question about resistors.

[scampo77]

im still not sure how to order parts myself from suppliers. i have 4 different resistors. marked, 22r1, 1000, 1002, 1003. i know they are all 0805 in size but im not sure how to verify this info from the data sheet. how many differnt sizes are there? 0805,1206,1210.....

when searching for these parts i measured old ones on the board and found the resistance on the 22r1 to be 22.4 ohms. i figured my multimeter could be out a little bit but when i went searching for replacement parts i found 22 ohm and 22.1 ohm resistors. is the "r1" like a decimal place? if i did interchange these resistors what would happen? would i even notice? even when i do find some they seem to be rated by wattage is a certain wattage indicitive of the physical diminsions?

the difference between surface mounted resistors and through hole resistors (the ones with the stripes). is this a manufacturing cost thing? or is it a sign of the age of the amplifier?

[Perry Babin]

If you order from Mouser, all of the information is on the page like the one I posted earlier.

There are many different sizes. The 0805 and 1206 are the most common in car amps.

They use the 'r' because it's easier to see and less likely to be missed.

If the resistor has 22r1 stamped on it, order a 22.1 ohm resistor. All resistors have a tolerance (±x%) so they will rarely read precisely the same as the marked value. Reading resistors in the board is not reliable. Most meters have trouble reading low value resistors accurately. Most meters will read ~0.2 ohms with the leads shorted together. This has to be subtracted from the displayed value.

Physical dimensions are related to wattage. Larger resistors can dissipate more heat. However, there are resistors that have smaller than average dimensions for a particular wattage. They are made of materials that can operate at higher temps without failure. In general, any 0805 can replace any other 0805 in car amplifiers.

SMD components are used because the amplifiers can be produced more cheaply when produced in large quantities. In some critical circuits (particularly those operating at very high frequencies), the SMD components offer advantages but for most audio circuits, there's no reason to use SMD over through-hole components.

[scampo77]

is it possible to replace one resistor with another size? like a 0805 with a 1206, or even a chip resistor with a through hole resistor?

the guy at the local radio shack said that the smd resistors have the numbers stamped on them break down like this. the first 3 numbers are the resistance and the 4th number is the number of zeros. is this true?

that would make a 1003 resistor a 100,000 ohm resistor and a 1004 a 1,000,000 ohm resistor.

[Perry Babin]

You can replace SMD resistors with through-hole resistors but you have to be very careful when soldering/desoldering so that you don't lift the pads on the board. If you do this, lay the resistors on the board and bend the leads so they touch the pads when the body of the through-hole resistor is laying flat on the circuit board. After soldering, tack the resistor to the board with a drop of adhesive. Since SMD resistors are readily available, it's better to order the correct part.

If you want to use the through-hole parts temporarily so you can continue troubleshooting, that would be OK but you should heat both pads at the same time when desoldering the resistor so you don't damage the pads.

What he told you is correct for 1% components. 5% components only have 3 characters/digits and the 3rd digit is the multiplier. One exception is when an R or decimal point is used. For those resistors, the digits to the right of the R/. are part of the resistor value and not a multiplier. From top to bottom, the following are 20 ohm 1%, 22.1 ohm 1% and 20 ohm 5%.