sourced current resistor

[fran1942]

Hello, I am connecting an AND gate IC from the output of one of my Picaxe 08m pins. This AND gate is then connected to a BJT transistor (the transistor controls current flowing to an LED)
I understand I need a resistor to limit current between the AND gate and the BJT transistor base.
Would I be correct to calculate it like this:

4.9V (Picaxe source voltage) / (say a 1KOhm resistor)
= to give me 4.9mA at the base of the transistor ?

I am just not sure how many amps I should be feeding into the base of the transistor ?

Thanks for any help.

 

[westaust55]

Suggest that you have a look at:
Post 4 here: http://www.picaxeforum.co.uk/showthread.php?17622-Calculating-chip-current&p=161014
And
Post 4 here: http://www.picaxeforum.co.uk/showthread.php?19011

Edit: the above 2 links give step by step details/calcs to determine the resistor value based on transistor data

 

[Goeytex]

The current going to the base of the transistor depends upon

1. Transistor Collector Current
2. Transistor HFE (Gain)
3. Current capability of the Logic Gate

How much current does the LED draw ?
What Logic Series ? And gate part number ?
What transistor part number ?

Provide the above and someone can show you how to calculate pretty close.

 

[MFB]

When calculating the value of the base resistor you can't just use the full supply voltage. Assuming the maximum PICAXE output is 5V, the base resistor will only have about 4.3 volts across it because of the 0.7V base-emitter drop of the transistor.

Anyway, unless you require more than 10mA output current, you probably don't need a transistor and can drive the LED directly from the gate. When calculating the value of the (gate-LED) current limiting resistor, don't forget that the LED will drop about 1.5 volts depending on the colour used.

 

[geoff07]

It is good to understand the theory but there are some simple rules-of-thumb.

Fed from a 5v Picaxe output pin:
Bright led of the normal type (i.e. not high power): 330 ohms.
Dim led (ditto) 1.2k-22k ohms. Good for battery life.

High power leds: PWM via a mosfet and driver chip.

 

[Goeytex]

I'm guessing that fran wouldn't be using a transistor if the LED was a low power (15 ma) one.
However, it could be that the logic gate (eg CD4000 series) cannot drive even a low power LED
and therefore a transistor is needed.

There's a lot we don't know.

 

[MFB]

Agreed, there is a lot we don't know. Even which gate technology is being used. If its CMOS then gates can be connected in parallel to increase current drive to the LED.