How to make a voltage switch?

[Sherry]

Hey ,
I have recently started learning about Transistors, MOSFETs, and Diodes etc. We are given a circuit with an AC input voltage (50Hz voltage rectifier) and it should give a 9V DC output.
I would appreciate if anyone can brief me on the principles of PNP transistors along with the bridge and zener diode's effects on the output, so I can decide on the transistor and the values of the components in my project.

By the way, I was trying to work with Traxmaker, but could not export a zener diode! What do I do now?

Cheers

 

[akg]

as you know a rectifier will convert an ac signal to a 'pulsating' dc . to smoothen that you'll need a (large)filter capacitor . a zener is used for low power voltage regulators , and is always used in reverse bias. when reverse biased a 5.1v zener will keep ~5.1v across its terminals .

well that's a short and crude explanation.

see this https://www.electro-tech-online.com/threads/stabilised-power-supply-with-current-limiting.158/

 

[kambliarup]

For transistor operation check this out
https://www.electro-tech-online.com/threads/transistor-operation.33/

 

[audioguru]

If a PNP transistor is used as an emitter-follower to buffer the voltage from the zener diode, then the output voltage would be about 9.3V if a 10V zener diode is used. The voltage is negative, which is odd. An NPN emitter-follower would make a positive output voltage.

 

[Roff]

If a PNP transistor is used as an emitter-follower to buffer the voltage from the zener diode, then the output voltage would be about 9.3V if a 10V zener diode is used. The voltage is negative, which is odd. An NPN emitter-follower would make a positive output voltage.

AG, what circuit are you referring to?

 

[Russlk]

Is there a transformer in the circuit, or is it using pulse width modulation to reduce the input 220 VAC to 9 vDC? What is the load current?

 

[RODALCO]

For safety if you are new to the field use a transformer to step down the mains voltage to 12 volts ac, then experiment with a rectifier and capacitor to get the required voltage.

12 volts ac will give you 16.92 - 1.4 (vdrop diodes) = 15.52 volts across your capacitor.
You can use a LM 317 to get 9 volts or use a voltage divider to bias a zener which can drive a 2 N 3055 transistor to give you the required output
To get 9 volts , zener should be around 9.6 volts, 0.6 v is lost in the base to emitter part to drive the transistor.

You can use a lower transformer voltage if you use the transistor option.
A regualtor like the LM 317 or 7809 will require 3 volts higher at the input to regulate correctly.

 

[RODALCO]

Some more to add

Depending upon current drawn by the load, you need to put the regulator or series pass transistor on a good heat sink.
The voltage drop across it times the current through it equals watts dissipated in heat.

say 15.52 volts in, 9 volts out at 1 amp.
difference 6.52 volts x 1 amp is 6.5 Watts!!

 

[eblc1388]

That advice by RODALCO is especially true if the transformer output is rated for many amps. The filtered voltage will then stay fairly high and your regulator operates fairly hot.

 

[Sherry]

This is getting a bit complicated!Today my instructor was advising us not to use the AC power supply at first, he said we shall start with a DC and then after we found the values for our components, we can change that to an AC.
Even now I am facing other problems, like how to get the minimize PCB. I am working with the traxmaker to do so, but am not sure how to get the foot prints of the components!

 

[Hero999]

Create the footprints yourself. Have a look at the datasheets and measure them.

 

[Sherry]

Is there a transformer in the circuit, or is it using pulse width modulation to reduce the input 220 VAC to 9 vDC? What is the load current?

Hey, could u plz check ur email!
Thnx