Hypothetical Question

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Dennis Alwon

Dennis Alwon

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This is not a trick question and I have no idea where this thought came from but I wondered whether the trany would notice the difference if DC was injected in it.

Someone asked where would you get 120V DC -- that doesn't matter as it is hypothetical
 
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GoldDigger

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Dennis Alwon said:
This is not a trick question and I have no idea where this thought came from but I wondered whether the trany would notice the difference if DC was injected in it.

Someone asked where would you get 120V DC -- that doesn't matter as it is hypothetical
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I would have to say that, like any other transformer subjected to DC at their rated 60Hz voltage, it would emit hypothetical magic smoke.
Also applies to induction motors....
 
K8MHZ

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Dennis Alwon said:
This is not a trick question and I have no idea where this thought came from but I wondered whether the trany would notice the difference if DC was injected in it.

Someone asked where would you get 120V DC -- that doesn't matter as it is hypothetical
Click to expand...

The transformer can only convert from 120 VAC to (for instance) 24 VAC. Transformers can't be used to convert DC voltage. Tesla discovered that a couple years before I was born.

Putting DC on a coil makes it into an electromagnet. There is no expanding and collapsing magnetic field like there is in AC, and that is needed to induce current into the secondary coil.
 
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fmtjfw

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K8MHZ said:
The transformer can only convert from 120 VAC to (for instance) 24 VAC. Transformers can't be used to convert DC voltage. Tesla discovered that a couple years before I was born.

Putting DC on a coil makes it into an electromagnet. There is no expanding and collapsing magnetic field like there is in AC, and that is needed to induce current into the secondary coil.
Click to expand...

This imposition of DC on AC transformers was a real issue when Mines started to use rectifiers to create DC from AC utility services. My father, an EE for a POCO, wrote a paper on core saturation in distribution transformers caused by 1/2 wave rectifiers.
 
K8MHZ

K8MHZ

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Pulsating DC works like AC on transformers. That's how automobile coils work. Make and break produces the required magnetic field movement in this case.

So hypothetically, you could use DC pulsating at 60hz with about a 70 percent duty cycle and the doorbell would hypothetically work just fine.
 
Besoeker

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K8MHZ said:
Pulsating DC works like AC on transformers. That's how automobile coils work. Make and break produces the required magnetic field movement in this case.
Click to expand...
Break the current in an inductive circuit and you create a high voltage transient - e=Ldi/dt. Few primary turns and many secondary turns and you get enough volts for a spark plug spark.

K8MHZ said:
So hypothetically, you could use DC pulsating at 60hz with about a 70 percent duty cycle and the doorbell would hypothetically work just fine.
Click to expand...
Not really. Transformers need both positive an negative half cycles to reset the flux. Pulsing DC doesn't do that.
 
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mike_kilroy

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Confucius says "Since Dc don't work like AC, it don't matter."

This is one my standard new sales engineer hire questions... MOST get it wrong!!!!!!!!!!! "Take a set of jumper cables, hook them to your car battery, put them into the primary of a 120V 1ph to 24v transformer, what will the output be?" Sad.
 
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