Discussion in 'Electronics Homework Help' started by wojnarj, Dec 15, 2013.

1. ### wojnarj

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Dec 15, 2013
Hello,

I am having trouble finding the formula to find the answer. If any one has the formula please send it to me; since I already have the answer:

A MOSFET has a "on" resistance of .4 Ohms and is rated for 7 A. If the supply voltage is 9 V, what is the minimum resistance of a device that it can safely switch?

Thank you.

2. ### Harald KappModeratorModerator

10,768
2,425
Nov 17, 2011
Welcome on the forum.

From the way the question is phrased, I think this is homework, so I moved the thread.

Start with the supply voltage and the max. current. From that you can calculate the resistance of the complete circuit.
Using the resistance of the complete circuit it is easy to find the resistance of the load, knowing the on resistance of the MOSFET.

If you post your findings, we will be glad to check them.

3. ### wojnarj

12
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Dec 15, 2013
Not Home Work... I wish, but its not

Hello

I wish this was my home work, but it is not. I downloaded a booked "Electronic circuits and systems" and this was check yourself question on page thirteen. I got all the ones I knew right, but I don't know this formula.
I will approach this problem from your suggestion.

Any help is much appreciated.

thank you again

Jason

4. ### wojnarj

12
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Dec 15, 2013
Hello Again

So I am not a hobbyist or student of electronics. Actually, I am the level before NOOB; which is boob. LOL

ok so my supply voltage is 9 and max current should be 15ma from a 9V.

R=9/.15 = 60R which doesn't seem right.
or R=9/15 = 6A which I am not sure of.. well the formula I am sure but not the 15.
SO, official I am stuck... I can do figure out current for a resistor to battery, voltage and current dividers and ohms law... but I am not sure what formula or formulas to find the answer of .886R. can any give me a nudge on the supply voltage and max current and let me try from there?

thank you

5. ### davennModerator

13,709
1,909
Sep 5, 2009
0.15 = 150 mA NOT 15mA

0.015 = 15mA

try again with that figure

Dave

6. ### wojnarj

12
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Dec 15, 2013
Thank you

r=9v/15ma = 600R and .135W
but how does the 7A come into play?

Again, I am not a electronics noob; but I do appreciate all the help and patience the forum has

thank you

7. ### Harald KappModeratorModerator

10,768
2,425
Nov 17, 2011
Where do you get that figure from?
The only current stated in the question is 7A. Use that value.

I'll leave the thread in the homework section. It kind of is homework, even if it comes from a book's Q&A chapter.

8. ### mursal

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Dec 13, 2013
What picture do you see in your head when you read, 9Volts supply and 0.4R across the closed switch controlling 7Amps through the load resistor? Don't worry what type of switch it is, its just a switch, that when closed has 0.4R internal resistance.

If you don't see any (picture), try and draw it on an old scrap of paper, it will help you put it into your long term memory.

Do it first with a perfect switch, with no resistance, see how you get on using Ohms law.
V/I = R

MOSFET = Glorified Switch (First approximation)

Last edited: Dec 16, 2013
9. ### wojnarj

12
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Dec 15, 2013
R=V/I

R=9V/7A = 1.2857R
1.2857-.4 = .886R

I tried and thought of different formulas, but sad to say I did not think of subtracting .4 until the gentleman nudged my brain.

I feel dumb for nit thinking about these way. I do thank everyone for the help and nudging.

Now, back to the book to learn.

thank you
Jason Wojnar