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EH Polyphase transformer dead

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EH Polyphase transformer dead

PostPosted: Wed Jul 01, 2015 4:47 am
by Adam Inglis
Would one of the technical types known to frequent these regions mind casting an eye over this grotty old schematic?

Down the bottom you'll see the PSU - transformer supplying a half-wave rectifier then a power transistor with zener on it's base.
My transformer has gone phhttt and I can't find any info on the original (mine was 240 volts, not 115). From the schematic I would have thought that one with 18 volt secondaries would suffice, no?

Re: EH Polyphase transformer dead

PostPosted: Wed Jul 01, 2015 9:01 am
by Folderol
That's actually a full-wave rectifier - a posh one using a centre tapped transformer rather than cheapo one using a bridge rectifier. However, it's quite difficult to get true centre tapped windings these days. They are usually dual windings you have to connect together to form the centre tap.

All that said, 15V + 15V secondary will do the job. This will actually give you about 20V when recitifed and smoothed, which will be enough headroom for the voltage regulator without cooking it.

What's less obvious is the power rating. I'd advise trying find one that physically slightly larger (if there is room) to be on the safe side.

Re: EH Polyphase transformer dead

PostPosted: Wed Jul 01, 2015 9:55 am
by ef37a
Hi Will,
Don't know why you think the old full wave circuit is "posh"?
It was a necessary evil in the days of valve rectifiers (which are themselves pretty evil these days!)

Costly, you needed twice the final voltage. Bigger and more copper loss than an equivalent single bridge jobby.

The power demand is trivial. Rule of thumb is, iirc, 2000mic' for every amp? That puts Iout at no more than 100mA. A 15V ~10va toroid should hiss that job and replace the diodes with a 1 amp bridge. I would change the res' cap as well. 470mfd would not hurt?

Not sure what all that gubbins does coming off the base/zener but I would be tempted to leave the circuit all as is but get the 15V from a 78/15 regulator. Such transistor regs are not S/C proof!

Dave.

Re: EH Polyphase transformer dead

PostPosted: Wed Jul 01, 2015 4:47 pm
by Folderol
Sorry Dave. I disagree with you on most points.

First, if it ain't broke, don't fix it. Why change any of the circuit at all when all that's needed is a replacement transformer.

A tapped winding is posh because it's more expensive. It's also better as you only have one diode V drop instead of two, and as you know peak drop could easily be over a volt per diode depending on the Tx impedance and cap.

As each winding is only conducting on half cycles they can be half the cross section area of a single one.

That extra bit on the drawing is a delayed unmute. See where B goes.

Re: EH Polyphase transformer dead

PostPosted: Wed Jul 01, 2015 5:17 pm
by ef37a
Folderol wrote:Sorry Dave. I disagree with you on most points.

First, if it ain't broke, don't fix it. Why change any of the circuit at all when all that's needed is a replacement transformer.

A tapped winding is posh because it's more expensive. It's also better as you only have one diode V drop instead of two, and as you know peak drop could easily be over a volt per diode depending on the Tx impedance and cap.

As each winding is only conducting on half cycles they can be half the cross section area of a single one.

That extra bit on the drawing is a delayed unmute. See where B goes.

You still need two windings with more bulk and the extra diode drops are of no import for 15V supplies, for lower voltages maybe.
Come on Will, noboby uses a two diode circuit these days for a single supply!

Dave.

Re: EH Polyphase transformer dead

PostPosted: Wed Jul 01, 2015 11:16 pm
by Folderol
ef37a wrote:
Come on Will, noboby uses a two diode circuit these days for a single supply!

Dave.
I do, frequently. Check up on RS & Farnell. It's virtually impossible to get a single secondary transformer in the 3 - 20VA range!

Re: EH Polyphase transformer dead

PostPosted: Wed Jul 01, 2015 11:52 pm
by Hugh Robjohns
Im with Mr F. Why reinvent the wheel when it's already there and working.

If you were designing from scratch then a standard regulator would be the way to go, of course, but that's not the case here.

And yes, the gubbings off the power supply feeding point B is a power-up mute.

H

Re: EH Polyphase transformer dead

PostPosted: Thu Jul 02, 2015 5:51 am
by Adam Inglis
Thank you gentlemen.

Dave says
ef37a wrote:Costly, you needed twice the final voltage. Bigger and more copper loss than an equivalent single bridge jobby.

You mean, 15-0-15, do you ? As Will recommends?

Re: EH Polyphase transformer dead

PostPosted: Thu Jul 02, 2015 7:58 am
by ef37a
Adam Inglis wrote:Thank you gentlemen.

Dave says
ef37a wrote:Costly, you needed twice the final voltage. Bigger and more copper loss than an equivalent single bridge jobby.

You mean, 15-0-15, do you ? As Will recommends?

I suggested a single 0-15V and replacing the two diodes with a small bridge but much depends upon what you can find. Most stock transformers are twin secondaries, i.e. 0-15+0-15 and you would put them in series* and the junction becomes "0".

As Will says, that is the easiest solution if you find such a component.

*Now you see the folly? You are generating 30V AC to get a 15V result and so using double the copper, winding time and more winding space.
Jusfort! If you can find an old Blackstar AC pedal supply they used a single 16V @800mA traff. Suit you Sir!

Dave.

Re: EH Polyphase transformer dead

PostPosted: Thu Jul 02, 2015 8:12 am
by ef37a
Folderol wrote:
ef37a wrote:
Come on Will, noboby uses a two diode circuit these days for a single supply!

Dave.
I do, frequently. Check up on RS & Farnell. It's virtually impossible to get a single secondary transformer in the 3 - 20VA range!

Oh! You old fashioned thing! I just parallel them and use a bridge!

Dave.

Warning! Highly esoteric discussion with little practical application

PostPosted: Thu Jul 02, 2015 10:47 am
by Folderol
ef37a wrote:
Folderol wrote:
ef37a wrote:
Come on Will, noboby uses a two diode circuit these days for a single supply!

Dave.
I do, frequently. Check up on RS & Farnell. It's virtually impossible to get a single secondary transformer in the 3 - 20VA range!

Oh! You old fashioned thing! I just parallel them and use a bridge!

Dave.

If you are forced to use a 2 winding transformer for a single output you should get one with half the required voltage and wire in series.

My old colledge tutor would have revelled in this! He would have immediately jumped in with, "The only transformer windings that should be paralleled are bifilar or dual bobbin ones". Both of these are rare as hens teeth and damned expensive.

The reasoning being, any other winding form will not give identical halves. The copper length (hence resistance) will be different, as will be the flux density - less of an issue with toroids. So, the current distribution in the windings will be different and the total power less than you'd expect - for some small value of 'less' :)

Conversely with series layered windings the effect is as if there was no break. There are no differential currents and the winding resistance is simply the sum of the two.

In a rectifier situation (push-pull v bridge) the winding differences mean that, under load, there will be slightly different voltage peaks between half-cycles, whereas these will be equal coming from a bridge.

Because I've sometimes had more free time than is good for me, I've actually tested this, and if you create an approriate DC offset then wind a 'scope up to maximum sensitivity you can indeed see the difference.

P.S.
I found yesterday's heat so exhausting that today I'm doing as little as possible to recover - aren't you the lucky ones :)

Re: Warning! Highly esoteric discussion with little practical application

PostPosted: Thu Jul 02, 2015 12:16 pm
by ef37a
Ok Will.
I will concede the point about paralleling windings..IN THEORY!

In practice we rarely care about small losses and in any case transformer speccs ususally SAY the windings can be series or parallel and we have been doing it for decades for 230/115 dual wound traffs!



Dave.

Re: EH polyphase power supply

PostPosted: Fri Jan 04, 2019 5:50 am
by Adam Inglis
Getting back to this after just a few years!

NY resolutions... ;-)

I desoldered the pass transistor and hooked up my bench DC supply to ensure the circuit is working as expected (it is), and the current draw was never much more than 50 mA.

Folderol wrote:If you are forced to use a 2 winding transformer for a single output you should get one with half the required voltage and wire in series.

But 'ang on, won't I then need to convert the two diodes to a bridge?

I've found a 9 -0- 9 v transformer that just might fit physically and has plenty of current capability. I wired it up but got 0 volts at the connection of the two diodes, so I'm clearly missing something ....!!

Re: EH Polyphase transformer dead

PostPosted: Fri Jan 04, 2019 11:55 am
by Folderol
ZZZzzz {ploink}
Eh? Wassat?
Oh er,

Well yes, for a 9-0-9 you'd have to go bridge to get the required voltage.

I'm not at all clear what way you were connecting to get 0V :?

Re: EH Polyphase transformer dead

PostPosted: Sun Jan 06, 2019 9:11 am
by Adam Inglis
Thanks Will!
Sorry, must've been having a brainfart - thinking this "posh" full wave rectifier was capable of some magic I wasn't familiar with.
Anyway, bridge is in, and it's working.
Despite the rating, I'm getting 20 v across the secondaries in series, and measuring 16.4 v at the power transistor's emitter, which looks suspiciously like two diode drops above the expected voltage. I presume that can be tolerated.

Unfortunately the transformer just won't quite fit physically into the case, so I continue my search ... or maybe look at some possible mods to the outer case of the traff, or to the way the PCB is mounted.