Evaluating Unknown Power Transformers

[wrenchone]

I've been picking up lots of nondescript transformers for amplifier projects, and a fair number of them have unknown ratings. I plan to load the windings to estimate their current rating. What is generally the accepted droop for a tube type transformer winding at max current? I would think it would be around 5% from the zero load voltage, but correct me if this is not the standard practice.

[Giaime]

5% is a bit too small, that's a value suitable for low voltage low DCR transformers. For high voltage transformer where you can have up to 150ohm DCR for a 300V winding (like a tranny I have here sitting on the desk), you can expect up to 10% voltage drop.

As always, check if the transformer is overheating, smell is a bad sign.

[Tom Bavis]

You can estimate the rating from the secondary resistance. The primary does come into it too - usually a transformer will be designed for about the same primary and seconday losses. For a start, allow 2 Watts per winding for resistive loss and see what current that is... for larger transformers, maybe 3-4W.

For a heater windings, I use a 6AS7 as a test load (2.5A) and see what the drop is ...

[wrenchone]

It sounds like there should be a compromise here - 5% droop for low voltage/high current windings ( filaments), and 10% for HV windings. Of course, I'm going to back off if the thing gets hot.... What I need now is one of those great, big slider resistors so I can load up the transformer and take my life in my hands.....
What I'll probably actually do is set up a switched resistor matrix like the one I built at work to test power factor correctors. Come to think about it, since the voltage drop is depedent on the winding resistance alone, there's nothing wrong with checking out the transformer at reduced line voltage, making whatever resistor bank I build up more versatile. The only thing wrong with this approach is that you won't be testing the primary at full volt-seconds to detect any skimpiness there, but you can also tell about that by running the transformer unloaded at full input voltage plus a few percent. If the thing gets too hot, then either the design was skimpy, or you've got a problem like a shorted turn. A shorted turn would also show up big time in the unloaded transformer exciting current. Ah, the joys of vintage transformers....

[kevinkr]

Properly designed power transformers for tube applications will generally deliver their rated current at some recognizable nominal voltage - this is particularly true of the filament windings, and in many cases you can also determine the secondary rating as well.

You need to know the nominal voltage rating of the primary which will usually be 110V (usually ancient) , 115V, 117V and on more recent ones 120V.

You will need a variac and an accurate meter or two, and a load box.
As you load the "tut" you should monitor the primary voltage and adjust to maintain it constant at the value chosen.

A fairly easy way to make the determination of primary design voltage would be to identify the 5V rectifier winding. (Usually yellow leads.)

Most rectifiers are either 2A or 3A filaments, although some windings may support two rectifiers. (rare and usually 4A in my experience.)

Through a process of iteration you will find both a primary voltage and filament load resistance that puts you fairly close to 5V. Note this is not absolute because with the other windings unloaded even at the nominal load the voltage might be a few % high, but it should get you close. It should not be less the 5V and should be less than 5% high if you are in the ballpark.

Once you know the primary voltage you can use the same technique to load the 6.3V windings and determine the rated current.

The high voltage secondary is a little trickier, a possibly useful rule of thumb (and like all r-o-t use judgement here) is that the no load to full load regulation may be something like 10%, however in many instances there are relatively standard secondary voltages that may fall out at specific load currents.

(Note that more modern transformers are often designed to 20% no load to full load in order to save both copper and core size, and hence cost - highly likely in inexpensive SS gear imho..)

All of this assumes resistive loads, and should not be considered absolutely definitive. It is not possible to know the insulation class of the particular transformer, and therefore except in the case of the rectifier filament winding I would be tempted to derate the transformer by 10% - 20% just to be safe - this is good practice anyway imo..

Note YMMV, and this is just one possible approach to making the determination.

Needless to say make sure that the possibility of contact with hazardous voltages is minimized and use a fuse in the primary circuit sized appropriately for the transformer being tested.
Caution is always warranted with high voltage.

[Johan Potgieter]

Wrenchone,

You have several pieces of good advice here, it would seem that designs differ from manufacturer to manufacturer.

I have measured known transformers over the years with the very purpose of establishing a rule-of-thumb should I need one. In my experience the average copper loss per secondary seems to have been about 3 - 4%. (That would mean an efficiency of about 85% taking primary copper resistance as well as iron loss into account.) The h.v. winding is then easiest to estimate: You calculate 4% of the open voltage and divide that by the winding resistance to give current, as Tom Bavis indicated. This method is not useful to calculate 5V and 6V currents because of the low winding resistance. In that respect Kevinkr's method is probably best - or just guess what type of circuit the estimated h.v. current would probably support.

There is also a basic equation for estimating the wattage from the core section area, viz. A = [sq.root(V.A)]/5.58 where V.A is the voltamp (watt) and A the core section in sq. inches. I have however found this to be generous.

In the end it would also depend on your climate! In India you would probably need to run transformers cooler than in Canada.

Regards.

[wrenchone]

The rub in India is that the mains voltages go all over the map, with really scary surges. It's a challenge for both linear and switching power supplies. Motorized automatic tap changers are common household items there to help keep the voltage within reasonable bounds.
Australia used to have a reputation for high mains voltages, though that may have been in areas at the head end of a long transmission line.

[blacklight]

^Would this not be fixed by a sine-wave UPS unit?

[wrenchone]

UPS supplies cost lots of money for something big enough to power a household. They may even be wiped out by the same sustained 300V surges they are trying to protect against if they are not designed to handle Indian mains. The motorized tap-changer is a cheap, bullet-proof Neanderthal-style approach that can stand up to sustained abuse.

[DigitalJunkie]

Weight,and core size can give you a ball-park figure,and if you can see the gauge of the windings,you can guesstimate from that aswell.

There's a bit of info here..
http://geek.scorpiorising.ca/windingtransformers.html