# Blown Fuse indicator - 24vdc

#### gcsier

Joined Feb 20, 2021
1
Hi, everyone

I am designing a PCB and I need help to specify some components to a blown fuse indicator.
I am using the schematic in the picture.
I know that my system is 24Vdc. Could anyone help me to know which transistor, diode and resistor should I use?
My led is 2V, 20mA.
I was using transistor PNP BC558 and diode IN4148, but I want to know if it is right.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,749
The transistors are graded A, B and C, so the value of R2 depends on the grade.

R1 is easy: R<= (24-2-0.6)/20mA; 20 mA is 0.020 Amps. Wattage > (I^2)*R1; probaby 1/4 to 1/2 Watt.

R2 depends on the grade/ That determines Hfe. For the A series device, Hfe (min) is 125. You want the base current o be
Ib = Ic/(Hfe*10); Ic = 20mA; Hfe 124; 10 is just 10x

So R2 <= (24-2-0.6)/(20mA/125*10); 24 = power suppply; 2=vf of LED; 0.6 = a typical diode drop; 125 typical Hfe of A grade A device.

The diode really doesn't matter much. I'd probably use a 1n4001. 50V 1A. The 1n4148 is a small signal diode.

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#### crutschow

Joined Mar 14, 2008
27,198
What's the purpose of the diode?
It would seem that it's not needed.

#### Ian0

Joined Aug 7, 2020
2,196
What's the purpose of the diode?
It would seem that it's not needed.
Assume R_load is zero, being the short circuit that's just blown the fuse!

#### LowQCab

Joined Nov 6, 2012
525
Put this across the Fuse-Holder Terminals ........
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#### crutschow

Joined Mar 14, 2008
27,198
Assume R_load is zero, being the short circuit that's just blown the fuse!
Okay.
See that now.