Bridge Rectifier (BR) :
1 Bridge Rectifier (BR) Slide 2:
2 120/25V
Transformer 120V
Variac 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 Bridge Rectifier(BR) + – +
≈ 28Vac rms
– 1 4 3 2 Equivalent DC load resistance RL +
≈ 28√2Vdc ≈ 40Vdc
− Iac Idc Be extra careful that you observe the polarity markings on the electrolytic capacitor Variac, Transformer, BR Hookup :
3 Variac, Transformer, BR 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). Slide 4:
4 Example of Assumed State Analysis +
Vac
– Consider the Vac > 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 RL Thus, our assumed state is correct +
– Slide 5:
5 Example of Assumed State Analysis +
10V
– 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” RL +
11V
– +
11V
– 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 − 1V + The bottom diode is reverse biased, and thus confirmed to be “off” Current cannot flow backward through the bottom diode, so it must be “off” Thus our assumed state is correct Auctioneering circuit Assumed State Analysis :
6 Assumed State Analysis Consider the Vac > 0 case + – +
Vac
– 1 4 3 2 RL What are the states of the diodes – on or off? We make an intelligent guess that I is flowing out of the source + node. 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. I cannot flow into diode #4, so diode #4 must be “off.” If current is flowing, then diode #1 must be “on.” Assumed State Analysis, cont. :
7 Assumed State Analysis, cont. +
Vac > 0
– 1 2 RL A check of voltages confirms that diode #4 is indeed reverse biased as we have assumed We see that KVL (Vac = I • RL ) is satisfied Thus, our assumed states are correct A check of voltages confirms that diode #3 is indeed reverse biased as we have assumed The same process can be repeated for Vac < 0, where it can be seen that diodes #3 and #4 are “on,” and diodes #1 and #2 are “off” AC and DC Waveforms for a Resistive Load :
8 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! Bridge Rectifier. :
9 Bridge Rectifier. 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. DC-Side Voltage and Current for Two Different Load Levels :
10 DC-Side Voltage and Current for Two Different Load Levels Approximate Formula for DC Ripple Voltage :
11 Approximate Formula for DC Ripple Voltage Energy given up by capacitor as its voltage drops from Vpeak to Vmin Energy consumed by constant load power P during the same time interval Approximate Formula for DC Ripple Voltage, cont. :
12 Approximate Formula for DC Ripple Voltage, cont. AC Current Waveform :
13 AC Current Waveform Schematic :
14 Schematic Mounting the Toggle Switch :
15 Mounting the Toggle Switch Careful! :
16 Careful! Thermistor Characteristics :
17 Thermistor Characteristics Slide 18:
18 Thermistor in series with 470Ω resistor
Series combination energized by 2.5Vdc
The voltage across the 470Ω resistor then changes with temperature as shown below Measuring the temperature on the backside of a solar panel Slide 19:
19 Measuring Diode Losses with an Oscilloscope T cond i(t) v(t) Slide 20:
20 Forward Voltage on One Diode Forward voltage on one diode Forward voltage on one diode AC Current Waveform :
21 AC Current Waveform The shape is nearly triangular, so the average value is approximately one-half the peak View this by connecting the oscilloscope probe directly across the barrel of the 0.01Ω current-sensing resistor