Heating element temp, does mains AC vs rectified DC matter?

Thread Starter

myall

Joined Mar 7, 2018
12
(hopefully not dumb questions)

I was wondering if AC vs DC matters if you connect to a resistive heating element?

What I mean is... say I take resistive heating element (heater, hair dryer...) and connect it to 110Vac mains and the element uses 100 watts of power

Now same heating element with mains power full bridge rectified... Would it still use exactly the same 100 watts of power?

Also would adding a large capacitor after the full bridge rectifier do anything?
 

Dodgydave

Joined Jun 22, 2012
11,320
It won't make any difference AC or DC, as long as the voltage is the same, putting a capacitor after the bridge rectifier will increase the voltage.
 

AlbertHall

Joined Jun 4, 2014
12,385
With just the bridge rectifier the power will be the same (except for the small voltage drop across the diodes).
If you add a large smoothing capacitor this will raise the average voltage by a factor up to 1.4 times. This will double the power in the heater and probably cause damage.
 

crutschow

Joined Mar 14, 2008
34,707
Below is the LTspice sim of the three scenarios for a 100V peak AC and a 100Ω load demonstrating what AlbertHall stated:
The rectified output power (green trace) is basically the same as the unrectified output (red trace) with their average (effective) power being near 50W.
The rectified filtered output (yellow trace) has an average power near 100W.

1712842007780.png
 

dl324

Joined Mar 30, 2015
17,017
Now same heating element with mains power full bridge rectified... Would it still use exactly the same 100 watts of power?
110VAC and 110VDC would deliver the same power, but the bridge rectifier will have a voltage drop so AC would be more efficient from a power and cost perspective.
 

nsaspook

Joined Aug 27, 2009
13,443
Even with the same amount of power to a resistive load it matters with the switching control at higher voltages. DC has a nasty tendency to feed continuous (no reversing zero crosses) arcs when interrupting load power.
 

KeithWalker

Joined Jul 10, 2017
3,147
Even with the same amount of power to a resistive load it matters with the switching control at higher voltages. DC has a nasty tendency to feed continuous (no reversing zero crosses) arcs when interrupting load power.
But in this case, we are talking about full wave rectified AC. The voltage goes from peak voltage to zero on each half cycle so that does not apply.
I see Ron beat me to it.
 

nsaspook

Joined Aug 27, 2009
13,443
But in this case, we are talking about full wave rectified AC. The voltage goes from peak voltage to zero on each half cycle so that does not apply.
I see Ron beat me to it.
Yes but one effect with non rectified DC is that the anode and cathode arc melt ion/electron generation wells (the arc roots) don't swap with changing polarity. This also increase the likelihood of contact arcing on disconnects because the zero volts dwell time is short and the KE of the arc plasma charged particles stays going in one direction even when the electric field accelerations drops.

1712847319825.png1712847528579.png

I think more heat is generated at the anode than at the cathode (one or the other) in a typical DC arc.
 
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