# How to determine watts and ohms for circuit board resistors?

I am about to embark on my first soldering job to fix a module that is broken inside my truck (compass/temperature display). It is a well known design deficiency in this particular vehicle brand as the failure has happened a lot. In most cases, one of the two 51 ohm resistors overheats and for reasons explained in this article, which also, to a fair degree outlines the fix.

What confuses me is how he came up with the idea to replace a single 51 ohm resistor with three 1W 150 ohm resistors (the formula for calculating net resistance):

A little research told me that the resistors have values of 51 Ohms. Figuring in the 12V power supply, that means they are probably dissipating about 3 Watts each. That equates to a lot of heat in such a small package. No wonder they keep failing. Who thought this design was a good idea? I decided to replace the single resistor with three 1W 150 Ohm resistors in parallel, and extend them off the board and out into the air so they can cool by convection. In my case, it was only one of the two resistors that kept breaking loose. Some people only have trouble with one of them. Others have trouble with both of them. I only replaced the one that has repeatedly failed. If the other one ever fails I will replace it too.

I went to Radio Shack to get the supplies necessary but they didn't have 1W 150 Ohm resistors, only 1/2 W 150 ohm so I got those thinking that in such small wattages (?), it may be okay.

I am looking for an explanation how to, in such parameters (12V DC, small electricity usage, etc), you would calculate what wattage and ohmage is necessary for your resistor. I haven't done any electronics but have lots of experience with residential electrical work and how electric load is balanced and how circuits should be structured.

• The assumption that all 12V are across the $50\Omega$ resistor is probably wrong anyway. Such an underpowered resistor would fail in minutes, not months... – Vladimir Cravero Mar 15 '15 at 18:35
• That's not Ohm's law, since Ohm's law only defines the relationship between voltage, current, and resistance. The power part came along some time later, I believe, and may be credited to Joule. – EM Fields Mar 15 '15 at 18:41
• The part getting hot may not be the resistor (that looks like a 1206 package which would never survive 3W, by resistorguide.com/resistor-sizes-and-packages ) but the vacuum fluorescent display, which has a heater in it that is required to work. – pjc50 Mar 15 '15 at 18:44
• It's Ohm's law with one value substituted for a power calculation. Ohm's law is still the basis of the formula. – Majenko Mar 15 '15 at 18:44

The equation is basic Ohms Law with parts replaced for power calculation.

The formula is: $$P=\frac{V^2}{R}$$ As it's got 12V across the resistor, and the resistor measures 51Ω, that becomes: $$P=\frac{12^2}{51} = 2.82W$$ So you need at least 3W to be safe.

Placing resistors in parallel results in lower resistance. The long form of the formula for that is: $$\frac{1}{R_T} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + ... + \frac{1}{R_n}$$ So taking three 150Ω resistors in parallel gives you: $$\frac{1}{R_T} = \frac{1}{150} + \frac{1}{150} + \frac{1}{150}$$ $$\frac{1}{R_T} = 0.02$$ $$R_T = \frac{1}{0.02} = 50\Omega$$ If all the resistors are the same value then it is a simple as dividing the value by the number of resistors, and the power ratings just add together, 20 three 0.5W resistors would give 1.5W of power. Not enough for your application.

If you cannot get 1W resistors then you would be better with six 0.5W resistors of double the value - so 300Ω each.

• That's assuming there's 12V across the resistor. But why would there be 12V across the resistor? That would be a really dumb design, to place resistors directly across the power supply and waste power for no reason. – user253751 Jan 15 '16 at 6:46
• There is obviously a design flaw or these resistors wouldn't keep failing. No one here knows exactly what that resistor does or how it is wired. Yes it is highly unlikely that it is wired directly across the power, but without knowing more all that can be done is guess. Since a car is a (nominal) 12v system a worst case scenario would be 12v across the resistor. It probably isn't, but it can't really be much more (yes I know car electrics can read 15v or so with spikes over 60v) so working on that principle the assumption that there is 12v across it is perfectly valid. We are not concerned... – Majenko Jan 15 '16 at 10:01
• ... in this with the why of the circuit, only (as the question has asked) the why of the mathematics. How to calculate the values given the supplied (possibly incorrect, but worst case possibility) data. How to select the best components. In this, whether the assumption is correct or not (there is no way to validate it without discovering what the resistors actually do in the circuit), the mathematical theory is still correct and provedes all the information requested in the question. So it matters not if the resistor is across the power supply or not - one just has to assume that it could.. – Majenko Jan 15 '16 at 10:05
• ... be and work from there. – Majenko Jan 15 '16 at 10:05

two formulas you need: ohms law relates voltage, resistance and current: V = I * R power: P = V * I

so assuming the original circuit had 12V across a 51 ohm resistor, the current flowing in that resistor (ohms law) is I = V / R = 12 / 51 = 235ma. the power in that resistor is: P = V * I = 12 * .235 = 2.8W which is close enough to 3W as you noted.

when you paralle resistors the current divides and flows equally through each resistor. so 3 1/2W resistors in parallel, of the same value, is the same as using a resistor whose value is 1/3rd of each resistor, but can dissipate 3 times the power.

So 3 150ohm resistors looks like 1 50ohm resistor. and if each resistor is 1W then it looks like a 3W resistor. if you used 1/2W resistors then you get 1/2W * 3 = 1.5W. if you put 12V across this it will burn up eventually.

Wiring three 150 ohm resistors in parallel produces a resistance of 150/3, or 50 ohms. Close enough to that 51 ohm value. If the resistors have to handle 3 watts total, then each one has to be able to handle 1 watt. So those 1/2 watt resistors aren't adequate. But you could use six 300 ohm 1/2 watt resistors.