ELC4345, Fall2013 Diode Bridge Rectifier (DBR)

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ELC4345, Fall2013Diode Bridge Rectifier (DBR)
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Diode Bridge Rectifier(DBR)
Be extra careful that you observe the polarity
markings on the electrolytic capacitor
Important - never connect a DBR directly to
120Vac or directly to a variac. Use a 120/25V
transformer. Otherwise, you may overvoltage the
electrolytic capacitor
Equivalent DC load resistance RL
Idc

1
3
Iac
28v2Vdc 40Vdc -
120/25V Transformer
120V Variac
28Vac rms
4
2

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Variac, Transformer, DBR Hookup
The 120/25V transformer has separate input and
output windings, so the input voltage reference
is not passed through to the output (i.e., the
output voltage is isolated)
The variac is a one-winding transformer, with a
variable output tap. The output voltage
reference is the same as the input voltage
reference (i.e., the output voltage is not
isolated).
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Example of Assumed State Analysis
RL

Vac

• Consider the Vac gt 0 case

• We make an intelligent guess that I is flowing
  out of the source node.

• If current is flowing, then the diode must be
  on

• We see that KVL (Vac I RL ) is satisfied

• Thus, our assumed state is correct

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Example of Assumed State Analysis
11V
Auctioneering circuit
10V
11V
- 1V
RL

• We make an intelligent guess that I is flowing
  out of the 11V source

• If current is flowing, then the top diode must be
  on

• Current cannot flow backward through the bottom
  diode, so it must be off

• The bottom node of the load resistor is connected
  to the source reference, so there is a current
  path back to the 11V source

• KVL dictates that the load resistor has 11V
  across it

• The bottom diode is reverse biased, and thus
  confirmed to be off

• Thus our assumed state is correct

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Assumed State Analysis

1
3
What are the states of the diodes on or off?
RL
Vac
4
2

• Consider the Vac gt 0 case

• We make an intelligent guess that I is flowing
  out of the source node.

• I cannot flow into diode 4, so diode 4 must be
  off. If current is flowing, then diode 1 must
  be on.

• I cannot flow into diode 3, so diode 3 must be
  off. I flows through RL.

• I comes to the junction of diodes 2 and 4. We
  have already determined that diode 4 is off.
  If current is flowing, then diode 2 must be
  on, and I continues to the Vac terminal.

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Assumed State Analysis, cont.
1
RL
Vac gt 0
2

• A check of voltages confirms that diode 4 is
  indeed reverse biased as we have assumed

• A check of voltages confirms that diode 3 is
  indeed reverse biased as we have assumed

• We see that KVL (Vac I RL ) is satisfied

• Thus, our assumed states are correct

• The same process can be repeated for Vac lt 0,
  where it can be seen that diodes 3 and 4 are
  on, and diodes 1 and 2 are off

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AC and DC Waveforms for a Resistive Load
With a resistive load, the ac and dc current
waveforms have the same waveshapes as Vac and Vdc
shown above
Note DC does not mean constant!
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EE362L_Diode_Bridge_Rectifier.xls
Diode bridge conducting. AC system replenishing
capacitor energy.
Diode bridge off. Capacitor discharging into
load.
From the power grid point of view, this load is
not a good citizen. It draws power in big
gulps.
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DC-Side Voltage and Current for Two Different
Load Levels
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Approximate Formula for DC Ripple Voltage
Energy consumed by constant load power P during
the same time interval
Energy given up by capacitor as its voltage drops
from Vpeak to Vmin
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Approximate Formula for DC Ripple Voltage, cont.

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AC Current Waveform
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Schematic
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Mounting the Toggle Switch



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Careful!
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Thermistor Characteristics
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Measuring Diode Losses with an Oscilloscope

i(t)
v(t)
T

cond

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Forward Voltage on One Diode
Forward voltage on one diode
Forward voltage on one diode
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AC Current Waveform
View this by connecting the oscilloscope probe
directly across the barrel of the 0.01O
current-sensing resistor
The shape is nearly triangular, so the average
value is approximately one-half the peak