Transistor Hook Up

Does the load have to be on the collector side of a transistor? I figured to run http://www.sparkfun.com/datasheets/Components/LED/7-Segment/COM-09483-YSD-439AR6B-35%20indoor%20use.pdf this 7-seg I would use 4 PNPs to control which digit and then 7 NPNs to control the individual segments. But How do I hook up the PNP to the feed of the display if the gnd of the display needs to go through through the collector?

why do you need 4 PnP's?

(this is a "to get you thinking" kind of question)

you can use NpN's, and the load after, as long as the emmiter of the transistor eventually leads to ground (within limits)

The load can go before or after the transistor - but a lot depends on the type of transistor - NPN or PNP. Take a look at this page - got good easy to read info.

http://www.kpsec.freeuk.com/trancirc.htm

Unless you are just wishing to use transistors - would a LED driver chip work better than transistors?

Ken H>

I assumed I need one PNP for each of the digits common anodes which have the potential to pull 160mA each if all the segments were lit. Then I need a NPN transistor for each segment.

Ken H, I was already looking at your link but I got the impression that the load is always connected to the collector. If I used a NPN and went from VCC to the CA, what would I use from the 7-seg to complete the circuit to the collector?

with a npn, plug your collector into +5 base tru 1-10k resistor to arduino digital output emmiter to load (led + led resistor) to ground

Osgeld, so why ever use a PNP? Just if you wanted to sink your output as opposed to source it? If I can hook up both on either the collector or emitter.

with a npn, plug your collector into +5 base tru 1-10k resistor to arduino digital output emmiter to load (led + led resistor) to ground

Ahh, but then what happens to the base to emitter voltage as the transistor (tries) to turn on? I don't think this will do what you intended...

Does the load have to be on the collector side of a transistor?

Having the load on the NPN collector side allows a clear flowpath for the transistor base current to flow to ground. Putting a load on the emitter side of the transistor makes the base current have to flow thru the load, which may result in issues.

Does the load have to be on the collector side of a transistor?

The quick answer is no.
However, it is only when the load is in the collector that you will get any voltage gain. This is known as a common emitter amplifier because the emitter is the same for input and output (ground).

If you have the load in the emitter you have a common collector or emitter follower circuit. With this sort of arrangement the voltage gain is one, but you do get some current gain. That means the load will never experience the full voltage of the supply, the maximum voltage across the load will be the the voltage on the base minus the base emitter voltage drop, normally about 0.7V. If you bear in mind the output of a 5V logic circuit is typically 4.5V and can be as low as 3.5V then you will see that an emitter follower is not usually very useful. Mind you it does allow you to dispense with the base resistor and connect it straight to an arduino output pin.