Transformer Rating

[RF Burn]

I have just unearthed a mains transformer from my shed, it is a Gardners GR23033 oil filled jobbie with a secondary rated 600 - 0 - 600 Volts at 166 mA RMS. Primary power rating marked as 200 VA which makes sense (1200V x 0.166A = 199.2VA).
As HT transformers seem usually to be rated for the DC output current (after rectification), I am not quite sure what safe maximum DC output current (assuming full wave rectification, 2 diodes, capacitor reservoir) could be obtained from this beast.

Can anybody clear this one up?

Also, is there any reason why the secondary current is given as RMS and not DC?

Adrian

[kalee20]

Beats me! And usually HT secondaries are rated for one section being loaded, as with conventional rectifier only one section is supplying current at any instant. So would be 600 x something not 1200 x something.

Maybe someone at Gardners blundered? It's not unknown!

[turretslug]

Could they have been quoting basic and resistive AC loading of the secondary on the grounds that something like this could have been used with choke- or capacitor-input filtering and whoever was using it would apply the appropriate calculation/weighting factors? Quoting (or even implying) output DC voltages/currents might be seen as misleading otherwise.

[Ed_Dinning]

Any heater windings on this transformer as well?

Ed

[G6Tanuki]

These ratings I find confusing in many different ways.

The classic centre-tapped bi-phase HT transformer has the 'problem' that at any time only half the winding is in-use. Meaning that in the half-cycle it's not being used any heat that's been developed can dissipate. Yet the bi-phase approach means the maximum current in a 'half' is twice what it would be in a full-wave rectifier where the whole winding is in use (but with lower average current) at all times.

It's also worth considering the application: a Class-A hi-fi audio-amplifier might be expected to run for hours non-stop, putting a continuous and unvarying load on the transformer, whereas a transformer powering a "Class C" amateur-radio transmitter may only be expected to deliver full-power for a couple of minutes - and then at 50% duty-cycle if CW is being used - with maybe 5 or 10 minutes of no-load idle-time in between "overs".


As in everything, the answer is "It Depends"!

[Herald1360]

Uesful table attached.....

TUF is ratio of dc power out to TX VA rating.

[trobbins]

I've seen similar winding type rating, compared to rectified and presumably filtered type rating, in vintage PT's, so not too uncommon.

As turretslug pointed out, back then one looked up tables for different rectification and filtering deratings.

Imho, the thermal time constant of the winding far exceeds whether a winding is just conducting for half a mains cycle or not - it is the rms level that dominates, and if a really detailed rating is needed then the pulse shape of the winding current can be used to define a slightly increased winding resistance (but that then gets in to power dissipation and so the max winding operating temperature needs to be defined or some custom derating applied).

Although rms current is probably definable for a particular arrangement of rectifier and filter capacitance-load-frequency as per Schade's curves, the PSUD2 software is eminently suitable for calculating the winding rms for any common power supply scenario.

If it doesn't have heater windings then maybe a target application was for separate heater transformer with delayed B+.

[kalee20]

Originally Posted by G6Tanuki

These ratings I find confusing in many different ways.

The classic centre-tapped bi-phase HT transformer has the 'problem' that at any time only half the winding is in-use. Meaning that in the half-cycle it's not being used any heat that's been developed can dissipate. Yet the bi-phase approach means the maximum current in a 'half' is twice what it would be in a full-wave rectifier where the whole winding is in use (but with lower average current) at all times.

That's more or less right. Transformer utilisation for the bi-phase is not as good as for bridge.

If you have a transformer designed for a bridge, the secondary resistance is 1Ω and you load it with 1A, you will lose 1W.

If you change to bi-phase, you will have two secondaries. For the same 1W total, they must lose 0.5W each. Each secondary is passing the 1A load current for half the time, so the same 1Ω resistance is satisfactory (giving 1W for half the time, 0 for the other half. Average 0.5W).

But look! - the bridge transformer had a single 1Ω secondary. The bi-phase must have two secondaries, each of the same resistance as the bridge secondary. So twice as much space will be needed for the secondary windings - a bigger transformer.

Alternatively, you can say that for the same size transformer, it's not possible to get as much power if configured for bi-phase as it would be for bridge.

The advantage of bi-phase is that although the transformer may need to be upsized, the rectifiers and associated losses can be downsized. So overall, in a particular situation, there is often not a lot to choose between them.

[Herald1360]

And doing a bridge with valves leads to extra heater windings never mind the cost of the valves etc. It's a fair bet that the traditional small HT supply with two diodes in one bottle and a common cathode was the cheapest way to do it!

Was there ever a double diode with separate cathodes and a common anode?

[RF Burn]

Thanks for the replies chaps.

I am going through that painful excercise of deciding what to throw out or keep as we will be moving house and some of my 'accumulated stuff' is not going with us . . .

I have no idea what this transformer was used in, it was just something I picked up many years ago in my travels and has been sitting in a dark corner waiting for the 'Round Tuit' day when things become useful!

It is big (140mm x 130mm x 100mm), black, and heavy (about 7Kg).

I guess I must have kept it as the potential basis for an RF Power Amp supply, either for continuous (AM/FM) or intermittent (SSB) use.

It has no other windings such as heaters or auxiliaries.

It seems to me that the TUF chart given by Herald may be a good starting point for the DC output power which should give me a suitable maximum DC output current at whatever the DC output voltage turns out to be on load.

I do believe that these old potted oil-filled transformers were rated conservatively and will take a fair amount of abuse and short term overload, although in my youth I had one in a power supply get hot and burst, spilling oil all over my workbench and floor which I put down to possible (time developed) shorted turns as I was certainly not overloading it at the time!

Adrian

[kalee20]

Originally Posted by Herald1360

Was there ever a double diode with separate cathodes and a common anode?

Nearest I know is the EB91 / 6AL5 / D77 (separate anodes, separate cathodes). But it's a signal diode so puny rating as a rectifier.

There's also the U801 with 4 diodes. But there are only 2 independent cathodes. So you would still need a pair of them to make a bridge.

It would be possible to make a bridge rectifier on an octal base - 4 pins needed, plus 2 for heater. But I don't know of any.

Originally Posted by RF Burn

I am going through that painful excercise of deciding what to throw out or keep as we will be moving house and some of my 'accumulated stuff' is not going with us . . .

I have no idea what this transformer was used in...It is big (140mm x 130mm x 100mm), black, and heavy (about 7kg).

If you're moving house, keep it. You will need something to hold the door open while carrying furniture in.

Seriously... seems a strange transformer, I have not come across something like this with no other secondaries. Which means it may be difficult to find a home for it, which is a shame, as Gardners transformers were well regarded. May be worth posting in the 'offered' section on here

[ms660]

Originally Posted by Herald1360

Was there ever a double diode with separate cathodes and a common anode?

The 25Z6 could be configured as such in effect:

http://www.r-type.org/pdfs/25z6g.pdf

Lawrence.

[turretslug]

Originally Posted by kalee20

Seriously... seems a strange transformer, I have not come across something like this with no other secondaries. Which means it may be difficult to find a home for it, which is a shame, as Gardners transformers were well regarded. May be worth posting in the 'offered' section on here

It could always be offered on eBay- anything with "audiophile" possibilities will attract attention from those who would want to make, say, a stereo single-ended 211/845 amplifier with 1100-1200V anode supply using a bridge + thorough filtering. The 0V tap then becomes a "half-way" DC source with possibilities for powering the necessary high-amplitude driver stage. The antediluvian valve types favoured by the cognoscenti often have oddball filament requirements, e.g. 10V at a few amps, so generally require separate transformers anyway.

Yes, I know quite a few here think that this sort of thing is potty, dangerous and needlessly extravagant, but some will pay good money for something out of the ordinary like this that fulfills a requirement. I never liked these sealed-case and oil-filled transformers, they end up being about twice the volume and weight that they need to be, but again they are revered by some.

Good luck with finding a home for it, it probably cost HM Taxpayer a small fortune originally!

Colin