DC Voltage feed into a Full Wave Bridge Rectifier ? For the curious.

Discussion in 'General Electronics Chat' started by SteveHow, Oct 23, 2016.

  1. SteveHow

    Thread Starter Active Member

    Jul 2, 2012
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    Hi ladies and gentlemen,


    I purchased a Digitech RV 1000 Guitar Effects processor off this guy which are run from external power packs.
    And after I got it home, I found it had a rouge power supply connected. The proper power supply should be 9 Volts AC (Yes AC) 1.3 Amps, but the seller had a 15 Volt DC (Yes DC) 1.34 Amp connected.

    Technically this is a mismatch and my first reaction was, why didn't it french fry the unit. But the unit seemed to function fine. (But I wasn't going to risk it, regardless of the sellers assurance) But here is my question.

    What would be electronically happening in this scenario? The reason I ask is because at first I was thinking "Alarm bells" someone call the fire department...but later when thinking about the circuit, I thought "what, if any harm could this potentially do to the input circuit" after all the only real difference (Besides the 15Votls being on the high side) is that the Rectification is housed externally.

    So pooling my aging electronics theory, I wondered about doing some theory on it. Here is what I came up with. I would love any input.

    First for the unit to have an external AC supply pack, there is a Full Wave Rectifier internally. So what happens when a DC signal is connected to Full Wave Bridge Rectifier? Normally, an AC signal is connected to obtain a DC signal then filtered, regulated. And only on every half cycle are 2 of the 4 Diodes working at the given frequency. But feeding a DC signal in would mean that only half of the FWBR (Full Wave Bridge Rectifier) would ever see a signal. (The positive signal)

    So theoretically just how bad of a mismatch was this after all? (Some rough calculations)

    15Votts DC was being feed into the FWBR, minus 2 x 0.7 volts to forward bias the diodes. Therefore 15 Volts input minus 1.4Volts leave the units input as 13.6 Volts. So the Filtering Cap would have been reading around 13.6 Volts.

    Hmmmm??? Still way to high as far as I can see.

    Anyone know roughly what the output of a FWBR would see being feed 9 Volts AC ? Would it be 9V minus 1.4V leaving an output of 7.6Volts being seen on its output?

    This would be basic theory for some no doubt. But the point is that maybe feeding an electronic appliance a DC Voltage, (Under the right circumstances) even though, it calls for an AC supply, may not harm a circuit at all. In fact, it could be seen as a handy trick to have up your sleeve if you've misplaced your power supply and need something quick and handy. :)

    However, reversing the scenario, whereby feeding an electronic appliance an AC signal when I calls for a DC input could be a disaster. (No rectification?)

    Anyone additional thoughts on this, please feel free.

    Steve
     
    Last edited: Oct 24, 2016
  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    9V AC has a peak voltage of 12.7V, so 15V is a bit over the top but unlikely to be disastrous.
    If the device in question has a simple rectifier feeding a smoothing capacitor then all is well.
    Some devices with AC input use a voltage doubling rectifier to get matched positive and negative rails and that wouldn't work with a DC input.
     
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  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    DC source => AC input A-OK

    AC source => DC input fire and smoke.

    Your analysis is correct. Also, it is easier to filter a DC signal over an AC signal as it is already pretty damn well filtered as is.
     
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  4. AlbertHall

    AAC Fanatic!

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    OTOH an AC input may feed a transformer inside and then smoke will ensue with a DC input :eek:
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Not in this context where a unit uses a low voltage power source. Requiring a low voltage implies the designer put the step down transformer out of the unit.
     
  6. renemonte

    New Member

    Oct 7, 2016
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    try to insert a nine volt linear voltage regulator between the output of the bridge rectifier and the filer capacitor..try xx7809.
     
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  7. renemonte

    New Member

    Oct 7, 2016
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    insert a nine volt linear regulator between the output of the bridge rectifier and the filter capacitor..try xx7809
     
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  8. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    The specified 9V AC will produce a little over 11V after the rectifier so 9V is rather too low.
     
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  9. floomdoggle

    Senior Member

    Sep 1, 2008
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    An RV-1000?
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Maybe missing something but, wouldn't a bridge rectifier in the device just insure correct polarity to it's circuit? It would make DC input from an AC source. And insure correct polarity from a DC source. Especially in a guitar effect case where some are AC powered and some are DC powered and it could be connected to the wrong power by accident. And not all DC wall warts use the same connector polarity in them. The only down side would be the 2 diode drops at the input.
     
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  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Done very frequently to either allow a device to operate on AC or a guarantee that a DC device has the right polarity regardless of the polarity connected.
    In the latter case, Commonly known as steering diodes.
    Max.
     
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  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Couldn't remember the term, "steering diodes" when I posted.
     
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