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Controlling power to a load using transistor or otherwise

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mbarren

New Member
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

New Member
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

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Most Helpful Member
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

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

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MikeMl

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...
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:
 

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Nigel Goodwin

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Most Helpful Member
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.
 

MikeMl

Well-Known Member
Most Helpful Member
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

Super Moderator
Most Helpful Member
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.
 
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