7hc595 Voltage Out.

Having a look at the 7hc595 datasheet and I was scratching my head why at 100 ohms on the base, I was still only getting a total a 1.2 amps, then i realized it’s USB voltage, so it’s probably closer to 3.8v (i’ll check in a few minutes) but anyway I plan on increasing the voltage to 5.8v to drive the transistor harder…

6v a little excessive to do all the time?

What are you babbling about? Your thoughts are just all over the place in that question.

Are you driving a transistor from a '595 output? Is the 100 ohms a resistor
in the circuit or the approximate output resistance of the '595 (which seems
plausible).

You always need a base resistor to limit the current from the thing driving
the transistor and avoid cooking it or the transistor.

I think you really need to spend more time learning how things work (be analytic) and less time flailing. I'm wondering if more complex electronics is not in the cards for you right now.

A BJT transistor is not "driven more" by increasing the voltage. The Base pin is driven by current... plain and simple. Using higher voltage only means your don't know what you are doing.

Draw out on paper what you are trying to do. Take the time to analyse each problem and solution in your design. Also, keep in mind that is would be very difficult for a TTL signal to drive a power transistor directly. This is why you often see a low power, high gain transistor driving a low gain high power transistor. Sometimes a solution requires more than 2 or 3 parts.

Also. if your power source was USB based... there is commonly a current limiting power controller in the path.

pwillard:
I think you really need to spend more time learning how things work (be analytic) and less time flailing. I'm wondering if more complex electronics is not in the cards for you right now.

A BJT transistor is not "driven more" by increasing the voltage. The Base pin is driven by current... plain and simple. Using higher voltage only means your don't know what you are doing.

How insultive mike2.

If a bjt's current is not increased with voltage.... (how i'm holding back)

Explain why both my circuit and Ohm's law disagrees entirely! Go read a basic electronics.

4.2v/100
42ma

6v gives 62ma depending on the transistor and hfe, that extra current is a lot for a bjt... yes these transistors ARE heatsinked... i'll get back to you on how much current the 7hc draws from each pin.

In my other thread...

I'm aiming for a 400 ma draw from 2n2222 as i have seatsinked them all with heatsink plaster. Multiply x 3 x 5 10 watt leds, with usb voltage i'm getting a lot less via the pwm out of arduino to multiplex the rgb channels... otherswise i'd be driving the led's too hard...

So my options are dump the 2n2222 or increase the voltage.... 6v works but i've not checked the current draw arduino side, i better do.

See page 2:

(IC = 500 mAdc, IB = 50 mAdc)
To get 400mA thru a 2N2222 you need a lot more current than you can get from a HC595.

http://forum.arduino.cc/index.php?PHPSESSID=ojkosvqsv5f3s78vlubp4l5ls4&topic=160532.0

Lets see, a 74Ch595 puts out 6mA at 5V,

So what’s the plan to get a lot more current, and still keep total current out to <70mA?

CrossRoads:
Lets see, a 74Ch595 puts out 6mA at 5V,
http://www.ti.com/lit/ds/symlink/sn74hc595.pdf
So what’s the plan to get a lot more current, and still keep total current out to <70mA?

Shifting up to 6v … i’ll work out the current draw from the arduino later… (without resistor)

So you'll get 35mA from 2 or 3 pins on a port at a reduced voltage then:

  1. Although each I/O port can source more than the test conditions (20mA at VCC = 5V, 10mA at VCC = 3V) under steady state conditions (non-transient), the following must be observed:
    ATmega48A/PA/88A/PA/168A/PA/328/P:
    1] The sum of all IOH, for ports C0 - C5, D0- D4, ADC7, RESET should not exceed 150mA.
    2] The sum of all IOH, for ports B0 - B5, D5 - D7, ADC6, XTAL1, XTAL2 should not exceed 150mA.
    If IIOH exceeds the test condition, VOH may exceed the related specification. Pins are not guaranteed to source current greater than the listed test condition.

Here’s some data comparing a 2N3904 and a PN2222 (attached).