# More Questions for AC to DC; inductors

Discussion in 'The Projects Forum' started by Kris Blouch, Jul 8, 2014.

1. ### Kris Blouch Thread Starter New Member

Jul 8, 2014
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Ok, so I'm reading up on Inductors, and to convert ac to dc with an inductor it is as simple as putting it in line with the ac, according to Wikipedia anyway.

I understand the theory, while the positive part of the sign wave charges the inductor, it allows current to travel through the circuit on the other side of the inductor, but when the AC changes direction, the inductor resists this and basically "stops" the flow on the other side. So, sign wave goes in with peaks ranging from 1 to -1 and a square (ish) wave comes out ranging from 1 to 0. I imagine that with the right rating of inductor you could even have sign wave of 1 to -1 one go in and get a relatively flat .5 dc line coming out (half of wave lost to charging, then discharges enough to "overwhelm" ac and continue producing output).

All of this is me guessing. Can someone please explain more about this? Or link me to a thread where this has been explained? Why use 4 diodes instead of an inductor? Is it just because you'd have to have a very specific inductor that might be hard to find, (one perfectly rated for 120 v 60 hz ac) as opposed to more "generic" diodes? or does the switching diodes produce a cleaner signal than the square wave of the inductor.

Also, does voltage drop on the other side of the inductor as it is charging? It would basically act as a resistor in that situation, right? It's used in step up (boost) circuits, for it's discharge property, but couldn't it also be used in a similar way to step down voltage, as in, the voltage while it's charging is used, and then it's switched to bleed off circuit to discharge the extra current. So, two of these circuits working together could reduce voltage, switching between each other with the added benefit of storing energy instead of converting to heat as in a resistor.

Thanks for any help. I also understand I'm basically asking two questions here. Sorry about that.

-Kris

2. ### wayneh Expert

Sep 9, 2010
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I think you've misunderstood what you read in wikipedia. An inductor doesn't rectify AC to DC, you need diodes to accomplish that. It CAN allow the DC portion of an AC signal to pass, while resisting ("choking") the AC part of the signal. But the AC power that comes out your wall outlet, for instance, has no appreciable DC component.

Jul 18, 2013
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I think what you need to explore is both Inductive reactance and impedance, both have a an ohmic value, a value of which is based on a few parameters including inductance, frequency, etc.
Allied to this is LC tuned circuits, both series and parallel.
Max.

4. ### Kris Blouch Thread Starter New Member

Jul 8, 2014
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Perfect! I appreciate the guidance as to where to look. I'm going to read up on lc circuits and induction forces vs resistance. My physics is good (everything makes sense to me when broken into units and put in perspective of conservation of energy) but I'm very new to electrical circuits. Thanks for the help.

So this is what I read on Wiki "They [inductors/chokes] are used to block the flow of AC current while allowing DC to pass"-- this agrees with the first reply and I did make a mistake... when I was reading this I inferred from that statement that it's actually blocking the "negative" of the ac signal and allowing a dc current through from AC. My mistake! Thanks for the catch.

-Kris

Jul 18, 2013
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One example that used to be common in tube circuits in particular, was the Pi filter, so named because the series choke and two parallel capacitors placed at the output of the bridge or rectifier resembled the letter Pi. and was particularly effective of removing the bridge output ripple.
Max.

6. ### tcmtech Distinguished Member

Nov 4, 2013
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Actually you can rectify AC with inductors if they are magnetically biased to favor conductance in one direction and blocking in the other by means of having a fairly large permanent magnet as part of the inductors magnetic circuit.

7. ### wayneh Expert

Sep 9, 2010
13,427
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There's always a smarty-pants around here somewhere!

8. ### crutschow Expert

Mar 14, 2008
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I guess that might sorta work if the magnetic bias caused the core to saturate in one direction but not the other. Otherwise a static magnetic field has no affect on the current in an inductor or the inductance.

9. ### alfacliff Well-Known Member

Dec 13, 2013
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a static magnetic field does make a difference on an inductor, read up on "magnetic amplifiers" a magnetic field changed the inductance of an iron core inductor. try it with a small audio output transformer, measure the primary inductance and put a varying dc bias on the secondary.

10. ### tcmtech Distinguished Member

Nov 4, 2013
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2,445
Yep. It works very well and it's really old tech.

Large DC output arc welder power supplies used that method of current and voltage regulation years ago. Very simple very rugged and highly reliable.

shortbus likes this.