Controlling 12V with 5V (transistor)

[KansaiRobot]

I know this must be really basic but I am trying to control a 12V circuit with a PIC (obvious 5v)

so I tried the following in the attached picture

the idea is


I apply ON/OFF (5V/0V) to the base with a RB=82K and a RC=300Ω

I need to generate a signal ON/OFF (12V/0V) in the arrow (for a H-bridge...)

but in the point with the arrow I only get 12V to 8V (and inverted!!!!)

What am I doing wrong????

I tried to put the resistor in the emitter and it goes from 0v to 2V

Help please

Kansai

 

[misterT]

Try a totem pole. Here is a picture where totem pole is used to drive a MOSFET.

**broken link removed**

 

[ericgibbs]

I know this must be really basic but I am trying to control a 12V circuit with a PIC (obvious 5v)

so I tried the following in the attached picture

the idea is


I apply ON/OFF (5V/0V) to the base with a RB=82K and a RC=220Ω

I need to generate a signal ON/OFF (12V/0V) in the arrow (for a H-bridge...)

but in the point with the arrow I only get 12V to 8V (and inverted!!!!)

What am I doing wrong????

I tried to put the resistor in the emitter and it goes from 0v to 2V

Help please

Kansai

hi,
Your drawing does not show the transistor type ie: npn or pnp.
I suspect you are using a pnp, which you should not in that circuit.??

If you used a npn it can switch +12V with a 5V PIC signal, but it will be inverted.

As simple method is to use a 74LS07 IC.

EDIT:
attached pdf

 

[crutschow]

Try a totem pole. Here is a picture where totem pole is used to drive a MOSFET.

**broken link removed**

You need a series input base resistor to limit the current.

 

[shimniok]

Thanks for posting this question; I was just about to ask it myself.

My situation is a 5V Atmega168 controlling a 12V 2A motor.

I found this thread that may offer some help too, I don't know. https://www.electro-tech-online.com/threads/ttl-current-switching.31367/

 

[ILikeComputers]

Why don't you just use a mosfet? They can handle much more current.

 

[KansaiRobot]

Sorry . It is a NPN.
Anyway I suspect that my error was that I was not saturating the transistor.
I changed RB to 10K and now it works to some extent although inverted.

When I apply 5V to base I got 0.5V to the bridge
when I apply 0v to the base I got 12V to the bridge

I suppose I will invert the base signal to make it correct.

I am not calculating anything about currents... I am a little bit worried about that.
edit: I measured the current going to the bridge . goes to 0.0375A or 37.5mA


I tried changing RC to 550 and now I got 22mA and good 0.17v~12V.
so I guess changing RC reduces the current without affecting the voltage. News to me

hi,
Your drawing does not show the transistor type ie: npn or pnp.
I suspect you are using a pnp, which you should not in that circuit.??

If you used a npn it can switch +12V with a 5V PIC signal, but it will be inverted.

As simple method is to use a 74LS07 IC.

EDIT:
attached pdf

 

[edeca]

The two important questions are how much current do you need and how fast do you need to switch it?

I have previously driven 12v LED lights from a 5v PIC using just a 2N7000 FET. This has a low Vgs so I drove it directly from the uC. I'm unsure if that is a sensible way of driving a FET, perhaps someone can correct me if not. However, it worked

 

[ericgibbs]

I have previously driven 12v LED lights from a 5v PIC using just a 2N7000 FET. This has a low Vgs so I drove it directly from the uC. I'm unsure if that is a sensible way of driving a FET, perhaps someone can correct me if not. However, it worked

hi ed,
That will work OK.
If I was switching an inductive load with the FET, I would have a series 1K resistor from the PIC pin to the FET Gate.
In the event of a Drain/Gate breakdown the 12V will not drive enough into the PIC pin to destroy the PIC's internal clamp diode.