Controlling power to a load using transistor or otherwise

[mbarren]

Hi,

I spent some time searching for this and I'm finally here:

I want to control power to a load using my PIC uC. First thing that came to mind was to use a transistor and put the load between the 5v+ and collector. That didn't work out since there was a 0.8v drop across collector-emitter. My load needs 5v to operate and my main supply is 5v as well.
Is there a transistor setup with no voltage drop? Or a better way of doing what I want to do? A mere hint would be a great help.

The load (which is a GPS device) draws max. 90mA.

appreciate your help

 

[fernando_g]

You can always use a Mosfet with a low RDSon.
commonly available TO220 low voltage Mosfets have RDSon < 0.01Ω, thus your voltage drop would be negligible.

 

[Nigel Goodwin]

If you were getting 0.8V drop, you weren't driving the base hard enough - you will always get some drop, but a proper saturated transistor should be low enough to work for your application.

 

[ericgibbs]

hi,
A 2N2222 transistor will have a Vce sat of approx 0.1V at 90mA,,, with a 3 to 5mA base current.

 

[MikeMl]

...
I want to control power to a load using my PIC uC. First thing that came to mind was to use a transistor and put the load between the 5v+ and collector. That didn't work out since there was a 0.8v drop across collector-emitter. My load needs 5v to operate and my main supply is 5v as well.
Is there a transistor setup with no voltage drop? Or a better way of doing what I want to do? A mere hint would be a great help.

The load (which is a GPS device) draws max. 90mA.

The problem stems mostly from characteristic of a PIC port. PIC ports do a better job of SINKING current when they are Low compared to SOURCING current when High. Look at the attachment:

 

[BrownOut]

Should be a simple enough problem to solve:

 

[Nigel Goodwin]

The problem stems mostly from characteristic of a PIC port. PIC ports do a better job of SINKING current when they are Low compared to SOURCING current when High. Look at the attachment:

I've no idea what relevence the chart you posted has?, but it's nothing to do with it. Typical PIC's both SINK and SOURCE about 25-30mA per pin.

 

[colin mac]

Post the circuit you tried originally and someone will tell you what's wrong with it.

 

[MikeMl]

I've no idea what relevence the chart you posted has?, but it's nothing to do with it. Typical PIC's both SINK and SOURCE about 25-30mA per pin.

Nigel,

Depends on which PIC. The table I posted is plainly labeled PIC16F87X (where X=3,5,6 or 7), of which I have used dozens of. It plainly states that in the high state, with a 4.5V supply, the Voh is only 4.5-0.7=3.8V when 3mA is being sourced.

You need to download the data sheet for the specific PIC you are using.

 

[Nigel Goodwin]

Nigel,

Depends on which PIC. The table I posted is plainly labeled PIC16F87X (where X=3,5,6 or 7), of which I have used dozens of. It plainly states that in the high state, with a 4.5V supply, the Voh is only 4.5-0.7=3.8V when 3mA is being sourced.

I suggest you read what you posted again, it's not a listing of pin output current capabilities.