It really depends on the transformer's physical construction and where the limit on current is coming from. For example, small wall-wart transformers are primary inductance and resistance limited so that even a shorted secondary will not overheat the primary. A beefy torroid, on the other hand, might well have short circuit current capability on both the primary and secondary of 5-10 times the rated current and you're just thermally limited by total temperature rise and the size of the core. (Size of the core is also important when over-driving transformers as you don't want to run into saturation issues but that's usually more of a problem when you're pushing extra voltage.) You can often easily draw 2-3 times or more than the rated current, just for shorter and shorter periods of time, depending on cooling and acceptable temperature rise for the application.
You will certainly be able to draw more than 5 amps @ 20V but possibly not 10 amps. The best way to find out is very simple. Rig it up and test it!
Don't go straight to 10 A, try it in stages and let it run for an hour or more, monitoring the temperature of the transformer. If it barely gets warm, bump it up another amp or two and monitor the temperature rise.
Since you say that it looks more stout than the transformer in the Pyramid, I wouldn't be surprised if you actually CAN get 10 A continuous out of it with acceptable temperature rise.
This, however, brings me to another point. If you're using this for an amateur radio power supply, you aren't going to be sitting there drawing full power continuously. You only draw full power when transmitting and fully modulated. Back in the day, things like transmitting tubes would have separate ratings for continuous commercial duty, like a power supply in a factory that runs full load all day, vs. intermittent amateur service. Transformers intended for transmitters were likewise specified differently, depending on the target application.
It also depends on the type of rectifier set-up you choose. In this case, I'm assuming you'll use the two-diode full-wave with the CT grounded. Using choke input instead of capacitor should boost your average output capability by about 60% according to the old rule-of-thumb of "about 40% less than double" when intending to use two transformers in a 4-diode bridge, etc. by making the transformer conduct over more of the cycle instead of just on peaks when the voltage rises above the capacitor level. Capacitor-input is way "harder" on the transformer than a resistive AC or DC load or choke-input filter because the whole thing only conducts on peaks. Battery chargers are super-bad for that too, obviously, even with no
capacitor since the battery acts the same as a capacitor would as far as when current is drawn.
By the sounds of it, I would be surprised if your supply will not easily handle 7-12 A continuous with peaks during a transmit cycle to 20-25A since it sounds like this transformer is fairly conservatively rated. Transformer manufacturers even tell you that you can go well over the published current rating as long as you keep the temperature rise tolerable for the application. Things like intermittent operation or forced air cooling can significantly increase a stout transformer's ability to deliver over-nameplate-rated current. A comparison photo of the transformer would help judge the likelyhood but it is best to just test it yourself to characterize it fully for this type of operation.
Home
Help
Search
Login
Register
