# Current swing in bridge amp

Discussion in 'Electronic Basics' started by Tim, Jan 9, 2005.

1. ### TimGuest

Can someone please confirm or modify the following, given a bridged
audio amp outputting a sinewave to an inductive load.

1. Reading across the load, the _voltage_ will be seen to rise and
fall as a positive value every full cycle.

2. However, the _current_ changes direction every half cycle, at the
peak and trough of the sinewave.

For example, if two wires were arranged side-by-side so their EMF
cancels, they would effectively see twice the signal frequency in
terms of current flow.

IOW the cancellation frequency, being once in each direction, would be
twice the conventional sinewave frequency.

Thank you for any comments or corrections.

Tim

2. ### John PopelishGuest

Right. The current will change in one direction (decreasing, passing
through zero and increasing) as long as there is voltage across the
inductance of one polarity. As soon as the voltage reverses polarity,
the current stops increasing in that direction and starts to decrease,
pass through zero and increases in the other direction (all change in
one direction). The formula that relates inductive voltage to current
is V=L*(di/dt) with V in volts, L in henries and di/dt in amperes per
second.
Are you saying the same voltage is applied to both wires, but of
opposite polarity? If so, their currents would be equal but
opposite. No new frequencies.
No. Everything except power occurs at the same fundamental frequency.

3. ### CBarn24050Guest

Subject: Current swing in bridge amp
No. How could that possibly be? Firstly it should be obvious that the voltage
across the load must reverse if the current through it reverses, there will of
course be some phase shift between them. Secondly, to get a doubleing of
frequency you need to generate the second harmonic somehow, that means passing
through some non linear element, an inductor is linear, so no way there either.

4. ### BanGuest

The voltage changes signs as often as the current, it might be they are not
in phase, but of the same frequency, somehow you are imagining something
else.

5. ### Jim DentonGuest

Perhaps I did not explain my thoughts well enough.

To simplify, let's say there are two bridge outputs each driving a
separate length of wire. There is no reference to earth.

These wires are then laid side-by-side but in reverse directions, such
that when the current is following one way in one, it will be moving
oppositely in the other.

We then apply a 100Hz sinewave of the same phase to each, which, for
the purpose of this discussion, can be said to cancel each other out.

So then, while the applied frequency is 100Hz, there are actually two
_cancellations_ per cycle due to current reversal. One each
corresponding to the peak and trough of the sinewave. Hence it occurs
at 200Hz.

This is simply a thought experiment. I realize the effect is not
objectifiable.

Tim

6. ### John PopelishGuest

They do not cancel twice per cycle, they cancel continuously
throughout the cycle. Zero is zero. If the cancellation is not
perfect, then there is a residual that is a smaller (and possibly
phase shifted) version of the original waves.  