Create delay in transistor output.

[FMoura]

Happy new year for all.
I´m a little problem in my project.

I have a transistor (BC817) working as switch, but I need a little delay in your output.
For exemple:
When it allow current flux and the base goes to low state, I need it hold on flux for 0.5 second or 1 second.
I´m thinking in use a capacitor.



Carregador is between two ICs.

I can make like this...?



How calculate the optimum value to capacitor?!

tnx very much

 

[andre_luis]

The values you gave above for capacitor and resistor theoretically would yield a delay which seems correctly calculated, however, Q1 will reach conduction at Vbe (0,7v) quickly and therefore you must increase either R or C for a greatest value.

Even performing this adjustement, you must take care about:

• Output of previous stage must have an impedance much smaller than 4k7
• Slew rate on output will be too slow, and unless input of following stage of this circuit operate with smith trigger, may expect some problems.

+++

 

[barry]

1 second is not a "little" delay; that's a BIG delay. I would suggest you use an inexpensive timer (555 or similar) to get your time function. You might be able to use a MOSFET with an RC in place of your BJT, but I think there is just too much leakage in a BJT to use an RC to get the long delay you want.

But you SHOULD know how to calculate the delay of an RC. If you don't, you shouldn't be messing with transistors, that's a lot more complicated. (A simple rule-of-thumb is that it will take 5 time-constants for an RC to charge from 0 to 100%)

 

[FMoura]

1 second is not a "little" delay; that's a BIG delay. I would suggest you use an inexpensive timer (555 or similar) to get your time function. You might be able to use a MOSFET with an RC in place of your BJT, but I think there is just too much leakage in a BJT to use an RC to get the long delay you want.

But you SHOULD know how to calculate the delay of an RC. If you don't, you shouldn't be messing with transistors, that's a lot more complicated. (A simple rule-of-thumb is that it will take 5 time-constants for an RC to charge from 0 to 100%)

Yes, yes, I know how calculate. t=RC, but when I have a transistor in the output capacitor must not be so simple.

Well, I´m looking a simplest solution, for this I thought in RC. I don´t need precision in delay, need only start a uC, after, uC can maintain the transistor working.

Edit: I need put a resistor between capacitor and base transistor ?

tnx very much.

- - - Updated - - -

I did some calculations about minimum current in base in relationship at colector and I designed this circuit in protheus ->



It work well, tomorrow I will test in practice. Anyone have any consideration??

Ty for help

 

[andre_luis]

That´s probably what you need: A fast turn on, and a low turn off...





+++

 

[FMoura]

That´s probably what you need: A fast turn on, and a low turn off...


View attachment 100490


+++

Yes! Fast turn on and a low turn off. This is the point.

But I can connect capacitor direct in base of transistor ?!

 

[andre_luis]

I may be wrong, but presume this perhaps could not pose a problem, due biasing current on Q1:B is limited by R2.
Other point is that voltage on C1 never reaches Vbe, and after turn off, will retain approximately 0,7v indefinitely.



+++

 

[FMoura]

I may be wrong, but presume this perhaps could not pose a problem, due biasing current on Q1:B is limited by R2.
Other point is that voltage on C1 never reaches Vbe, and after turn off, will retain approximately 0,7v indefinitely.



+++

ahhhh, now I get the 0.7V you commented. This isn´t a problem.
About for the resistor between the transistor and capacitor, should be for limit the amount of current, not polarization.

Your circuit is more simple than mine so I will test it first.

 

[barry]

I definitely recommend the diode approach.