Using a transistor as a switch
Using a transistor as a switch
(OP)
Hello everyone.
I am new to Eng-Tips.com I mainly work on the mechanical side of a project.
I am currently working on an application requiring lights be turn on by the output of a PIC16C71 micro controller.
The source is 5Vdc @ 20mA and the switched output is 12Vdc @ 1A. I have tried a number of NPN transistors and Darlington pairs but have not been able to get the required results. One special requirement is that the load be in series with the emmiter to ground. This is because the lights share the same ground plan as the rest of the equipment. All of examples I have looked at have the load in series with the collector.
Any help is welcomed.
Thank you
I am new to Eng-Tips.com I mainly work on the mechanical side of a project.
I am currently working on an application requiring lights be turn on by the output of a PIC16C71 micro controller.
The source is 5Vdc @ 20mA and the switched output is 12Vdc @ 1A. I have tried a number of NPN transistors and Darlington pairs but have not been able to get the required results. One special requirement is that the load be in series with the emmiter to ground. This is because the lights share the same ground plan as the rest of the equipment. All of examples I have looked at have the load in series with the collector.
Any help is welcomed.
Thank you





RE: Using a transistor as a switch
RE: Using a transistor as a switch
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Using a transistor as a switch
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Using a transistor as a switch
RE: Using a transistor as a switch
http:/
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Using a transistor as a switch
High side drivers typically use a n-channel FET and require an isolated power supply that is higher than the circuit power supply by about 15V to turn it on. If you are constantly switching the device then this requirement is not a big deal but for constant-on it makes things more difficult.
You could also use a p-channel FET but they typically have a higher on resistance and more losses.
The simplest could be a PNP transistor if a little extra power loss in the transistor is OK for the circuit. If you are going to maximum efficiency then a FET is the way to go.
RE: Using a transistor as a switch
Teach me to take the Digikey description at face value.
Here is a bonafide HIGHSIDE DRIVER. (I checked!
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The nice thing is they are bullet proof. Over temp, and over current, non-destructive limiting.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Using a transistor as a switch
RE: Using a transistor as a switch
You're going to waste a LOT less heat using a FET than a BJT, which means smaller/cheaper component, less/no heatsinking, etc. Unless you're doing signal amplification, switches are best implemented using FETs due to their high input impedance and low on-resistance (particularly at high current levels).
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Using a transistor as a switch
PIC side:
OnSemi 2N7002 (N-Ch FET 60V 150mA)
Mouser:
http://ww
gate:PIC
drain: gate of the other FET
source:GND
Load Side:
Infineon SPP08P06P (P-Ch FET 60V 8.8A)
Mouser:
http://
gate:drain of the other FET
drain:load
source:12V rail
PIC pins will be likely tristated at power-up. Add a pull-down resistor of some 1Mohm at the driving pin, to prevent rattling your load at start.
Also add a pull-up resistor around 10K at the gate of the load side FET.
Good Luck!
RE: Using a transistor as a switch
My apologies for this brain cross-connection with another thread :)
RE: Using a transistor as a switch
This project has been a learning experence.
After let the smoke out of a few transistors I have ordered from Fairchild the FET’s that should bring this project to a close. The circuit is very much like the one you and Operahouse describe.
Again,thanks to all that have responded
RE: Using a transistor as a switch
RE: Using a transistor as a switch
They have saved my bacon on numerous occasions. I try to use them, or something similar when the project can afford it. A lot of what I do is industrial controls, so a little added cost to save a control system is usually a better trade off.
Besides they prevent the dread "prototype smoke".
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com