# how to calculate the value of one missing capacitor on a circuit with uc3843

Thread Starter

#### Mullins

Joined Dec 31, 2021
178
Hi to all.
Still here to ask for help
I have one Control board with one component burned. I saw one photo on internet and the component is one ceramic capacitor C1. I follow the line and saw that the capacitor is connected to pin 3 of uc3843. I made one draw to explain it. The r1 doesn't exist but this is the value on the dmm when I put the probe on pin 5 (Gnd) and pin 3. Kindly someone can tell me how to determinate the value of the capacitor? Should I replace the uc3843? The pcb have many cracked point. I saw many examples with uc3843 so I make a draw of my board obviously not all the board. C1 is the burned one. For the value of r2 I have to remove it from circuit and measure it. Not sure about color band...

#### Dodgydave

Joined Jun 22, 2012
11,303
Colours look like Yellow, Grey, Black, Orange, so that's 48 pF, 3%,
If it's a resistor then it's 480K.

Pin 3 is the current sense for the switch-mode chip, have you got the fill picture of the circuit board.

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Thread Starter

#### Mullins

Joined Dec 31, 2021
178
The First photo Is R2 resistore. The Maine problem Is the valute of the capacitor. The capacitor explode and Is not anymore on the board.

#### MisterBill2

Joined Jan 23, 2018
18,538
If a capacitor ( C1) exploded on the circuit board then certainly there was enough current into that pin #3 to do additional damage.

Thread Starter

#### Mullins

Joined Dec 31, 2021
178
Ok, surely there was too much current.
Since pin 3 is "sens" the current came from the "S" MOSFET terminal.
So do I have to go and look for the component that supplies power to the "s" pin of the mosfet?
Otherwise Did Uc3843 have an internal short cirquito that caused the capacitor to burn?

#### MisterBill2

Joined Jan 23, 2018
18,538
Are you certain that the missing component was a capacitor? The photo does not show enough area to allow even a guess as to what the missing item was. And the circuit shown does not make sense either.
So show the area around where the part was.

Thread Starter

#### Mullins

Joined Dec 31, 2021
178
Sorry @MisterBill2 your right. I looked better the photo and it was a resistor.

#### MisterBill2

Joined Jan 23, 2018
18,538
OK, and in the other half of the photo I see that the device has a marking that may indicate the value of the resistance. So with better magnification the value might be determined.
BUT there exists the problem that resistors in a correctly designed and built product to not "burn out for no reason." This means that some other part has failed and caused excess current to pass through the resistor. Probably it was quite a lot of current. So some other component must also be failed to allow that much current.
At this point having a circuit schematic diagram would allow quite a bit of diagnostic progress towards determining what part, or parts, would have failed and caused the problem. Without a diagram the task will be a big challenge.