re: Boomerang Power Supply

[Bret <echoplex@yahoo.com>]

Andy,
I disagree with your conclusion that using a dc source to an ac
(rectified) input of a device like the boomerang can possibly damage
the rectifier's diodes or that 'things will get a little hot'.

As far as I know rectifier circuits and diodes are rated by max current
and max voltage, not power. Diodes will fail if you exceed their
current rating, or if you exceed their breakdown voltage rating.  

Your 'half and half share' statement assumes a full wave rectifier, but
whether damage is done does not matter if we are talking about a full
wave or a half wave rectifier.  

As you note, in the case of the full wave circuit presented with an ac
input, the current is not split across 2 paths at the same time.  It is
not analogous to 2 paths that carry the same current at the same time
(like 2 wires in parallel).  At a given moment, half of the full wave
rectifier's diodes carry ALL of the current.  When the phase of the ac
input changes, the other half of the full wave rectifier circuit
carries ALL of the current.  So, each of the diodes in the bridge must
be rated to carry ALL of the current (plus margin).

The half wave rectifier is simpler.  In this circuit the current flows
through the diode half the time when an ac voltage is applied.  When
the same rms dc voltage is applied to the diode, the diode conducts
100% of the time (if forward biased) or None of the time (if reverse
biased).  The diode is fine with the dc input because the current
through the forward biased diode is the same as with an ac input, and
the dc voltage presented to the diode will not exceed the peak ac
voltage.  

When a 9v dc source is applied to the device's 9v ac (rectified) input,
the current through the rectifier is not increased (the device still
draws the same current), nor is the peak voltage increased from the
same device having ac input to the rectifier (the peak may even be less
for the dc case).  The difference the diodes will experience is the
duty cycle of the voltage and current.  Going to a 100% duty cycle will
not damage the diode or diodes because the current and voltage ratings
of the diode junctions are not exceeded.  

On an anecdotal note, the Roland GR-09 guitar synth expects an AC
input, but I have used a DC input (inside a car) and not seen any
problems.

bret
--- SoundFNR@aol.com wrote:
> > The fact that the boomerang uses ac input means that the boomerang
> >  rectifies and filters the ac to dc inside of the boomerang. 
> Plugging
> >  9v dc into a device that has rectifiers and filters on the power
> input
> >  will not do any harm.  It may not work, but it will do no harm.
> >  bret
> Caution, all the current will be flowing through half of the
> rectification 
> circuit
> all of the time (instead of the normal half & half share).
> If the power capacity of your rectification circuit isn't over
> specified 
> by a factor of 2 then things might get a little hot.
> So every chance you could blow up your gear.  
> Andy 
> 


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