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How can i drive logic mosfet from a PIC

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The Focus

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I can a drive with a logic level mosfet a lamp from a PIC.
and with mosfet are the right choice ?
 

Do you want to know what mosfet to use or do you want to know if mosfet is a good choice?
 

Which mosfet can i use for this purpose?

and how i connect the mosfet to the PIC output port
 

Which mosfet depends of the current and voltage needed to the lamp.

I guess you can connect the gate directly to the PIC.
 

An important thing to look for when you drive a MOSFET from logic levels is the Gate Source Threshold level.
Normal MOSFETs has too high a threshold voltage, so you need a special logic level MSOFET with a low theshold voltage.
The Gate - Source max threshold voltage of the MOSFET should be lower than the min Vout high from your PIC output pin.
Use an N-channel logic level MOSFET for the purpose.
Find a MOSFET that are able to handle the voltage and current from your lamp.
The lower Rds-on (drain source on resistance) the MOSFET have, the lower power you dissapate in the MOSFET, but the MOSFET also becomes more expensive and have a higher input capacity, which can become a problem for very high swithing frequencies.
But to switch on a lamp you should not worry about the capacity of the MOSFET.
 

be more explicite regarding the characteristics of the lamp.
//a
 

put a 10 ohm resistor in series with the mosfet gate to reduce ringing and to limit the current in/out of the mosfet during switching.

i heard you mention the word lamp. i am wondering if you are planning on using a mosfet to turn ON/OFF the lamp? be careful how you do this.. lamps are usually AC powered so your mosfet must be able to conduct in both directions. also, do not inturrupt the power to the lamp via the ground or neutral wire, but instead interrupt the hot wire. now you are high-side switching which takes some more thought as to how to turn ON the mosfet..

Mr.Cool
 

Guys,
This discussion is getting nowhere...
Focus has to tell us first what load (lamp: dc V and W?, AC W?) is hi going to switch On/Off.
Then we can advice.

Someone mentione usig MOSFET to operate on AC: frankly ... I will not comment that.
 

Mr.Cool said:
lamps are usually AC powered so your mosfet must be able to conduct in both directions.
What???
MOSFET able to conduct in both directions???
What about the built-in reverse voltage diode placed between drain and source in all MOSFET transistors? This diode will always conduct in one direction, so you can never break the voltage in one direction of a MOSFET. The built-in diode will always conduct when you reverse the voltage.

Mr.Cool said:
also, do not inturrupt the power to the lamp via the ground or neutral wire, but instead interrupt the hot wire.
What would the purpose of a high-side MOSFET be, unless you use a H-bridge with 4 MOSFETs?
 

Its a simple 12V lamp DC and maybe later a relay
 

The Puma said:
Its a simple 12V lamp DC and maybe later a relay
How do you know it's a 12V DC lamp? I can't see "The Focus" mention that anywhere.
 

Its a friend and we do togetter building some experiments
 

I have attached how the schematic should look like with either a relay or a lamp connected.
The resistor is mentioned in a previous post, you can leave it out if you like.
You could use the PIC's active pull-up to turn on the MOSFET instead of using totem-pole output.
Unsing the PIC's internal pull-up is probably the best solution.
 

What???
MOSFET able to conduct in both directions???
What about the built-in reverse voltage diode placed between drain and source in all MOSFET transistors? This diode will always conduct in one direction, so you can never break the voltage in one direction of a MOSFET. The built-in diode will always conduct when you reverse the voltage.

{ corrected Oct 31}
i am confused between mosfet and igbt. usually triacs are used for AC switch because you get the same impedance/voltage drop in both directions. using a single mosfet will be different response depending on which direction power flows. for some applications this matters. since it wasn't clear what the application specifics were, i thought i would point this out.


What would the purpose of a high-side MOSFET be, unless you use a H-bridge with 4 MOSFETs?

you can use a highside mosfet as a switch to break the power. think of a battery, the positive terminal is connected to a resistor and the other end of the resistor connects to the negative terminal of a battery. how to create a switch? you have 2 places in which to break the current. between the positive terminal & the resistor, or the negative terminal & the resistor. i say it is bad idea to break circuit between the negative terminal and resistor because from a safety point of view you are loosing your continuous connection to ground. thus it is safer to disconnect the circuit from between the positive terminal & the resistor. this improves safety, but complexity of circuit increases because now the mosfet (n-channel for example) has a source terminal that is *not* connected to ground. thus you need to High-Side switch this mosfet. there are many applications for highside switches other than an H-bridge.

Mr.Cool
 

Mr.Cool said:
i was thinking of a MOSFET without any internal diode.
I have never heard of such a MOSFET, can you plese give a link to one?



I can't see why it should be more safe to break the high voltage side than the GND side. It should make no difference, so why not do it at the GND side, which is easiest.
 

use the PHP125N06, it's very goog section.This mosfet TTL gate.
 

I have never heard of such a MOSFET, can you plese give a link to one?
no i can not, because they do not exist. i work with IGBTs mostly and have to add external diodes, i confuse myself and was thinking of igbts i guess... there are many igbts without antiparallel diode, but due to the way mosfets are made they contain the diode.

I can't see why it should be more safe to break the high voltage side than the GND side. It should make no difference, so why not do it at the GND side, which is easiest.
it is about maintaining a constant connection with the ground. it is exactly the same reason why all house wiring has a switch on the HOT wire and never the neutral.

Mr.Cool
 

Mr.Cool said:
I can't see why it should be more safe to break the high voltage side than the GND side. It should make no difference, so why not do it at the GND side, which is easiest.
it is about maintaining a constant connection with the ground. it is exactly the same reason why all house wiring has a switch on the HOT wire and never the neutral.

Mr.Cool
Yes, but there's a very big difference between house wirering and 12 V.
There's no safetey risk at 12 V. Both GND and 12V are isolated from the house wirering, and there's no risk by touching 12V.
You could just as well use the positive side as reference and run it all at -12V. It doesn't matter if wich wire you touch, GND or 12V.
 

use opto isolate TLP250 for separate GND
 

SPATAN said:
use opto isolate TLP250 for separate GND
Why do you want to isolate the GND???
I don't think there's any good reason to make it more complicated than shown in my schematic.
 
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