High side & Low side swiching?

Other Hardware General Electronics
1

Hi,

May I run this by you good folks before I connect up and fry something :slight_smile:

Baiscally I would like to switch the backlight LED on an LCD on and off via a transistor and I'm not to sure about the high/low side switching thing. The LCD LED in question shares the GND with the rest of the display so the only option open is high side switching - below is the circuit I propose but not sure it its correct? The transistor in there at the mo is a BC817 NPN : http://www.nxp.com/documents/data_sheet/BC817_BC817W_BC337.pdf (the symbol I have used if for a PNP - sorry about that).

BL.PNG

2

For High Side, you want to use PNP.
High into the Gate will be On Off, Low will be OffOn.

3

10K to the base is kind of small, maybe something smaller, like 1K.
How much current do you need?
PNP will also have fixed voltage loss from C to E.
Logic Level P-Channel MOSFET with low Rds (like down in the 10's of milliOhms) might be better.

4

Wow, that was quick, thanks CrossRoads.

The current in question is only 45mA... 60mA MAX. So it look like this circuit will work if I swap out the resistor to 1K and change the transistor to a PNP? Would you be able to recommend a part number - I will need to go out and buy some :slight_smile:

5

This one looks good

http://www.digikey.com/product-detail/en/ZTX553/ZTX553-ND/92534

15mA into the base, switch up to 150mA.

6

Thanks again - as I would like to move this to PCB eventually an SOT23 package option would be ideal, may I ask if this one is suitable for the schematic in question too?

http://www.nxp.com/documents/data_sheet/BC807_BC807W_BC327.pdf

..... or perhaps this one which seems to be a replacement for the above:

http://www.nxp.com/documents/data_sheet/BCW29_BCW30.pdf

7

Either look to be okay for switching 60mA.

8

FAB - I will get some on order and give it a try - thanks for helping me out with this basic question :slight_smile:

9

Glad to help.

10

CrossRoads:
For High Side, you want to use PNP.
High into the Gate will be On, Low will be Off.

slight correction, low into the base to turn on and high into the base to turn off, the emmiter is already high.

11

Thanks, good catch, I went back & fixed it.