how to chose the right resistor

Thread Starter

Mullins

Joined Dec 31, 2021
179
Hi to all.
I have one melf resistor to replace since the old one burned. I got an other control board and removed the same resistor. The color code show it's 1kohm resistor. According to the dimensions it must be a "MMA 0204". I gave power to the resistor with 12v and I got 0.012A. According to the formula P=V⋅I
I got 0.144W ( about 1/8W). If I look on Mouser electronics "MMA 0204" format is available from 0.25 (1/4W) and over. That means Mouser electronics doesn't have enough chose or is there some mistake on my calculations ? May exist "MMA 0204" 1/8W? Senza-titolo-1.jpg
 

Thread Starter

Mullins

Joined Dec 31, 2021
179
So, you are telling me that I can I replace it with any one as long as it is larger than the original one? 1kohm 1/2w can be replaced with 1kohm 0.5w, 1w....? Usually I put the same one?
 

Ian Rogers

Joined Dec 12, 2012
1,136
A few years back even thin film / carbon resistors were re-rated. all 1/4 watt changed because they were "too large"
the original max voltage reduced from 500v to 300v. I don't know if the same applies to SMT resistors, but who cares.. a1/4 watt will be fine.
 

MisterBill2

Joined Jan 23, 2018
18,539
If any of those resistors burned up, there must be some other component failed that caused too much current thru the resistor. So until that problem is fixed replacing the resistor will not solve any problem.
If you replace that resistor with another one of a high enough power rated resistor to not burn up, then some other components will burn up.
None of the resistors in the portion of the circuit shown could pass enough current to overheat unless some other part has failed.
 

Thread Starter

Mullins

Joined Dec 31, 2021
179
The board is mounted in a machine that vibrates too much. All the welds were cracked. Some of them very much. After the rectifier I found 2 burnt resistors in parallel of 1 and 2 ohm 1/2w. Then R1 and the mosfet are broken. UC3843 I can't test it but I'll replace it regardless. I vacuumed with the gun and then re-soldered ALL THE TIN SOLDERING, more than 5 hours. I also tested all the transistors and mosfets. Now I can only hope. According to my experience the problem arose from a cracked weld. I explained my hypothesis to the customer and he confirmed that the machine vibrates a lot and there is no way to move the board in to a less stressed place.
 

MisterBill2

Joined Jan 23, 2018
18,539
OK, machine vibration is a different case. That might be worse than the vibration on a one-banger BSA bike.
Hard Wax over all the parts would be the way to go. Use that instead of epoxy because it can be melted off with boiling hot water, when repairs need to be made.
 

MisterBill2

Joined Jan 23, 2018
18,539
It may also be possible that there is some sort of unbalance problem with the way the machine is assembled. That does happen on occasion.
Whem re-soldering the broken connections, using a bit more solder to provide a bit more support may be a good idea.
 
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