transistor as a switch

[ponsakthi]

hi all,
i am having bc 547 npn transistor,
i need it to act as a switch.
supply voltage is 3.3 volts.
please guide me in designing the circuit(like rb rc values etc)
i need to drive 300milli amp.
input will be 50 milli amp

 

[duffy]

It sounds like perhaps your beta is 60, yes?

 

[ericgibbs]

hi all,
i am having bc 547 npn transistor,
i need it to act as a switch.
supply voltage is 3.3 volts.
please guide me in designing the circuit(like rb rc values etc)
i need to drive 300milli amp.
input will be 50 milli amp

hi,
The BC547 has a Ic max of 100mA.

 

[ponsakthi]

even if it drives 100ma ,its enough.
my requirement is a 50 ma i/p should drive 100 m/a o/p device with a supply voltage of 3.3 volts

 

[ericgibbs]

even if it drives 100ma ,its enough.
my requirement is a 50 ma i/p should drive 100 m/a o/p device with a supply voltage of 3.3 volts

hi,
Why did you state a 300mA requirement if you only need 100mA.?

i need to drive 300milli amp.
input will be 50 milli amp

Also you do not need a 50mA base drive current to saturate a BC547 with an Ic=100mA.
It has an hFE of at least 100 at 100ma, so I would use a base drive of say 5mA max.

So, from 3.3V, allowing a Vbe of 0.7V = 2.6v/0.005 = 520R... pref value say of 470R.

 

[ponsakthi]

eric
in my board i have only a 3.3v supply with 50ma .... with this i need to drive
a component of 3.3v 100ma ....
please say me how to calculate rb and rc.
i am not going to connect the load across vcc and collector of transistor.
So i need to put a resistor (Rc) instead of connecting load.

hope u get it..

 

[ericgibbs]

eric
in my board i have only a 3.3v supply with 50ma .... with this i need to drive
a component of 3.3v 100ma ....
please say me how to calculate rb and rc.
i am not going to connect the load across vcc and collector of transistor.
So i need to put a resistor (Rc) instead of connecting load.

hope u get it..

hi,
If I understand correctly.
The 3.3V supply on your pcb is rated at only 30mA.?

If you need 3.3V at 100mA for an external device you will require another, external power supply.?

Is this what you mean.?

 

[ponsakthi]

ya,
but an transistor can do this current amplification know.

that 3.3 v 50 ma is from my micro controller .
and 3.3 v 100 ma is LCD .
i need to turn ON and OFF the LCD by my code.
But my micro controller cant directly do that because of low current rating.
that s why i go for a transistor switch.

 

[ericgibbs]

ya,
but an transistor can do this current amplification know.

that 3.3 v 50 ma is from my micro controller .
and 3.3 v 100 ma is LCD .
i need to turn ON and OFF the LCD by my code.
But my micro controller cant directly do that because of low current rating.
that s why i go for a transistor switch.

hi,
I dont understand what you mean by that 3.3 v 50 ma is from my micro controller are you talking of the MCU pin output current rating.?
If so use a 470R from the MCU pin to drive the base of a transistor.

When you say switch the LCD on/off do you mean the lcd backlight or the actual lcd display.??

EDIT:

 

[hoangson]

hi,
I dont understand what you mean by that 3.3 v 50 ma is from my micro controller are you talking of the MCU pin output current rating.?
If so use a 470R from the MCU pin to drive the base of a transistor.

When you say switch the LCD on/off do you mean the lcd backlight or the actual lcd display.??

EDIT:

oh ! I think he wants to control lcd backlight ! but he doesn't understanding to use lcd.....
I think if he wants to control lcd it's all depented on code in MCU.. to on/off lcd... and the current supply for Backlingt that can be controlled by a resistor variable..

 

[ponsakthi]

i need the LCD to get completely switched off ,both backlight and display.

 

[ericgibbs]

i need the LCD to get completely switched off ,both backlight and display.

hi,
What current is specified for the backlight leds for the lcd.?

Why do you want to power off the actual led.?

 

[ponsakthi]

eric,
your circuit is right but. i need just a wire coming out from the collector.
shall i replace the load in ur figure with some resistor and take a o/p wire from the collecor ....

 

[ponsakthi]

backlights drive around 200ma.
the lcd in my circuit wastes power.
so i need the lcd to get turned on only when i need

 

[ericgibbs]

backlights drive around 200ma.
the lcd in my circuit wastes power.
so i need the lcd to get turned on only when i need

hi,
If your backlight requires 200mA to power the leds, the BC547 will not be OK.

Does the led datasheet state the forward voltage drop of the lcd , leds.??

A typical lcd only draws about 1mA.!!! You will probably use more power in the switching circuit than you save.

 

[ponsakthi]

i am using an nokia 6100 . just now i checked data sheet.
the lcd backlight uses 5o ma.

 

[ponsakthi]

i mean i am using nokia 6100 lcd

 

[ericgibbs]

i am using an nokia 6100 . just now i checked data sheet.
the lcd backlight uses 5o ma.

I have looked at the lcd spec, it says its 7V.! at 40 to 50mA for the backlight its normally controlled by PWM.

The lcd logic power is so low 2mA, IMO its not worth the power saving

 

[ponsakthi]

what to do ericc.

 

[ponsakthi]

eric in port b if microcontroller, o/p current is 25 ma. lcd backlight is 50ma. how do i drive