Using a transistor as a switch 1

[mcrceng101]

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

 

[OperaHouse]

Use a PNP transistor with the emitter to 12V and collector to the light. A separate NPN transistor is driven by the PIC, its collector drives the base of the PNP with a 330 ohm resistor with a 1K resistor E to B to insure turnoff.

 

[MacGyverS2000]

Any particular reason you're using BJTs rather than FETs? FETs are perfect as on/off switches.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=IRF9Z30PBF-ND

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[sreid]

There are a lot of high side switches available that are used in 12 V automotive applications. Thes generally protect the switch from almost any external problem that might be encountered.

 

[itsmoked]

Okay then here you go. Better for newbies.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=IRSF3011-ND

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[LionelHutz]

That is a low side driver/MOSFET Keith.

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.

 

[itsmoked]

DOH! Thanks LionelHutz.
Teach me to take the Digikey description at face value.

Here is a bonafide HIGHSIDE DRIVER. (I checked!

)

http://www.irf.com/product-info/datasheets/data/ir3315pbf.pdf

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

 

[mcrceng101]

Thanks to all that have responded to this thread.To answer macgyver2000's question. I am not as familiar with FET's as I thought I was with BJT's.

 

[MacGyverS2000]

mrc,

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

 

[GonzaloEE]

After a lot of good suggestions. I'm just going further with this FET-version of that circuit Gunnar proposed:

PIC side:
OnSemi 2N7002 (N-Ch FET 60V 150mA)
Mouser:

http://www.mouser.com/Search/ProductDetail.aspx?qs=PNhECL2hiouLrcURjXOtOg==

gateIC
drain: gate of the other FET
source:GND

Load Side:
Infineon SPP08P06P (P-Ch FET 60V 8.8A)
Mouser:

http://www.mouser.com/Search/ProductDetail.aspx?qs=UzPslXLlT%2bfAjlzGpM%2bPNg==

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!

 

[GonzaloEE]

I meant the above circuit was based on that of Operahouse.
My apologies for this brain cross-connection with another thread

 

[mcrceng101]

Thank you GonzaloEE.

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

 

[LionelHutz]

That looks like a really handy device Keith.

 

[itsmoked]

Hi LionelHutz!

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