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output power against load

Nayia

Apr 14, 2012
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Apr 14, 2012
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what is the relationship between the output power with the load by using a transformer?
As I know, as the load increases the output power increases until to reach a maximum point and then falls. Is that due to the heat?
The transformer is 1:1 ratio.
 

Laplace

Apr 4, 2010
1,252
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Apr 4, 2010
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The general rule for power transfer is that maximum power is transferred to the load when the source impedance is the complex conjugate of the load impedance. But transformers are usually used only for resistive, and not reactive, impedance matching. In this case it seems that impedance matching is not the goal since the turns ratio is 1:1. Also, your mention of 'heat' would lead one to believe this is a power isolation transformer. So as the load draws more current the transformer core approaches magnetic saturation. As the load draws even more current the transformer becomes saturated, exceeds its specified ratings, and produces heat due to non-linear operation with a saturated core.
 

TedA

Sep 26, 2011
156
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Sep 26, 2011
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Nayia,

This is a complex subject. I would not hurt for you to tell us more about what you are doing. Practical answers depend in part on things like the frequencies involved, the power, any devices you expect to use, and no doubt something important I didn't think to ask about. Yeah, like permissible distortion.

Laplace's answer is rigorous, but not complete. It applies in a linear network. Antennas are usually linear, for all practical purposes. Power amplifiers are not. At least not when you are seeking the maximum power from them.

A tightly coupled transformer may saturate less under heavy load, if the voltage across the windings falls as the source bogs down. Unless the increase in load makes the alternator turn more slowly. Microwave oven transformers are different.

Common sense tells us that with no load at all, we have many volts but no amps, thus zero power. Similarly, a short circuit load leads to lots of amps but no volts. Again, no power. So the max power spot has to fall somewhere in between.

In a resistive linear network, this point can be easily calculated to be the load where the voltage across the load is one half the no load voltage. This is a special case of our Laplace's rule. In either case, it's called matching the impedances.

This is a useful concept when you are trying to couple your microwave dish to your LNA. Or the LNA to the mixer.

If you are trying to get more noise from your car stereo, this whole business does not apply at all. The network is not linear at the margins, and the margins are exactly where you get the maximum power.

Heating is another subject. This usually causes the power to gradually fall over time, after your power-up.

Hope some of this made sense.

Ted
 
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