• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Transformer current question

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
tranny current question

General question here regarding "old" used and unmarked NOS transformers.

I have no problem figuring out if some of these trannies I have are "shorted" or not.

HAve even set up an iso and lamp board for checking voltages.

But is there a simple way to determin or estimate the current capabilities of a given unmarked power transformer????

I have several that range in bulk all apparently in good order and would really like to use them rather than buy new or have them gather more dust.

Any helpful suggestions to some one who is somewhere between the apprentice and journeyman stage of this addiction.


I don't think there is any "easy" way of determining current rating. You can make a guesstimate based on physical size and previous use if known. Sometimes removing the shells and getting a peek at the wire used on the coils will help make a fair estimation compaired to other known transformers. And you could load them down for suspected currents and monitor heat build up and voltage drop. But many times it's a c-r-a-p shoot. (stupid sensor :dead: )

There are some generalisations you can use. For one, you can guess approximately 35VA per pound of iron in the core. This is rough, of course, but will give you a general idea based on mass. This is based on 60 cps use in the USA. On 50 cps the output capability would be less.

Also, you can load the transformer until the loaded output falls to about 90 percent of the unloaded output. Operate under these conditions for some time whilst monitoring the temperature. If everything goes well you can assume this amount of current is safe for the transformer. If it gets too hot to touch comfortably then the transformer is overloaded.

Yes, these are crude methods, but will get you in the right neighbourhood with transformers you don't have specs for.

Best, Wade
When in doubt, go for it. You shouldn't be building anything nice or critical with unspeced parts, but fun projects or research/fun amps, just guess based on weight and size. What's the worst that could happen? It dies? It's not a new part anyway... just something you had laying around. The learning and fun of building should make the risk of failure worth it. Again, do not put unspeced parts in anything nice, unless you KNOW you are running them under spec - like say a 10lbs transformer at 20VA or something ridiculous. And always be safe.
A quick estimate of capacity can be made from winding resistance. Allowing about 2 watts loss per winding - the corresponding current is 2/sqrt(R). Start with primary resistance to estimate overall capacity. Of course with a large transformer, the total loss can be more, and a small transformer with multiple windings can't take as much per winding.

The "10% drop" rule is good for small transformers, but larger ones may be at their temperature limit at 5% loss (A 200VA transformer will be dissipating 10W at 5% drop) Use SPICE or PSU designer to get an estimate of RMS current - usually considerably more than DC current..
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.