# Minimum wattage for resistors

#### spikespiegelbebop

Joined Nov 30, 2021
89

#### Irving

Joined Jan 30, 2016
3,312
That's because I'm choosing SMD components and the ones available seem to be too weak, look:
On what basis do you make that assertion?

Most thru-hole resistors are 1/8 or 1/4W as standard, but are rarely stressed at that level. Without going through the circuit and calculating it is hard to say if there's an issue with SMD parts.

#### crutschow

Joined Mar 14, 2008
31,524
You need to calculate/simulate the power dissipated in each resistor to determine the power rating needed.
The resistor rating should be at least twice the calculated dissipation.

#### spikespiegelbebop

Joined Nov 30, 2021
89
The author didn't specify which wattage should be used, so it means that ordinary 1/4 watts resistors (for example) would be enough, but I've never used SMDs, and there's this one, for example, saying 100 milliwatts, that's way less than 1 watt.

#### Dodgydave

Joined Jun 22, 2012
10,688
The yellow 220 ohms resistors R4,8, 12, need to be 1/2 W minimum, the rest can be 1/4W.

#### spikespiegelbebop

Joined Nov 30, 2021
89
Any tips on how to find those in JPCB library? Even when I specify the wattage, their search tool is pretty lame. I also accept suggestions on other PCB Assembly companies. There's PCB Way, but they're more expensive than JPCB.

#### ThePanMan

Joined Mar 13, 2020
534
Give a man a fish ? ? ?

The way to calculate the wattage a resistor will dissipate is to know the voltage and the resistance. From there you can calculate the amperage. Then once you know the amperage going through that resistor you multiply that by the voltage. Pretty straight forward. But not really. If you have series components the resistance changes. Calculating the wattage means you need to discern the total amperage through that leg of the circuit. As long as you know the total current through the leg of circuitry you're concerned about you can easily calculate the wattage.

Some people recommend double the wattage and select a resistor from that. That's fine if you have the real estate (space) for it. The reason why surface mount resistors can be 1/10th watt (100mA) is because of reducing the footprint of the circuit board, thus making things smaller and smaller. The smaller it can be made the lower the amperage it will take. And the longer it will last on a charge. Smaller also means more powerful computation capability. Or more features that can be loaded onto a smaller and smaller circuit board.

Back when I started tinkering with circuits it was all 100% wiring. There were standoffs that you soldered a resistor lead to and then a wire. Run that wire to where it needed to go - and so on. Printed circuit boards drastically simplified circuitry because it could be printed onto a card and etch away the unwanted connections. Then drill through the circuit at given places and plate that hole with copper, then you could stick your components in where they go. That eliminated mis-wiring a circuit. With through hole components PCB's (Printed Circuit Boards) got a lot smaller. Then came surface mount components which allowed for even smaller PCB's.

OK, way way off topic. Sorry. I go by a standard practice: If I expect to dissipate 750mW ideally I'd go with 133% above, meaning 750mW X 1.33. If I was more concerned about managing heat I'd opt for 150% above the calculated wattage. Some people just go double and don't worry about it. In the case of my example 1 watt would be good enough, it meets my 133% rule. 750mW x 1.33 = 997.5mW. So I'd use a 1 watt in most cases. If there were a concern for heat buildup I'd opt for a 2 watt resistor because 750mW X 1.5 = 1.125W. Two watts is twice the size body resistor and larger leads. That would require a larger plated through hole (PT hole). Using oversized PT holes wastes solder and the connection is not as strong. Using a PT hole that the lead barely fits through can cause stress to the board itself and potentially cause cracks.

So use double the wattage if you want and if it doesn't become a concern for assembly. Otherwise if you can use the 133% rule in my example you'd be fine. For SMD (surface mount devices) wattage is more of a concern because the pads on a board may already be set for a certain physical size resistor, and using a higher wattage resistor means using a larger resistor, and therefore a component that might not fit on the circuit board. If you're building your own board or having a board house construct your board make sure you have the correct size components for your design. If you're designing then it's easy to specify a larger (higher wattage) resistor.

Hopefully now you know how to fish for yourself.

#### ThePanMan

Joined Mar 13, 2020
534
The yellow 220 ohms resistors R4,8, 12, need to be 1/2 W minimum, the rest can be 1/4W.
Given that he showed examples of SMD I'd think 1/4 watt resistors versus 1/2 watt, the size in his design may be more critical than simply stabbing a larger resistor on the board. But since the TS is asking about board houses it would appear he's in the design stage. Knowing how to size a resistor should be pretty basic, long before one takes on the task of designing a SMPCB.