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.

Does capacitor trigger transistor?

Status
Not open for further replies.

SimonTHK

Member
If I have a capacitor + attached to transister "in" + will the transister then "trigger"?

I know that a transistor is current controlled, so will it start only by a capacitor or will I need a resistor in between to draw some current? Cause a capacitor itself shouldnt draw any current.... I think.


My project is to charge a capacitor to "trigger" the transistor for some time. The transistor will then open for current and turn on a 12v relay.
 
I think you might be confusing a transistor with a thyristor?

Transistors don't trigger.

A thyristor triggers when the gate voltage exceeds a certain threshold.
 
Any capacitor charged to more than 0.6V will 'trigger' a transistor.

The amount of time it will remain triggered depends on the voltage, the internal resistance of the capacitor and transistor's base-emitter junction.

If the capacitor is large, the transistor will be destroyed because the sharp current surge will destroy the base junction. This can be mitigated by adding a base resistor.

The current drawn by the emitter load and the base current will determine the saturation voltage.

EDIT:
I think this is going to be a long thread.

Please post a schematic illustrating what you mean by using a capacitor to trigger a transistor.
 
Last edited:
A fast drawing, I might need a transistor more, but just to get a quick overview of my idea how the capacitor should work.

The actual idea is to make a timer (not an accurate timer).

Nothing should happen before the switch is pushed just a half second, and then charge the capacitor that will lead current to the transistor for some time.

I want to do this because right now I just use big capacitors to held a short time. This might make me able to get more time with a smaller capacitor. But can it work? In my head, the capacitor should be able to charge without sending the charge current through the transistor, tus not damaging the transistor.

I wont use this timer IC :) I want to make this work. Ill go to bed, but am back tomorrow morning.
Night
 

Attachments

  • untitled..JPG
    untitled..JPG
    14.8 KB · Views: 522
You might want to give this a read and then think about placing a resistor in series with your switch so the capacitor can charge through the resistor. You may also want to think about your source voltage to your switch too because as Hero mentions once you get to about .6 volt your transistor is going to turn on.

Ron
 
It is a "burn out" circuit.
The circuit as shown will charge the 1nf capacitor as fast as possible then the unlimited current will burn out the base-emitter junction of the transistor.
Then when the transistor turns off the high voltage flyback spike will destroy the transistor even more.
 
or no, I wont make a drawing right now. But I would have placed a 24k resistor in between the capacitor and transister to lead 0,5ma current to transistor.
 
You might want to give this a read and then think about placing a resistor in series with your switch so the capacitor can charge through the resistor. You may also want to think about your source voltage to your switch too because as Hero mentions once you get to about .6 volt your transistor is going to turn on.

Ron

The capacitor should charge as fast as possible, so I dont like the idea of slowing it with resistor :)
 
The capacitor should charge as fast as possible, so I dont like the idea of slowing it with resistor :)

Whatever as I have no clue what you are trying to do? However Audioguru sums the circuit description (as it was drawn) up nicely:

It is a "burn out" circuit.
The circuit as shown will charge the 1nf capacitor as fast as possible then the unlimited current will burn out the base-emitter junction of the transistor.
Then when the transistor turns off the high voltage flyback spike will destroy the transistor even more.

You will need plenty of transistors. :)

Ron
 
Whatever as I have no clue what you are trying to do? However Audioguru sums the circuit description (as it was drawn) up nicely:



You will need plenty of transistors. :)

Ron

Ok but what if I put 24k resistor in between transitor and capacitor.

12v/0,0005ma= 24000 ohm

Then it should maximum get 0,0005ma to transistor, or am I wrong?
 
A very crude timer can be made using a single transistor.

The delay will depend on how long it takes for the capacitor's voltage to drop below the level required to provide enough base current to switch the relay on.

The delay will depend on the capacitance, base resistor, power supply voltage, transistor gain and the relay.

EDIT:
A better timer circuit can be made using two transistors, an arrangement known as the monostable multivibrator (see Wikipedia).

An even better monostable can be made using the 555 timer IC.
 

Attachments

  • Crude BJT RC time&#1.PNG
    Crude BJT RC time&#1.PNG
    748 bytes · Views: 1,136
Last edited:
If I have a capacitor + attached to transister "in" + will the transister then "trigger"?

I know that a transistor is current controlled, so will it start only by a capacitor or will I need a resistor in between to draw some current? Cause a capacitor itself shouldnt draw any current.... I think.


My project is to charge a capacitor to "trigger" the transistor for some time. The transistor will then open for current and turn on a 12v relay.

Back to your original post and I am still not sure what you are trying to do? Anyway read ahat Hero posted in detail and look at the circuit he posted.

Additionally as to "triggering" a transistor the only transistor type I have heard the term triggering used with is a uni junction transistor. A Google of Uni Junction transistor circuits will bring up some interesting results. Start here if that trips your trigger but I would focus on what Hero is saying in his post.

Ron
 
A very crude timer can be made using a single transistor.

The delay will depend on how long it takes for the capacitor's voltage to drop below the level required to provide enough base current to switch the relay on.

The delay will depend on the capacitance, base resistor, power supply voltage, transistor gain and the relay.

EDIT:
A better timer circuit can be made using two transistors, an arrangement known as the monostable multivibrator (see Wikipedia).

An even better monostable can be made using the 555 timer IC.


This is very much what I had in mind.

Though I am new and might did misunderstand how a transistor works. Now I can go read but I can as well put up two options.

1. A transistor works just like a relay and open up for an external current source to run through it.

2. A transistor works like a relay but also choose the amplification of the current to run through, by changing the current run to P (NPN transistor).

2 is correct right?

thanks in advance and thanks for the good help in general.
 
Last edited:
Neither are really correct.

In simple terms a transistor can act as a switch, when the base voltage rises above about 0.6V it will start to turn on, allowing current to flow through the collector to the emitter.

In reality it's not that simple because the transistor will not be fully on if the base voltage is above 0.6V but the current is tiny. The higher the base current, the more the transistor will turn on, meaning the lower the voltage between the emitter and collector will fall. Eventually the collector voltage will reach a point when a further increase in the base current won't reduce the voltage any more, this is known as the saturation voltage. As a general rule of thumb this occurs when the bas current is one tenth of the collector current but it varies widely from transistor to transistor. Driving transistors into saturation is desirable because it keeps the power loss as low as possible.
 
A NPN transistor connected as a common emitter amplifier: the base needs current to do its thing.
The collector cannot output voltage, it can only cause more or less current to be drawn through its load resistor. If a voltage is applied to the base resistor a current now flows into the base (base emitter junction). If a resistor is connected between the collector and a positive supply voltage: the collector current flowing through the collector or load resistor causes a voltage to be dropped across said load resistor.

This page is a good read on how a transistor works and is kept very simple to understand.

Ron
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top