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fuse checker problem


Guest xchan6
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I am currently building a fuse checker circuit. Features are that if the fuse is ok, the green LED will light, if not, the red LED will together with a buzzer.

attachment contains the diagram.


My problem is, when fuse is ok, the buzzer will make a sound (lesser sound than fuse is out), but the RED LED is not glowing. When fuse is out, all is well.

Can anyone suggest tell me which is wrong with the circuit? i kinda designed it myself plus some reading.

post-108919-14279144605874_thumb.png

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Guest rajeshr

hi,
The buzzer is connected next to voltage source so small sound will be there. I guess D3 is green LED which glows when Fuse is ok as the circuit closed. When Fuse not ok that circuit opens. The current in PNP transistor flows from collector to emitter when voltage at the base is reduced when fuse is not ok.

Connect PNP transistor with collector on upside and emitter on downside. it should fix things for you.

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thank you for your replies i appreciate it and put it to consideration. however, i found the solution myself. The problem lies within the RED led, it was paralleled to R1 in series with Base-Emitter junction of the NPN transistor which is a technical problem. LED has its own constant voltage drop (~2.1), which is different to a Base-Emitter Junction voltage drop (~0.7). That is why, when fuse is ok, red led is not glowing since no current will flow through it due to BE junction's lower voltage drop. It showed that the buzzer is not working well but its the circuit itself was.

My solution was to swap the position of R3 and D1, thus making R3 adjust its voltage drop with accordance to BE junction's voltage drop, making the flow of current correct.

If something is wrong with what i stated above, i would love to know it since i really want to learn more. thank you guys.

EDIT: something was wrong with my attachment, i forgot to include the GREEN LED. New attachment is added XD

post-108919-14279144606024_thumb.png

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