# measuring rise time percentages 10%-90%

Discussion in 'General Electronics Chat' started by walters, Aug 1, 2005.

1. ### waltersBanned

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1.) check the fuse
2.) check the cord
3.) check the power supply is getting voltage on the secordary

2. ### checkmateNew Member

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This is right. But it was not what I asked.
My question was, starting from -1V, the voltage rose to 6V. At what voltage is it when the voltage has risen by 20%? 80%? What you answered was from 0V to 6V, not -1V to 6V.

3. ### heathtechNew Member

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Very good!
How would you check the fuse with an instrument?
There are two easy ways of doing this with a meter. Do you know them?

BTW, sorry checkmate for interupting your other subject, but I figure if he wants to fix things, he needs to know how to identify faults.

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5. ### waltersBanned

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Either on Ohms setting checking the ohms of the fuse or continuity check it to see if there is a break in the fuse wire

20% = 1 volt
80%= 4 volts

6. ### checkmateNew Member

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How did you arrive at this? Then 100% is 5V? 0% is 0V? Then that's 0V to 5V, still not the required -1V to 6V.

7. ### heathtechNew Member

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Actually, both of these are essentially the same test. But you did get one possibility correct.

The other check would be on the live circuit with a voltmeter. This is assuming the fuse is a line input fuse. Do you know how to check the voltage drop across a suspected open fuse in a live circuit? What reading would you expect if the fuse was good? What reading would you expect if the fuse was bad? Let's assume the line input voltage is 110 VAC.

8. ### waltersBanned

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How much voltage drop does a fuse have?
i have never done a voltage test on a fuse
some are fast or slow blow is but thats for current testing

20%= 1.4
80%= 5.6

9. ### checkmateNew Member

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You are almost there, but it is still not correct. This are the values for 0V to 7V, not -1V to 6V. Can you figure out why?

10. ### waltersBanned

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So how do i do it then this is tricky because its -1 to X

is it -7 or 7 for 100% or what is the 100% thats my probrom from -1 to 6 how do i get it?

11. ### heathtechNew Member

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OK, great, this is ESSENTIAL knowledge when troubleshooting electronics.

A good fuse has zero voltage drop. A bad fuse shows full voltage supply dropping across it. Do you understand why this is?

variations of this technique can be used to troubleshoot many, many faults in circuits, with slight rule changes with differing devices. This takes cummulated months and years experience.

12. ### waltersBanned

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Well really the meter is in parallel so it reads the full 110volts if the fuse is open

Fuse is good= the meter reads ? ?

13. ### checkmateNew Member

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All right, I'll give you the method, and you tell me the answer. For any range of X1 to X2, the Y% value is given by
X1 + [(X2-X1) * (Y/100)]
So, what is the 80% value for the range (-1,6)?

14. ### heathtechNew Member

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If the fuse is good, you will read zero volts on the meter. Can you explain why?

I hope Checkmate is not distracted by this simple little quiz I am giving you. But this is all Essential that you understand if you want to be a tech.

15. ### waltersBanned

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X1 + [(X2-X1) * (Y/100)]

6-(-1)=7 * 7/100 + X1

This is hard stuff

16. ### waltersBanned

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If the fuse is good, you will read zero volts on the meter. Can you explain why?

Because the meter is in parallel so there is no voltage drop or current going through the meter because of the impedanced,resistance of the input circuit so current flows throught the fuse instead to take the less opposition of currrent flow

17. ### StyxActive Member

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not really.

I'm not going to do it for you BUT.

-1V to 6V is a 7V rise yes
so 20% of 7V is?

that will give you the 20% rise, but the volts are not rising from zero are they... so what do you think the voltage is?

[/quote]

18. ### waltersBanned

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-1V to 6V is a 7V rise yes
so 20% of 7V is?

that will give you the 20% rise, but the volts are not rising from zero are they... so what do you think the voltage is?

This is the hard part what is it 0.4 i have to minus 1 volt?

19. ### checkmateNew Member

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CORRECT! Well done. So what's 30%? Given the range (-1,6)?

20. ### heathtechNew Member

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AAHA! A very good explanation and well thought out!

Another way of looking at it is that a fuse is simply a wire, and there is no POTENTIAL DIFFERENCE between one end of the fuse and the other (with exception to the very slight resistance created by the length of the wire) However, when the fuse is open, it will have nearly INFINITE resistance to current flow, with only a very miniscule possibility that the air gap isn't acting as a sufficient INSULATOR) This means there is FULL POTENTIAL DIFFERENCE between the two terminals of the fuse, thus Full supply voltage (110 VAC) will drop across the fuse with zero current flow.

Did you understand all of this before I quized you or did you learn something new?

21. ### StyxActive Member

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Finally we are getting somewhere!!!