burnt resistors on PCB changing them to higher wattage

When repairing PCB of different designs or modules

If you find alot of 1/4 watt resistors burnt , some repair tech. will change the 1/4 resistor to 1/2 watt resistors so they won't get burnt up again so they can handle more current, but now the 1/2 watt resistors can handle all these extra current and voltage but now further down the chain the IC's melt or other 1/4 watt resistors burn

Its weird how where the 1/4 resistors are at in the schematic that have more current

From looking at a schematic how would you know which 1/4 resistors are going to get MORE CURRENT than others?

1/4 watt resistor list
1.) Resistors to ground? have more current?
2.) Resistors that sink current?
3.)

Which resistors in a network or design can STRESS the 1/4 watt resistor?

Usually all resistors 1/4 watt are used in your application with a margen of security. If they get burn and you change for a high power, you are solving the problematic not the problem. SOmething is wrong in somewhere that provoques the increase of current, that is what you must find and solve. And you must use the 1/4 W resistors. If not as you found, that will provoque another problem.

It does not depend on the position or other thigs, but only from the Vrms applied and the resistance, with the formula P=V*V/R
Where V is the RMS voltage across the resistor.

Mandi

Ya all the replies are true, but while designing we need to take care if the current flow is more then increase the width of the track so that the current is limited. if you have thin tracks and use more current the tracks will burn causing problems to components. 1/2 W resistor have less current carrying capacity, compared to wire wound or 1/2 W resistors. Using 1/2 watt resistor should also have the design accordingly. Thou the design may not change drastically,

Added after 1 minutes:

Ya all the replies are true, but while designing we need to take care if the current flow is more then increase the width of the track so that the current is limited. if you have thin tracks and use more current the tracks will burn causing problems to components. 1/2 W resistor have less current carrying capacity, compared to wire wound or 1/2 W resistors. Using 1/2 watt resistor should also have the design accordingly. Thou the design may not change drastically,

How would i know where the "increase of current" is coming from or what is causing it?

If you are using regulated power supply then you can measure it there, If you doubt you can feel few components gets too much heated in the board, or if you have any IC's sharing a common input voltage then if there is component problem or short then they will draw more current or there will be drop in vlotage at the VCC pin of each IC. Or if you sense if your circuit is not working then remove all teh IC's and measure the volatge at VCC pins, then insert Ic one by one and keep measuring the Vcc. if you find any variation in the supply voltage then you catch the culprit on whose addition to circuit caused this problem.

Possibly the circuit has a specific defect, that causes overload of the said resistors. In this case supplying resistors with higher power capability would be as helpful as using a bigger bumper at a car with a defective brake.

Or the the circuit has a design flaw, that the developer didn't realize. In this case, you have to understand the circuit better than the original designer. Not impossible at all, but requiring a lot of design experience.

Or the circuit is used in a way, that wasn't inteded respectively foreseen by the designer, but could be achieved by smaller changes. If the the necessary changes are obvious, you actually may succeed by simply replacing some resistors.

walters said:

How would i know where the "increase of current" is coming from or what is causing it?

Click to expand...

if you learn the basic principle about electronic and then from the circuit schematic you can analyze where the most current is going to

1. look to the load, is the resistance too small? if it was, then you should put the series resistor for limiting the current

2. but if the load only can work with a 'huge' current, then you should change your resistor with higher power capabilities

3. another possibility is that the circuit had the wrong path in somewhere (i don't know where), then it's better you put your circuit here

3. another possibility is that the circuit had the wrong path in somewhere (i don't know where), then it's better you put your circuit here

Can you explain What this point means. If there is circuit problem what can wattage of resistor do for that, your complete circuit gets short and components get damaged.
If the signal track is wrong then it will not affect the components as they dont cause breakdown of the circuit.

1. look to the load, is the resistance too small? if it was, then you should put the series resistor for limiting the current.
If there happens to be problem at the input side due to over-voltage to regulator then you get excess current in the tracks. No load resistance comes into picture at that time. and more over how much a current will be limited if you put a 1/2 watt resistor instead of 1/4 watt resistor, i really dont understand.

is st point has no value then I need not comment the 2nd point as it holds the same. Might be i have forgotten basics of electronics after working in the industry for many years. Please let me know if i am wrong anywhere.

What is the difference in current carrying and current limiting capacity between 1/2 watt and 1/4watt. please explain.

current carrying and current limiting capacity between 1/2 watt and 1/4watt.

1,) the 1/2 watt resistor is not going to current limit but be able to take the increase of current from the design flaw


I don't know why in certain areas on the board only some resistors burn up , all i can think of it that there is points where current sources ADD or voltage source ADD to have more current

Or since the board uses IC's op-amps these "increase the current" so maybe thats where the increase of current comes from and the resistor wattage rates can't handle it?

you are now thinking as a professional and are coming to the right track. you will catch the correct point from here on.

Your analysis / assumption is somewhat correct. keep it up. try to think in the same lines you are very near to the answer.