Choosing a transistor

[The Master]

Thanx for pointing that one out. Its nice and cheap too.

I do still have a few questions though. I dont want to just use the one you picked, i want to understand why you picked that one.

I see in the datasheet Collector-Emitter Voltage: 30V and Collector Current: 500mA so i guess thats one of the first things i should check. Both of those value must be equal to or greater than the requirement for the load.

Is it the Base-Emitter On Voltage that says how many volts you need to put into the base to make it turn fully on?

Why did you use the 10K and 100K resistors? The 100K looks to be a simple grounding resistor that keeps the base grounded while there is no input from the PIC. The PIC im using (and i thought all pics) grounded output pins when you set them to low. In this case would i still need that resistor?

Where you have put the 10K i normally have a 1K or nothing at all. I assume its to limit the current that gets to the base but how do you know what value resistor to use?

[Luciano]

Hi,

The MPSA14 is a Darlington transistor. (See datasheet page 1).

http://www.rapidonline.com/netalogue/specs/81-0142.pdf


The Darlington transistor (often called a Darlington pair) is a semiconductor
device which combines two bipolar transistors in a single device so that the
current amplified by the first is amplified further by the second. This
configuration gives a high current gain and can take less space than two
separate transistors because the two transistors can use a shared collector.

* * *

The 100k resistor can be omitted if you can afford to have the LEDs in an
unpredictable state during the initialization of the microcontroller.

The 10k resistor must be present.

Best regards,

Luciano

[The Master]

That makes sense. I wont need the 100K resistor then. It takes less than a second for the PIC to do whatever it needs to do on startup and if all the LEDs flash slightly as the PIC turns on then it doesnt really matter. I was reading something in the PIC datasheets about which pins are in what state when it first turns on so i guess that will affect it but its still not a problem. The circuits are going to be designed so all the LEDs can be on at the same time so theres no danger of using too much current.

Thanx for everyones help!

[rmteo]

If you need more than 1 transistor, for about 0.50c you can get 7 darlingtons (50V/500mA, diodes etc.) in a 16-pin package:
http://focus.ti.com/docs/prod/folder...uln2003ai.html

[skimask]

If you need more than 1 transistor, for about 0.50c you can get 7 darlingtons (50V/500mA, diodes etc.) in a 16-pin package:
http://focus.ti.com/docs/prod/folder...uln2003ai.html

But if you look at Figure 15 of the datasheet, you'll see you're limited to a little over 100mA of collector current across all 7 outputs at 100% duty cycle.
And besides, he doesn't have 'the room' for a 16PDIP or TO-220's.
Sounds like it's time for a redesign...

[The Master]

Usually when i have the need for transistors i also have the need to dot them around the PCB so they can be squeezed into small areas. Having an array of them in a DIL package may be smaller in all but it cant be split up to fit into the little spaces i have spare. As it happens ive redesigned this circuit and all the transistors are in a line but i do only need 4 of them.

Normally a TO220 package wouldnt be a problem especially if i need a lot of current but this time i only need 200mA and i dont have the vertical space for them. There will be 2 PCBs that are only 10mm apart. One of the voltage regulators will be soldered to the underside of the PCB and both of them will be folded over to be parallel with the PCB.

There will be a bigger version of this light with more LEDs but for this one im limited with space. Ive redesigned the circuit to use 4 of those transistors Luciano recommended and they all fit on fine

[The Master]

I just noticed something. Do you mean MPSA14 or MPSA13? The datasheet you posted is for MPSA13 and thats what ive been reading. The order code you posted is for MPSA14 though. In the datasheet for the MPSA14 it says Ic is 1.2A. The datasheet ive been looking at says that Ic means collector current and that one is only 500mA. Both look suitable to me except the MPSA13 is a little cheaper.