Correct wattage

[ldboehm]

I’m rebuilding this old flex circuit from an early 1980s car. It’s a 12v circuit from the dash cluster.
I’ve identified the correct resistors: 22 (1w), 150 (2W) & 470 (1W) ohm.
But it seems like the ones I received are a lot smaller than the originals. Has technology changed so much that the resistors can be smaller now or did I order the wrong ones?
TIA

[PaulHutch]

@ldboehm

Has technology changed so much that the resistors can be smaller now or did I order the wrong ones?

Yes it has, the newer materials used allowed for a lot of miniaturization during the 1980s and later.

FYI - an early 80s car uses 1970s era component technologies, due to a 5 to 10 year design/qualification to production cycle for autos of the time. A lot of the long timeframe was because there were essentially zero micro computers for engineering so things like PCBs were almost certainly laid out by skilled draftsman using tape, transfers & ink on film.

[Robert_Fay]

@ldboehm we run into this a lot and I actually wrote a post to help show how some resistors with the same watt value are smaller than others.

Physical size of a through hole resistor

One call our customer service department has been getting more of recently is, people thinking they have gotten the wrong resistor because the body of the resistor is smaller than similar resistors they have received in the past. Often when the customer reviews the physical specifications in the data sheet they find they have the correct part. Many of the through hole resistor manufacturers have started to come out with mini lines to meet an ever growing demand for smaller and smaller parts. Yo…

[ldboehm]

Is there a way for me to calculate the correct wattage for each resistor?
I know it is a 12v circuit for a car, and the resistor values of 22, 150 and 470

[PaulHutch]

You need to know how much voltage is dropped, or current is passed, through each resistor. It’s virtually certain that none of the resistors has 12V across them.

I know of three ways to do this:

• Assume 12V across each resistor - An over the top method where all resistors will have way too high a rating, easily 2 to 50 times too high. Then use this formula (combination of Watt’s & Ohm’s laws): P = E²/R (Watts = Voltage²/Ohms) = 12V²/22ohms = 144/22 = 6.54 so round to 10 watts
• The technicians way - measure the voltage across the resistor while the circuit is operating then use the combination of Watt’s & Ohm’s laws formulas above.
• Use the schematic diagram of the complete circuits to calculate the voltage drops/current flows through each resistor.

[AaronRollens]

Paul, I just want to thank you for the great answers like this that you provide. You are part of what makes our community great.

[ldboehm]

Paul,
Thanks for the excellent reply and I appreciate the help as I’m learning.
I’m basically reverse engineering a circuit and there’s no schematic for it.
I do think that 12v is being fed to the dash cluster but agree 10 Watts seems very large.
The resistors in this circuit are being used to drive LEDs.
Judging from the size ( I know that’s not always accurate, but these are all from the late 1970s/80s) I think they are 1/2W or 1W
Knowing this is for LEDs, I’m assuming there’s no issue if I use 1W instead of 1/2W.
-Larry

[PaulHutch]

@ldboehm

Knowing this is for LEDs, I’m assuming there’s no issue if I use 1W instead of 1/2W.

Generally yes, and when they are just involved in LED driving almost certainly yes.

Other than physical size problems (e.g. air flow, fitting), another consideration that can make a higher value unsuitable is material type. Sometimes going higher involves changing from a film type to a wire wound type resistor, and for some circuits that can be a big problem due to the inductance of most wire wound resistors.

So in summary, with LED driver circuits, when sticking with the same type resistor (film vs. wire wound), and assuming it’s physically close to the same size, then you can use any higher wattage.