Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

12v to 9V question

Status
Not open for further replies.

nicksydney

New Member
Hi,

I'm planning to build this circuit (**broken link removed**) and I have a question that I'm confused about, what is the function of connecting the middle of C2/R1 to the base of the transistor ?, I thought the base of the ECG184 is control by Z1 ?

Thanks for your help

Cheers
 
The transistor cannot function unless current is flowing up and into the base. If you had only Z1, but not R1, this would be impossible. Current cannot flow from the collector to the base as this junction is reverse biased. So, R1 provides the base current. It also provides the current that flows downwards through Z1.

It is correct to say that Z1 controls the VOLTAGE at the base terminal, but it does nothing to provide the current for that terminal.
 
The transistor cannot function unless current is flowing up and into the base

Cool..I understand this part

Current cannot flow from the collector to the base as this junction is reverse biased

Ok my understanding is current mostly flow from collector and base to emitter (when the emitter is connected to GND), now in the circuit I notice that the collector and emitter are both connected to (+) how does this work ?

It also provides the current that flows downwards through Z1

Correct me if my understanding is wrong, but the current does not flow "through" Z1 because it's diode ?

Thanks
 
The diode is not an ordinary diode that would be reverse-biased and therefore would not conduct.
Instead the diode is a Zener diode that conducts when its reverse voltage equals or exceeds its rating (9.1V or 10V). It needs current from R1 to conduct.
The base of the transistor also needs current from R1 to conduct.
 
Last edited:
Ok my understanding is current mostly flow from collector and base to emitter (when the emitter is connected to GND), now in the circuit I notice that the collector and emitter are both connected to (+) how does this work ?

The + at the left, connected to the collector is the highest voltage. The voltage we see coming from the emitter will be lower because the current there flows through the transistor from collector to emitter and the transistor has some voltage drop, so the voltage at the emitter must be lower. In fact the voltage at the emitter must be the voltage across Z1 (9.1V) minus the base-emitter voltage drop of the transistor which will be about 0.7 volts, or 8.4 V.

Correct me if my understanding is wrong, but the current does not flow "through" Z1 because it's diode ?

Actually this is not a regular diode, it is a zener diode. The schematic has the wrong symbol for this part. A zener diode does indeed conduct current in the reverse direction as long as its zener voltage theshold has been exceeded at its cathode.
 
The circuit is in part an emitter follower, when you connect +v to the collector and the load to the emitter then the emitter voltage will 'follow' the volatge on the base - a bit, the advantge of this is that only a small reference current is required to regulate a larger emitter current (collector current/hfe).
The resistor biases both the zener diode so that it can maintain its rated voltage and the base of the tranny at the same time, the cap is needed to filter noise from the zener and the incomming supply, zeners are often used as a noise source they make a fair bit so the cap is needed and preferable a good quality one.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top