help to find out the value of a base resister to a transistor

[Andrew Crook]

I need some help to find out the value of a base resister to a transistor.
The only thing is that the transistor is turn on via an rc circuit therefore
the current and voltage will vary over time. I guess I just use the highest
values?

Andrew

 

[Paul Burridge]

I need some help to find out the value of a base resister to a transistor.
The only thing is that the transistor is turn on via an rc circuit therefore
the current and voltage will vary over time. I guess I just use the highest
values?

Can you post the schematic?

 

[Andrew Crook]

here is the basics.....

|
R1 _____
| |
|________R2___TR1
| |
CP1 |
| |____ logic line
|


R1 and CP1 will be chosen for the correct time, R2 needs to be found. TR1
has not yet been chosen but controls a logic line, so most cheap transistors
will do..

many thanks

Andrew

Can you post the schematic?

Winston Churchill

 

[Paul Burridge]

here is the basics.....

|
R1 _____
| |
|________R2___TR1
| |
CP1 |
| |____ logic line
|


R1 and CP1 will be chosen for the correct time, R2 needs to be found. TR1
has not yet been chosen but controls a logic line, so most cheap transistors
will do..

Well, there may be some geniuses here that can come up with a figure
for you, but AFAIC, there's still not enough info shown. For instance
you've got R2 in series with R1 so its value will be in some way
related. Yet you decline to define R1 at this stage. All you really
want is hard saturation when you want the logic line to go high, so
why not just experiment with different values of R2?

 

[Jim Thompson]

Well, there may be some geniuses here that can come up with a figure
for you, but AFAIC, there's still not enough info shown. For instance
you've got R2 in series with R1 so its value will be in some way
related. Yet you decline to define R1 at this stage. All you really
want is hard saturation when you want the logic line to go high, so
why not just experiment with different values of R2?

The usual method is to set "forced-beta" to about 10... that is, RB =
10*RC

...Jim Thompson

 

[Andrew Crook]

I was thinking of R1=10M and CP1=10microparads, I would then have to find
R2. I don't wish to damage any transistors
thats why I don't like trial and error. Thats why I was looking for a
numerical method of calculating R2 and also T=R1/CP1 may be changed,
so a numerical method would be better.

Andrew

Well, there may be some geniuses here that can come up with a figure
for you, but AFAIC, there's still not enough info shown. For instance
you've got R2 in series with R1 so its value will be in some way
related. Yet you decline to define R1 at this stage. All you really
want is hard saturation when you want the logic line to go high, so
why not just experiment with different values of R2?

Winston Churchill

 

[bg]

You need to know how much collector current is needed and what is the
minimum beta of the transistor. From that you calculate base current Ib =
Ic/beta. Multiply Ib by about 10 times to insure it is more than enough to
turn the transistor on.
The base resistor is Rb = (supply voltage - .6 volts)/ base current. When
you get to this point you can experiment with values of C that will give you
the time delay needed without worrying about damage. Or look up RC time
delay on google, for the math. The delay will not be very accurate because
the base voltage varies with temperature.
Andrew Crook wrote in message ...

 

[Andrew Crook]

grate many thanks

could you suggest a transistor to switch a CMOS logic line? I presume a bog
standard one will do

Andi

 

[John Popelish]

grate many thanks

could you suggest a transistor to switch a CMOS logic line? I presume a bog
standard one will do

Andi

Any transistor will do as long as its collector can swing through the
logic range. Are there any frequency concerns?

 

[Andrew Crook]

thanks for your reply

no

Andi

 

[Andrew Crook]

I don't have a multimeter that reads current, Ic would be the current from a
CMOS ic (4093) with a logical high output and running on a 9v supply any
aprox ideas.

I have some bc337-16 transistors but could not find the min beta from the
PDF.

thanx for all your help so far and a merry Christmas

Andrew