Aboutcleggsan Expertise All technical areas of Electronics Engineering.
Experience BSEE, MBA, Design, R&D, University Research.
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Broad teaching experience; work experience mostly in consumer electronics and conversion from analog to digital technologies. Pioneer in digital audio at all levels.
Question QUESTION: I am trying to get a solid grasp on how to accurately determine the maximum current draw I can get on the HV winding of a given transformer. I'm going to phrase this question somewhat indirectly in the context of a known circuit application. The transformer in question is rated at 250-0-250 @60ma. In the circuit it supplies there is @325 VDC across a 1K dropping resistor. Solving for current gives me 325ma. Why does this not burn up the HV transformer winding? I must be either missing some critical info on how to get the maximum current rating from the full load specs(250-0-250 @60ma) or making an error in my calculation. From the full load specs, how do I determine the max current draw for this particular transformer? Thanks
ANSWER: Without knowing the circuit (schematic diagram) I cannot give you a very specific answer, but let me make a few generic comments which may lead to sufficient answer.
BTW what do you mean by HV winding? You state the transformer in question is 250-0-250; is that the primary side or the secondary side? Is it a toriod coil or iron core EI lam transformer? Are you bridge rectifying, half wave or full wave? These are questions to know so I can determine what is going on to get a 325 dc?
And, let us not confuse ratings with maximum. Even though you give 60ma as the rating for the transformer in question, that does not mean you are limited to that amount; you could be running it overload condition which will, over time, overheat the transformer and eventually cause a burnout, but it will produce extra current until it fails. The 325vdc across the power resistor dissipates 105 watts. That is a very hot resistor, by the way. So, the input must be at least 105 watts (can't create energy).
The theory says either you are running the transformer way over it ratings or you have miscalculated the load. Ohms law and power conservation must be maintained. Can't create energy in a passive circuit.
Can you send a schematic or explain to me what the circuit is doing: Drive supply, transformer connections, rectifier configuration and filtering being used. Then I can envision what you are doing. You can attach a drawing with your next email.
It's a basic tube type power supply with full wave tube rectifier. The HV winding I was referring to was the high voltage secondary. The actual power transformer used was actually rated at 270-0-270 @ 55ma, potted military style, probably EI lam. Since this circuit was a commercial device I assume they were within the safety margin for the chosen transformer so I must be making an error in applying ohm's law to the 1K resistor to get the current being drawn at that point in the circuit. Overall, what I'd like to know is the correct method for estimating what the total current draw on the high voltage winding would be. I understand how to calculate current draw on the rectifier and filament windings using filament current data from a tube manual, but for the HV winding I'm not clear on how to get an accurate estimate. For getting current measurements without using an ammeter I was always taught to measure the voltage drop across a known resistor, then solve for current using ohm's law.
Answer That makes all the difference in the world. This is a typical vacuum tube amplifier. The power supply is very traditional and the current being fed by the power supply is simple to estimate. Please look at the ripple reducer filter at the output of the rectifiers.
There are 40mfd caps on either side of the R of the RC filter. This resistor is 1k ohm. The voltages on the schematic show a voltage drop of 20 volts across it (345 - 325). The current, therefore, is 20v/1,000 or 20ma. Or about P = 20ma X 320v = 6.4 watts to the amp circuit. (Which is plenty for the 600 Ohm output).
This is a nice old "peerless" tube amp and if you bias it up nicely it will give really good sound. Keep in mind it has iron core transformers in the front end and at the output stage. Some people don't like the sound of transformers because the introduce eddy current distortions and other artifacts, but if the push-pull balance is good it produces mostly even harmonics which sound good - as opposed to odd harmonics which sound dissident. The odd harmonics get canceled out in a circuit like this when balanced good.
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