OK I have had a sleep and time to try and digest but need to do more reading and playing before it sinks in and I may start to understand this better. I wish I had a variable frequency ac power supply to play with and measure things, as I only seem to learn by trying and seeing the results, but I do not have such gear.
I found an article in the early hours and had a read of it, it is on audio OPT transformers and it talked about power de rating as in the following:-
I give an example: suppose you select an OPT which can handle 50 Watts at 30 Hz.
Then this transformer can handle 50/2 = 25 Watt at 30/1.414 = 21.2 Hz.
Or the transformer can handle 50*2 = 100 Watt at 30*1.414 = 42.4 Hz.
The rule behind all this is: “the power capability doubles when the frequency is a
factor 1.414 larger. The power capability halves when the lowest frequency is divided
by 1.414 (square root 2)”.
This reduces the VA of the transformer by a factor of 16 which is much, much more than I thought it would.
Somewhere else that the voltage ratio was dependant on the turns ratio.
I was thinking that if I applied the above for a 50 Hz transformer to get to 12.5 Hz
50Hz/1.414 = 35.35Hz. VA = 20
35.35Hz/1.414 = 25Hz. VA = 10
25Hz/1.414 = 17.67Hz. VA = 5
17.67/1.414 = 12.5Hz. VA= 2.5
Current wise 40 VA at 230 Volts means the primary current could normally be up to 173mA rms.
de rate to 2.5 watts and it drops to 11mA.
I want a peak secondary current of 100mA which is 35.4mA rms the primary current would be 35.4 x (48/230) = 7.4mA
I need 50 Volts peak to peak which as you say is 18 Volts ish rms, the turns ratio is 230/48 so the primary voltage required is 86 Volts rms, Things I do not know is if i could apply the same rules for sawtooth as one does for sine wave. So I generally thought the transformer may still have a chance? No No I had not considered any standing dc current.
I need to try something to progress the project in some way, so I may as well build the EL84 stage and rewind a transformer, provide an air-gap by separating the E and I laminations and stick as much copper in there as I can. Not sure what else to try.
Adrian
I found an article in the early hours and had a read of it, it is on audio OPT transformers and it talked about power de rating as in the following:-
I give an example: suppose you select an OPT which can handle 50 Watts at 30 Hz.
Then this transformer can handle 50/2 = 25 Watt at 30/1.414 = 21.2 Hz.
Or the transformer can handle 50*2 = 100 Watt at 30*1.414 = 42.4 Hz.
The rule behind all this is: “the power capability doubles when the frequency is a
factor 1.414 larger. The power capability halves when the lowest frequency is divided
by 1.414 (square root 2)”.
This reduces the VA of the transformer by a factor of 16 which is much, much more than I thought it would.
Somewhere else that the voltage ratio was dependant on the turns ratio.
I was thinking that if I applied the above for a 50 Hz transformer to get to 12.5 Hz
50Hz/1.414 = 35.35Hz. VA = 20
35.35Hz/1.414 = 25Hz. VA = 10
25Hz/1.414 = 17.67Hz. VA = 5
17.67/1.414 = 12.5Hz. VA= 2.5
Current wise 40 VA at 230 Volts means the primary current could normally be up to 173mA rms.
de rate to 2.5 watts and it drops to 11mA.
I want a peak secondary current of 100mA which is 35.4mA rms the primary current would be 35.4 x (48/230) = 7.4mA
I need 50 Volts peak to peak which as you say is 18 Volts ish rms, the turns ratio is 230/48 so the primary voltage required is 86 Volts rms, Things I do not know is if i could apply the same rules for sawtooth as one does for sine wave. So I generally thought the transformer may still have a chance? No No I had not considered any standing dc current.
I need to try something to progress the project in some way, so I may as well build the EL84 stage and rewind a transformer, provide an air-gap by separating the E and I laminations and stick as much copper in there as I can. Not sure what else to try.
Adrian
Learning as I go!
Youtube EF91 Valve
Youtube EF91 Valve
