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In this circuit opamp compares the output voltage divided by R1 and R2 with the zener voltage so the output voltage will be Vload=Vzener*(R1+R2)/R1 and transistor will conduct just enough current to control the output vltage at this level ..
The whole control current in this case is conducted by the transistor ..
Regards,
IanP
how about the Vcc and Vee for the op amp, do we need to connect the Vin as Vcc, and Ground as Vee? so in open loop, the output would be Vcc or 0 Volt? is it the collector of the transistor in reverse biased? if the output from comparator is Vcc, the base current would be in forward biased, so, what will happen to the transistor? is it saturate, cut off, or active?? and is it the function of R3 there as series resistor to the zener, so it limit the current to the zener diode, and prevent exceeding current to flow into the zener?
Vcc=Vin
Vee=GND
In "open loop" situation the opamp will try to forse the output to go HIGH, but real opamps have current limit in output stages (≈10-20mA).
Depending on currents in the circuit it may be enough (Ic≈β*Ib) to saturate transistor ..
As you said, R3 limits current to the zener diode ..
Regards,
IanP
The advantage is that a shunt regulator can usually be designed to work with a lower voltage overhead than a series regulator, eg a typical 7805 series regulator needs Vin to be a minimum of about 3V avove the load voltage.
The disadvantage is that in typical applications shunt regulators.are less efficient than series regulators. More power is wasted as heat.;
The advantage is that a shunt regulator can usually be designed to work with a lower voltage overhead than a series regulator, eg a typical 7805 series regulator needs Vin to be a minimum of about 3V avove the load voltage.
The disadvantage is that in typical applications shunt regulators.are less efficient than series regulators. More power is wasted as heat.;
when using a PNP transistor as pass element, the voltage drop across the pass element can be in the 100mV range. There are several so-called Low Drop Out (LDO) regulators that also need far below 1V overhead.
Agree with the efficiency, especially with varying loads.
ok thanks at all,,problem solved,,i didn't read the whole article
To develop a voltage for the adjustable set-point, we used a 15 V, 1 W zener diode and a 4.7 K trim-pot. To calculate the series resistor for the zener, we just used:
VOLTAGE DROP (20 - 15) V
R = ------------ = ------------- = 250 Ohms. We used 200 Ohms.
ZENER CURRENT 20 mA
WATTS = (5V) x (5V) / 200 = 125 mW (Use 1/4 Watt)
and just use 5.1V 20mA zener
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