VAC, 15.8V peak, take-away 2V for diode loss, you get 13.8V max. Taking 1V ripple for 1000uFd/1A, a 6,000uFd cap puts you near 12.6VDC, but 1V ripple around a preamp is worse than well-wired 6.3VAC. Also the ripple-current is brutal. So you should be thinking 220,000(!)uFd, 0.1 Ohms (10 Watts!), another 220,000uFd. The "0.1 Ohms" should be an array of 0.5 Ohm 5W parts so you can use more/less to get the right result (11.4V-12.8V at 12V-wired heaters; preamp tubes don't need a full 12.60V heat). Even then it is a VERY brutal build. Every *milliOhm* matters!! The PT, rectifer, and first cap have to be essentially adjacent, butted, or copper costs (and radiated EMI) will soar.

I'd even be thinking of using a *switching* supply. 12V 6A is not a huge switcher. 0.1 Ohms and 1,000uFd each side ought to take the hypersonic squeal off.

blankrazor

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Re: Power transformer VA, max current for transformer
« Reply #11 on: January 02, 2014, 10:35:55 PM »
If I go DC heaters might I just look into voltage regulators instead? I haven't used them before but in this situation might they just be easier? Any experience building larger mixers with AC heaters and their noise getting into the signal wires? I know the older consoles had them so it is possible. The tubes I was planning on using don't take 12V heaters only 6V. The transformer I am using is just extra and not something that I am having built. It was off of an older Baldwin organ and powered a lot of el84s and 12ax7's. It probably weighs about 15 lbs. I guess worst case scenario is it doesn't work and I try a different power supply.

Orpheus

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Re: Power transformer VA, max current for transformer
« Reply #12 on: January 03, 2014, 12:56:28 PM »
What is it that you are planning on building? Also on topic to your original question, aside from the very advanced explanations given previously, although the primary is rated for total current drawn, each secondary is separate and has its own ratings. The wires of the 6.3 tap do not increase in current handling capability if the 5v tap is unused.

Using regulators in this application is very tricky due to the high currents used and the waste heat associated with them.

PRR

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Re: Power transformer VA, max current for transformer
« Reply #13 on: January 04, 2014, 12:37:50 AM »
> tubes I was planning on using don't take 12V heaters only 6V.

If more-than-one tube, you wire two 6V bottles in series for 12V.

> The wires of the 6.3 tap do not increase in current handling capability if the 5v tap is unused.

Arguably they "may". See Reply #1. While sag may be an issue, in heavy iron the issue is often heat. Not-loading the large 5V winding avoids some heat, so we can stand a little more heat in another winding. How much more? See CJ's Reply #4. In an emergency, I would guess "about half of what's not-used in other winding", but sniff for cooked varnish.

Some firmer notion of what blankrazor is really doing would help frame useful answers, but he's reluctant to offer information (number of tubes, heat demand per tube, size of chassis, alltogether or modular.....).

blankrazor

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Re: Power transformer VA, max current for transformer
« Reply #14 on: January 04, 2014, 02:03:31 PM »
I am sorry things have sounded vague so far, I am planning on powering 8 preamps and 4 line amps from one external power supply drawing about 9 amps of heater and 200ma for B+. The chasis would be about 28 in. by 24 in. Mixing would be done by passive resistive mixing made up by the 4 line amps. I wasn't planning on making the amps modular. I was planning on using a single 6n6p for each line amp and using 6j7 and 6c5 for the preamps. I am starting to think maybe I should use a single smaller power supply for now rated for a single amp just for testing each amp, then tackle the power supply for all of them afterwards.
« Last Edit: January 04, 2014, 02:31:52 PM by blankrazor »

Speedskater

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Re: Power transformer VA, max current for transformer
« Reply #15 on: January 07, 2014, 11:13:16 AM »
Just add a 1.3V @ 6A buck/boost transformer.

(No I don't have a transformer like that in my spares box)
Kevin


 

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