mosfet basics

[Lston]

As a newbie I am still trying to understand schematics and mosfets.You can find endless technical info on the web but I just want to know how do I hook up an N channel enhancement mode mosfet? From the schematic I uploaded is this right?
I need two power sources.One fairly small (power source 1) to supply voltage to control the gate with it's positive going to the gate and it's negative going to the source and to the negative of power source 2.
Then power source 2's positve go's to the drain.
Then take the positive from the drain and go to the positive of whatever I want to drive like a motor ect. and the negative from the source and go the negative of the motor.

[Al_The_Man]

The two voltage sources are shown for explanation purposes, in practice the voltage sources can be one common one where whatever is driving the gate shares the same supply with the drain load side, or an example of separate supplies would be where the gate is driven by a 5v TTL circuit and the drain load fed from a 24v DC supply, the commons of both supplies connected together.
In your circuit, the drain load would be inserted where the black arrow is shown.
Al.

[pminmo]

Assuming your wanting to use the mosfet as a switch. A voltage potential greater than the source by the amount defined for the particular mosfet (P/N) for the switched current. To derive the minimum look at the "transfer characteristics". For example, IRL540, source at 0V, and you want to switch 5A. Curves show that 4.5V at the drain as a minimum (depending on temperature), but for reliability a higher value should be used. If the same part has the source at 40V, then to turn it on it would have to 44.5V at the gate minimum. Realistically off the shelf mosfet drivers would use a voltage difference maybe 8 to 12V at the gate. For the mosfet to be off, the gate should be less than 2V of the same value of the source. In the middle of that region the mosfet becomes a linear device.

[Lston]

For the TTL setup, like this?

[pminmo]

No

[Al_The_Man]

Originally Posted by Lston For the TTL setup, like this?

If you go back to my first post, I mentioned that the load is inserted where the arrow is, IOW substitute the arrow with your motor or load.
The load is in series between drain and power.
Do a search here for 2N7000 for examples of the connection.
Al.

[Lston]

If the positive from the 24V goes to the negative of the motor and the negative of the motor goes to the drain,Where does the negative of the 24V go?The positive of the 5V?

[John3]

Think of the MOSFET as a switch to ground (when the source is grounded).

The Source is connected to ground.

The Drain is the Load side of the switch.

When the MOSFET is turned on, it conducts positive voltage from the Drain to the Source / ground.

The gate-to-source voltage controls whether and how much the switch is turned on.

Zero Volts between gate and source and the switch is open (off). 5-10 volts and the Drain-to-source switch is closed (low resistance).

Between zero and 5 volts the MOSFET acts as a variable resistor of sorts. The higher the voltage the lower the resistance.

John

[NC Cams]

Another way to look at it.

Let's say you only have 1 @ 12v batter to work with. You need want the fet to switch the 12V to a light or some other current using device.

A logic level fet (IE: IRLZ44) can be enhanced with 5v so all you'd need to do would be to come up with a decent source of 5v (a simple regulator IC would do as not much current is needed) and a storage capacitor. Feed that voltage to a gate driver (totem pole or fet driver) and 5v will swith the 12 as needed.

Let's say you have a regular fet (IRFZ44) these are enhanceable with 5 to nearly 18 volts. At 5 volts, however, they are NOT as fully enhanced as they are at 10 or better yet, 12 or 15-18vdc.

The IRFZ44 will turn on quite effectively at 12 volts - recall, we only have a 12v battery for EVERYTHING. IT will do so even better if you boost the gate voltage to 15-18 volts with a charge pump of some kind.

DO NOT exceed 20Vdc Vgs with regular fets and do NOT exceed Vgs of 10vdc with logic levels.

Folks may take exception to this method but we used it QUITE effectively in our R/C car speed controllers and it worked great. Especially when you are/were pulling at/near peak possible current thru the fet(s)...

[Lston]

Thanks John3,you explained that very well but I believe I have the theory of the operation down I just don't understand the wiring for a 5V TTL controlling a larger source like 24V going to a motor.Could you draw a sketch,not a schematic, that is what I can't figure out, anything just a rough MS paint drawing?

[Al_The_Man]

http://cnczone.com/forums/showthread...92&page=6&pp=5

[John3]

Originally Posted by Lston Thanks John3,you explained that very well but I believe I have the theory of the operation down I just don't understand the wiring for a 5V TTL controlling a larger source like 24V going to a motor.Could you draw a sketch,not a schematic, that is what I can't figure out, anything just a rough MS paint drawing?

Lston,

With all due respects and not to be a smartass... if you had "the theory of operation down" you would "understand the wiring for a 5V TTL controlling a larger source" voltage.

Maybe this will help?

Even though they are tied common at the source, you should think of the gate-source control circuit as being independent of the Drain-source output switch circuit.

This is pretty crude, but think of the MOSFET as a SPST-NO relay with one coil wire and one switch wire tied together at the ground (source) connection.

To control the MOSFET Switch (relay) you apply positive voltage to the gate (coil) to energize it. The reference side of this control voltage is ground or the source (other coil wire).

The Drain-Source (relay contacts) circuit acts as a switch of positive voltage/current to ground.

The voltage/current switched at the Drain-source (contacts) can be and is usually independent of the voltage that is used to control the MOSFET gate (relay coil). The drain-source circuit can switch lower or higher voltages than the gate drive voltage, and it can control currents from the micro-amp range to hundreds of amps depending on which MOSFET is used and the operating conditions.

This is a very simplistic view of the MOSFET as a switch/relay. At zero gate-to-source voltage the MOSFET is OFF (open circuit). At 5 / 10 /20 volts between the gate and source (depending on the particular model) the MOSFET is ON (Low Resistance) and is conducting current from the drain into the source.

Unlike a relay there is no exact point where the MOSFET turns from OFF to ON. In the middle range between full off and full on, it operates as a linear analog device with Drain-to-Source resistance dropping for increasing Gate-to-source control voltage. The analog voltage range/points are determined by many factors, construction/layout of the chip, doping of the materials, temperature, drain current... etc. This characteristic is know as the Transconductance of the FET.


So, think of using a MOSFET as a relay with a common coil and contact lead.

Does this make it as clear as mud?

John