Discussion in 'General Electronics Discussion' started by Danny Daviss, Mar 16, 2013.

1. ### Danny Daviss

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Mar 16, 2013
Why does over-loading an output cause transistors to blow or have excessive current to flow through them?

Overload i mean a load on the output of the circuit, why does overload cause excessive current to flow through the transistors? Example if u plug to many things in your ac outlet it will overload the circuit or ac outlet causing it to catch on fire. I guess the load draws to much current but why would the transistors have excessive current flow or heat up?

why do components overheat and blow if the output is overloaded? the overloading is drawing to much current but why would the components before the output overheat and blow?

2. ### Harald KappModeratorModerator

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Nov 17, 2011

Over = too much = component can't take the load.

That's what ratings are for.

3. ### Danny Daviss

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Mar 16, 2013
If the overload is taking or drawing all of the current , then there is less current going through the components in the circuit before the overload right?

So the components get damaged or heat up , why? I don't get it

I understand it puts stress and it heats up the components in the circuit before the overload

The part I don't get is the components before the overload have less current and voltage , because the overload is drawing it all?

4. ### BobK

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Jan 5, 2010
No. If the output is going through a transistor, like the pass transistor in a power supply, or the output transistor in an audio amp, then ALL of the current going through the load also goes through the output transitor. Too much current through it can blowit.

Bo

5. ### Danny Daviss

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Mar 16, 2013
Yes it blows the output transistors but i don't understand why overloading increases the current or the dc supply voltage/current to raise
i would think overloading would decrease the current through the output transistors because the overload would take all over the current right?

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Aug 13, 2011
You may be getting confused by semantics. An overload is not an increase in resistance to current flow, just the opposite. It might be better to call it an overcurrent condition, but terminology develops in illogical ways.

7. ### Danny Daviss

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Mar 16, 2013
So overloading means parallel resistance = low resistance on the output cause it to increase current? Does the dc supply voltage raise up to when overloading?

8. ### davennModerator

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Sep 5, 2009
not necessarily in parallel ... not unless you have added extra resistance in parallel
The DC voltage CANNOT increase if its from a fixed voltage supply basic Ohms Law
I = V/R

Also what you havent picked up through the previous comments is that current is the same regardless of where in the circuit its measured... before or after the load or any device like a transistor that is switching ( controlling ) that load

Dave

9. ### Danny Daviss

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Mar 16, 2013
How is the output transistors controlling the load?

I don't understand why the current increases and blows the output transistors

The overload causes the current to increase why and how so?

10. ### davennModerator

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Sep 5, 2009
well there may or may not be a transistor controlling a load ... depends on the circuit
you hinted that your circuit has a transistor controlling a load ....

as I said last time... could be either ... depends on the situation

because the transistor may not be rated for that extra current flow

I told you last time Ohms Law I = V/R

so if you have say 10V supply across a 10 Ohm resistor ( load) then there will be 1A of current .... 10/10 = 1
But if you keep the voltage supply the same but halve the resistance, then the current will double
eg.... 10V supply across a 5 Ohm resistor ( load) then there will be 2A of current .... 10/5 = 2

Dave

11. ### Danny Daviss

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Mar 16, 2013

Overloading means the outputs has lower resistance so the current gets doubled right?

Yes the output transistors have to have a high amperage rating for extra current flow

12. ### davennModerator

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Sep 5, 2009
No, that was just an example of how the current increases

No, read my last statement .... the load resistance could be changing anything
3/4 1/2 1/4 or even 0 Ohms aor anything in between

you still havent shown any circuit specific to these questions
And you need to find out why the overload is occurring .... what is happening with the load ???

Dave

Last edited: Mar 16, 2013
13. ### Danny Daviss

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Mar 16, 2013
Wait , I just want to understand the concept of this

Are you saying that overloading , changes the resistances on the output down , the resistance doesn't get added or go up right?

10V / 8 ohms =

Overloading this output of 8 ohms down to 2 ohms would be overloading the circuit because the output transformer and output transistors are matched to 8 ohms not 2 ohms right? this causing the current to increase up

14. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
No water analogy, but I can give you a car analogy...

Imagine you have a car that is rated for a 1000kg load. Imagine you put 1100kg in it, or 1200kG, or 1500kg, or 2000kg, or 10,000kg.

At huge overloads it will fail immediately. At lesser overloads it might work for a while, but get damaged if you go over a small bump.

It might not even fail with some level of overload, but it may wear out faster and fail (early) sometime later under a normal load.

15. ### Danny Daviss

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Mar 16, 2013
I do understand that analogy , but with electronics its different that's why I don't understand the overloading theory applied to electronics

Yes I understand it stress or wears out the output transistors

It sounds like overloading is close to low resistances or a short circuit, its a parallel path that increases current

16. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Perhaps after you've studied Ohms law, you will realise...

17. ### Danny Daviss

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Mar 16, 2013
ohms states that is R is low value that V and I are high, and V and I are proportional together

18. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
No it doesn't

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Nov 17, 2011
20. ### Danny Daviss

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Mar 16, 2013
I Have what am i missing about this, how do u guys look at ohms law? So voltage and current are not proportional together?