Mosfet operation

[AxelD]

Hi,

As far as i understand, a voltage applied to the base of a mosfet translates into a flow of current from the drain to source of the mosfet - provided the voltage is higher than the threshold voltage.

Can someone confirm that my understanding is correct and can someone confirm that the current (from drain to source) is independent of the load. Ive attached a rough drawing with R1 being the load.

Thanks

 

[aljamri]

https://en.wikipedia.org/wiki/MOSFET

 

[Willbe]

a voltage applied to the base of a mosfet translates into a flow of current from the drain to source of the mosfet

the current (from drain to source) is independent of the load

Yes.
From the graph in the link, in the saturation region the device acts like a voltage-controlled Current Source [whereas a bipolar junction transistor, and a transformer, and possibly a relay, act like current-controlled Current Sources].

 

[AxelD]

Thank you.

The reason i ask is that ive built a circuit to control two fans and im trying to fully understand its operation.

Ive included the circuit in the following thread (when i had a question on heat levels):

https://www.electro-tech-online.com/threads/transistor-heat-levels.41369/

So from your response and my understanding of the mosfet operation, changing the value of the POT on the drain has no impact on the current through it and into the base of the transistor (which in turn controls the current through the fans)

Then why does this circuit work so well? As i change the POT the current through the fans moves from 75ma to 15ma.

 

[andy257]

Thank you.

The reason i ask is that ive built a circuit to control two fans and im trying to fully understand its operation.

Ive included the circuit in the following thread (when i had a question on heat levels):

https://www.electro-tech-online.com/threads/transistor-heat-levels.41369/

So from your response and my understanding of the mosfet operation, changing the value of the POT on the drain has no impact on the current through it and into the base of the transistor (which in turn controls the current through the fans)

Then why does this circuit work so well? As i change the POT the current through the fans moves from 75ma to 15ma.

Just think of your mosfet as a switch. Looking at your circuit the mosfet will be fully saturated therefore its switched hard on. Your current path is then determined via your resistor network. Changing the value of the pot will alter the series current through that path.

 

[AxelD]

Thanks.
Thats how i originally looked at it before the confusion...

 

[Hero999]

[whereas a bipolar junction transistor, and a transformer, and possibly a relay, act like current-controlled Current Sources].

A BJT yes but I can't see how a relay and transformer act like current controlled sources.

 

[Willbe]

A BJT yes but I can't see how a relay and transformer act like current controlled sources.

IMHO:

A transformer works based on current (but has no power gain).

A relay is current controlled (the armature moves based on the product of the coil turns times the amps through the coil) and it has power gain.

Analog meter movements are current controlled.

Vacuum tubes and FETs are voltage controlled. I'm not sure about the + input of an op amp configured as a voltage follower.

Device outputs can be Current Sources or Voltage Sources or something in the middle. A car battery and wall outlet approximate Ideal Voltage Sources and a flourescent ballast approximates its dual, an Ideal Current Source.

 

[crutschow]

[whereas a bipolar junction transistor, and a transformer, and possibly a relay, act like current-controlled Current Sources].

A transformer typically is used to transform voltages as a voltage-controlled voltage source (no power gain), but it can be used in current mode to sense AC currents in circuits. A relay is generally a voltage-controlled (or sometimes current-controlled) switch.

 

[Willbe]

A transformer typically is used to transform voltages as a voltage-controlled voltage source (no power gain), but it can be used in current mode to sense AC currents in circuits. A relay is generally a voltage-controlled (or sometimes current-controlled) switch.

IMHO, at the most basic level, transformers and relays depend on [current flow] x [coil turns] to generate magnetic flux; some voltage is necessary into the inputs of these things in order to generate the required flux because of the resistance of the wire used.
Superconducting transformers and relays, if there are such things, would have zero voltage in, since E = IZ and Z is zero.
I think the "prime mover" for these things is current [amp-turns], but the design equations should say so.

In the steady state, I think that voltage controlled things like vacuum tubes and MOSFETs do not draw current from the control grid or gate. They work with the voltage just being present. I guess that makes them electrostatically operated.

 

[crutschow]

Transformers do not operate the same as relays.

A mechanical relay does require a current to generate magnetic flux to move the armature. This would be true even for an ideal relay (using superconducting wire for example)

An ideal transformer, when driven by an AC voltage, draws no current if the output has no load. It would appear as an infinite inductor. There is current only when the output load draws current.

In practice, of course, the transformer inductance is finite and there is a small magnetizing current, but that does not make it a current device, at least not by usual definitions of that designation (an ideal current device has zero input impedance and infinite output impedance). The magnetizing current also has nothing to do with the wire resistance.

Whether something is called a voltage device or current device depends upon how close it is to either ideal. If it has a relatively low input impedance, such as a bipolar transistor base input, it's usually called a current controlled device. If it has a relatively high input impedance, such as a FET or standard mechanical relay, it's usually called a voltage controlled device. (Thus most relay coils are rated by the voltage they require for operation: 5V, 12V, 28V, etc.)

 

[Willbe]

Transformers do not operate the same as relays.

A mechanical relay does require a current to generate magnetic flux to move the armature. This would be true even for an ideal relay (using superconducting wire for example)

An ideal transformer, when driven by an AC voltage, draws no current if the output has no load. It would appear as an infinite inductor. There is current only when the output load draws current.

In practice, of course, the transformer inductance is finite and there is a small magnetizing current, but that does not make it a current device, at least not by usual definitions of that designation (an ideal current device has zero input impedance and infinite output impedance). The magnetizing current also has nothing to do with the wire resistance.

Whether something is called a voltage device or current device depends upon how close it is to either ideal. If it has a relatively low input impedance, such as a bipolar transistor base input, it's usually called a current controlled device. If it has a relatively high input impedance, such as a FET or standard mechanical relay, it's usually called a voltage controlled device. (Thus most relay coils are rated by the voltage they require for operation: 5V, 12V, 28V, etc.)

I have to sleep on this. . .

 

[Willbe]

OK.
Instead of sleeping on it I've decided to raise my blood alcohol level such that I am granted profound insights into the nature of the physical world.

Transformers do not operate the same as relays.
A mechanical relay does require a current to generate magnetic flux to move the armature. This would be true even for an ideal relay (using superconducting wire for example)

Agreed.

An ideal transformer, when driven by an AC voltage, draws no current if the output has no load.

It would appear as an infinite inductor.

. .?. . .I guess. . .but see below

There is current only when the output load draws current.

An ideal transformer has zero input resistance. I don't kinow about the impedance, because of the mutual inductance effect.
If the input impedance is zero and you put an Ideal Voltage Source across it what happens is indeterminate.

In practice, of course, the transformer inductance is finite and there is a small magnetizing current, but that does not make it a current device, at least not by usual definitions of that designation (an ideal current device has zero input impedance and infinite output impedance).

An "ideal current device" would then be a current-controlled current source, approximated by a BJT transistor.
The output resistance of an ideal transformer would be zero. The output impedance is probably dependent on the mutual inductance effect.
I'll have to look this one up when I'm sober.

The magnetizing current also has nothing to do with the wire resistance.

It does if it's fed by a voltage source.

Whether something is called a voltage device or current device depends upon how close it is to either ideal.

Since, if you open-circuit a current source or short-circuit a voltage source the effects are equally dire, and since there are not many voltage sources of infinite value in series with resistors of infinite value such that the V/I ratio is, let's say, 1 A, I'd say we all are more concerned about short circuiting a voltage source, so most of our real world sources are more like voltage sources.

If it has a relatively low input impedance, such as a bipolar transistor base input, it's usually called a current controlled device. If it has a relatively high input impedance, such as a FET or standard mechanical relay, it's usually called a voltage controlled device. (Thus most relay coils are rated by the voltage they require for operation: 5V, 12V, 28V, etc.)

I think if "Current Sources" were more commonly available relays would be rated by current drawn and not voltage applied; "Nominal" vs. "Actual". They are designing for a spec'ed number of amp-turns.

If motors were depended upon as Constant Torque Sources they might be rated by current drawn.
If they are depended upon as Constant Speed Sources, then a voltage rating is more appropriate.

I think I'll have another drink!

 

[crutschow]

An ideal transformer, when driven by an AC voltage, draws no current if the output has no load.

It would appear as an infinite inductor.

. .?. . .I guess. . .but see below

There is current only when the output load draws current.

An ideal transformer has zero input resistance. I don't kinow about the impedance, because of the mutual inductance effect.
If the input impedance is zero and you put an Ideal Voltage Source across it what happens is indeterminate.

The input impedance of an ideal transformer is infinite if there's no output load. Any output load impedance is, of course, reflected back to the input inversely by the square of the turns ratio.

The magnetizing current also has nothing to do with the wire resistance.

It does if it's fed by a voltage source.

The magnetizing current is determined by the inductance of the transformer, and by the frequency and voltage of the input signal. For most practical transformers the winding resistance is so low that it has no effect on this current.

I think I'll have another drink!

Have two.

 

[Wond3rboy]

i have a question which is actually relevant to the post!!

The channel in a fet is insulated from the gate so how do we get a replica of the input signak when using it as an amplifier?cause as far as i know we only bring it in to conduction via the gate and it does not in any way transmit any currecnt or voltage on to the channel.

 

[Hero999]

I don't understand your question.

The charicteristics of the channel changed when exposed to an electric field - see the Wikipedia article.

 

[ramakrishna.s1]

ramakrishna

i think when voltage applied across the base of mosfet transistor , then current flows from source to drain and it requie less time and high switching speed when compared to bipolar transistor.

 

[Hero999]

high switching speed when compared to bipolar transistor.

Not always, it depends on the resistance and inductance of the circuit driving the gate.

 

[Claude Abraham]

A transformer is both an I to I converter, and a V to V converter. It simulataneously transforms both current and voltage in unison. A voltage xfmr is connected across the power line and is driven by a constant voltage source. The secondary voltage Vs, is equal to the primary voltage Vp, multiplied by the turns ratio. Vp/Np = Vs/Ns.

The secondary current, Is, is established when loaded. The primary current Ip is the secondary current Is multiplied by the inverse of the turns ratio. Np*Ip = Ns*Is.

A current transformer is placed in series with the power source and load. The primary current Ip is established by the source and the load. Then, Is is Ip times the inverse of the turns ratio. Np*Ip = Ns*Is. Vs is determined by the terminating resistance, Rs, and Is. Then Vp is determined by Vs, as Vp/Np = Vs/Ns.

A xfmr simultaneously transforms both I and V. With a VT, Vp is the fixed quantity, determined by the value of the constant voltage source driving the primary. Vs, Is, and then Ip, depend on load and turns ratios. With a CT, Ip is fixed, and Is, Vs, and Vp are determined by turns ratios and loading.

It is an I-I and V-V converter.

 

[MrAl]

Hi,


Actually, a transformer can be used for any of these:

1. voltage controlled voltage source
2. current controlled current source
3. voltage controlled current source
4. current controlled voltage source

Which one do we choose?

Well, most generally, the transformer is a POWER converter.
It takes one form of power and converts it to another.
It's only after it's been inserted into a particular circuit that
it may take on a new meaning, such as "current transformer".
Of course there are also transformers manufactured with
an intended end application and are so named as such,
but that doesnt mean they have to be used that way either,
or i should say that doesnt mean that they WILL be used
in that way because after all, their operation is more
flexible than that as they cant really be truely classified
that way.

The basic operation requires current to generate a magnetic
field, so we might be tempted to call the primary a
'current controlled magnetic source', but then we would
have to call the secondary a 'magnetically controlled current source'
because it is not current that causes current in the output,
it's the changing magnetic field.