amplify voltage

Details Message
Author
Qazi Wajid
Posted
12-May-2011 14:37 GMT
Toolset
C51
amplify voltage

hi

i m using 89c51 , the output voltage of port1 is approx 5 volt, but i need 12 voltage as my requirement, i used simple NPN transistor, but the output is approx same with base voltage, actually i want to use it for digital clock, where 6 LEDs in one series, so 42 LEDs are in one segment,
so how can i get 12 voltage as output for 7 segment, as i told the base voltage is 4.5-5 voltage

Author
PerYo Westermark
Posted
12-May-2011 14:54 GMT
Toolset
C51
RE: amplify voltage

A transistor works well - but must have suitable amplification. And you obviously need 12V somewhere - the output voltage from the processor can turn on/off the transistor but the base voltage will not be amplified. A bipolar transistor doesn't amplify voltages. They amplify current. So they can modulate a current from a separate power supply by controlling the amount of base current.

There are large numbers of examples of how to drive relays from a 8051 chip. This is basically the same thing - you just make use of multiple LED (and a series resistor for current control) instead of the relay coil.

And 12V will probably not be enough supply voltage. You need 6 times the rated voltage of the used LED. Then you need a tiny bit of voltage for the losses in the transistor. But you also needs a couple of volts over the series resistor to control the resulting LED current. Too small voltage over the resistor means that you will get very large variances in light intensity depending on production and temperature variances in the individual LED. If your LED need 2V each, then you should have 15V or more. The more voltage you have over the resistor, the better current control you will get. An alternative is to use a dedicated LED driver chip. They are designed as constant-current generators, taking care of supplying each LED chain with a well regulated current.

I guess "42 LEDs are in one segment" should actually say "42 LEDs for each 7-segment digit".

Author
Andrew Neil
Posted
12-May-2011 15:10 GMT
Toolset
None
basic electronics

This has nothing at all to do with Keil!

It would be far more appropriate at http://www.8052.com/forum