# DIODES IN MOSFET

#### jjb135

Joined May 17, 2021
13

Silicon Diode = 0.7V

#### ericgibbs

Joined Jan 29, 2010
12,919
hi jjb,
Welcome to AAC.
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#### jjb135

Joined May 17, 2021
13
Hi Eric,

Thanks for the response. So far I had assumed Ir=V/R. Therefore potentially 0.7/510 = 0.0013. However, to my understanding this is incorrect. Thus not being able to work out (a) (ii) I sadly cannot attempt to complete the other questions.

Thanks

#### ericgibbs

Joined Jan 29, 2010
12,919
hi jj,
OK,
Consider this part, You should know a diode conducts in one direction and blocks current in the opposite direction
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#### jjb135

Joined May 17, 2021
13
The current will flow through the path of least resistance which could result in fusing the circuit?

#### ericgibbs

Joined Jan 29, 2010
12,919
Come on jj a fellow Yorkshire man can do better than that...

Consider the Diodes a form of Logic Gate.

You don't want either switch to operate both the circuit paths..

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#### jjb135

Joined May 17, 2021
13
result in an open circuit? Slightly confused by the last comment

#### ericgibbs

Joined Jan 29, 2010
12,919
hi jj,
Look at this edited drawing, L1 and L2 are 5v lamps,
Consider what the switch action for the lamps would be with and without the diodes.??
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#### jjb135

Joined May 17, 2021
13
the current would not reach the lamps?

#### ericgibbs

Joined Jan 29, 2010
12,919
hi jj,
Look at these 3 images.
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BTW: Can you say which course and year you are in.? help us to give better answers.

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#### jjb135

Joined May 17, 2021
13
Thanks for the images and help - all very much appreciated. Apologies I am pretty useless at this - I am trying to catch up 2/3 years worth of physics within a few weeks before an exam in 2 days. I study MME (Music, multimedia and Electronics) and am in 1st year.

#### ericgibbs

Joined Jan 29, 2010
12,919
OK, so do you now understand the action and purpose of those diodes.?
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#### jjb135

Joined May 17, 2021
13
I believe so, I understand that diodes either cause or prevent current from flowing and if I'm not mistaken if they were replaced by a short circuit in this case this would lead to an open circuit and current not being able to flow throughout is that not correct??

Once again, Thanks for all of your help Eric

#### ericgibbs

Joined Jan 29, 2010
12,919
hi jj,
Think of diode as a switch that will only allow current flow thru it in one way [ in the direction of the 'arrow'] and not in the opposite direction.

What is the next step in the questions.?
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#### jjb135

Joined May 17, 2021
13
What would go wrong? = in the case of the homework; Q1 would not get turned on?

#### ericgibbs

Joined Jan 29, 2010
12,919
No,
look at the image 285c , both lamps ON, . and Gate to 0V, ,, Source to +5v

Don't forget its a P type FET

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#### jjb135

Joined May 17, 2021
13
Oh yes I meant to say that - so both lamps are on? Would they take up too much current and so the transistor won't function...? I'm sorry but I'm still not quite sure what would go wrong.

#### ericgibbs

Joined Jan 29, 2010
12,919
hi jj,
Its the definition of wrong thats misleading.
It should say what would be the effect on the circuit operation, if the diodes were removed/ shorted out

Consider the just the switches and the 'R' resistor.

The switches are connected as an OR gate, either one ON would connect the Gate to 0V and turn on the P FET.
BUT
the logic inside the U1 unit would not know which switch has been switched ON, as either switch would pull both logic lines Low.

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I think you are going to have a problem with this Course, catch up.

#### jjb135

Joined May 17, 2021
13
Yes I do see what you mean.

Thank you for all your help Eric, even though it may have caused the both of us a little frustration.

'think' maybe an understatement as I am fully aware of my lack of knowledge on the module but I shall nevertheless prevail in attempting to understand the world of circuit analysis and design.

Many Thanks for your guidance and truthful advice. Any further tips would be much appreciated.

p.s I would be curious as to what you do for a living/ area of speciality is?

#### ericgibbs

Joined Jan 29, 2010
12,919
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