Mosfets

[Dismantle.Repair.]

Hi,

This is my first post! I'm a complete beginner to electronics and I'm trying to learn about MOSFETS.

So far I can see that there are 2 types N-Types and P-Types.

As far as I understand each of these has 3 pins - Gate, Source and Drain.

My question is does current flow from Drain to Source in both N and P-types, or does the current flow differ?

If it differs what happens if the Drain and Source are incorrectly wired.

Thanks in advance

 

[Diver300]

In an N-channel MOSFET the current normally flows from Drain to Source. The MOSFET controls the current, depending on the Gate voltage.

MOSFETS contain a diode, so they will conduct in reverse whatever the gate voltage, generally with a voltage drop of about 0.6 V. However, the MOSFET will conduct in reverse if the Gate voltage is raised and then the voltage drop in reverse may be less than 0.6 V.

The current normally flows from Source to Drain in a P-channel MOSFET.

 

[Dismantle.Repair.]

Thank you for your quick reply.

So basically a P-channel MOSFET could be used to do the same job as an n-channel MOSFET (I know the gate logic would be switched) if the drain and source terminals were wired the other way around?

Out of interest what happens if you incorrectly wire the source and the drain?

Thanks once again

 

[shortbus=]

Mosfets when turned on will pass current in either direction. But when turned off, they will allow current to pass IF they are not wired correct with regard to drain and source. Its the internal diode that will allow this current to flow. Look at the electrical symbol of mosfets to see the diode.

 

[crutschow]

So basically a P-channel MOSFET could be used to do the same job as an n-channel MOSFET (I know the gate logic would be switched) if the drain and source terminals were wired the other way around?

If by "gate logic would be switched" you mean the polarity of the signal. N-MOSFETs require a positive gate-source voltage to turn on and P-MOSFETs require a negative gate-source voltage to turn on.

Both types turn off if the gate-source (absolute) voltage is less than the threshold voltage.

This brings up a common misconception. The MOSFET transistor "threshold voltage" is the voltage where the transistor just begins to turn on, not where it is fully turned on. To fully turn on a MOSFET requires a Vgs of 10V (absolute) for a typical MOSFET or 5V for "logic-level" type MOSFETS.

Another difference is that N-MOSFETs typically have a lower on-resistance than a P-MOSFET for a give transistor size.

 

[Dismantle.Repair.]

Thanks everyone for your help so far. It might have been easier if I explained what I'm trying to achieve.

I'm trying to replicate the logic of a switch that is found in a DI box. In order to effectively open or close the transistor I want to use an Arduino boards digital output(s). (Connected to the logic MOSFET's gate).

The following links shows the connections when the switch is both opened and closed as well as replicating the logic.

**broken link removed**

**broken link removed**

**broken link removed**

I was hoping that as long as the MOSFETs were reversed the logic could be replicated.

Once again thanks for your replies.

 

[Diver300]

If a DI box is one of these:-

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

then you are probably switching signals not power.

It is simplest to use a relay. You need a logic level MOSFET to drive a small DPDT relay that has a 5V coil. (With a freewheel diode)

It is easy to test, as you can hear the relay click. The contacts of the relay will do exactly what the switch does. It then doesn't matter what voltages or currents you are switching so long as they are less than the relay contact ratings.

 

[Dismantle.Repair.]

Thanks for your reply,

so I've got this right it would be a device like this.

https://uk.rs-online.com/web/p/electromechanical-relays/0515537/

Then the Arduino connects to a logic level Mosfet which controls the DPDT relay on and off as required?

 

[Diver300]

That relay would be fine.

In fact, it might be possible to run it directly from the Arduino as the coil current is only 10 mA. You would have to check the specifications.

With a 10 mA load, a bipolar transistor would work fine. You do need a resistor to limit the base current with on of those.

 

[Dismantle.Repair.]

Thanks once again

Sorry but this is all completely new to me.

The Arduino gives out a 40mA current on each digital output at 5Volts. I had planned to use a logic Mosfet as it integrates nicely with Arduino. (Eventually I would like to build some simple software that would control the switch and the others on the DI box - probably in processing as it interfaces with Arduino so well)

What is the purpose of a bipolar transistor ? Also what is base current?

Could it be done without a bipolar transistor by using a logic MOSFET.

Thanks for your patience. (This is for a College project)

 

[Diver300]

If the output of the Arduino is 40 mA, you don't need any current amplification, so just connect the output to the relay coil.

A transistor, bipolar or MOSFET is to increase the current. That would be useful for larger relays.

Base current is the control current on a bipolar transistor. If there is no base current the transistor will turn off. The base is roughly equivalent to the gate of a MOSFET

 

[shortbus=]

Unless the relay has "make before break" contacts, won't there be a loud 'pop' when it switches? Maxim and others have CMOS switch ICs that can do this.

 

[Dismantle.Repair.]

Thanks for your replies.

I definitely need to do some more research into the various alternatives.

(Could you give me an example Shortbus ?)

Diver300 if the relay coil is 10mA then a resistor to limit the Arduino output (40mA) would do the trick?

 

[Diver300]

Diver300 if the relay coil is 10mA then a resistor to limit the Arduino output (40mA) would do the trick?

NO!

The relay coils is designed to be powered from 5 V. The current it takes from a 5 V supply is 10 mA, give or take. It does not need any current limiting.

The output of the Arduino is 5 V when turned on, and can supply a load of up to 40 mA. So it will supply the relay easily.

Similarly, your 12V car battery supply 500A to crank the car engine. The bulb in the interior light takes 1/2 A or so. It is just wired straight the battery and it takes the current that it needs without anything in the way that reduces the current. There may be fuses, but they do nothing unless there is a fault.

I mentioned a resistor to limit the current if you were going to use a bipolar transistor. The voltage on the base of a bipolar transistor is 0.7 V, for that sort of load you need to limit the current.

(A MOSFET or bipolar transistor would be needed if you had to supply a relay coil that took more than 40 mA)

 

[Dismantle.Repair.]

Ok, starting to make sense now!

So literally connect the 5V logic output to the relay.

Thanks for your patience once again I'm new to all this!

 

[Diver300]

Ok, starting to make sense now!

So literally connect the 5V logic output to the relay.

Yes. I would suggest that you add a flyback diode as well, or you might damage the Arduino when it turns off.

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

 

[shortbus=]

Thanks for your replies.

I definitely need to do some more research into the various alternatives.

(Could you give me an example Shortbus ?)

Check these out, you will need to match your specifications to the switches. **broken link removed**