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. You will find that certain series of resistors are able to have the same resistance values and power ratings in a package size that is smaller that previously built.
Take for example Stackpole part S100KQCT-ND and Stackpole part CF14JT100KCT-ND as you see in the below chart there two parts are electrically the same part, however physically the S100KQCT-ND is much smaller.
|
CF14JT100KCT-ND |
S100KQCT-ND |
| Resistance |
100k |
100k |
| Power |
1/4 watt |
1/4 watt |
| Tolerance |
5% |
5% |
| Composition |
Carbon Film |
Carbon Film |
| Features |
Flame retardant Coating
Safety |
Flame retardant Coating
Safety |
| Temp Coefficient |
0/-850ppm°C |
0/-850ppm°C |
| Oper Temp |
-55°C to 155°C |
-55°C to 155°C |
| Physical Size |
2.3mm x 6mm |
1.7mm x 3.3mm |
Here is a picture of the two resistors mentioned above next to one another.
If you have concerns over the values of your specific resistor please refer to the individual data sheet.
I struggled with this on my last PCB project that I did in KiCad. There are so many size choices and even being careful I still made an error and ended up with resistor footprints that were too small.
Fortunately DigiKey had a resistor that would fit. I just ordered them and threw the others in the parts bin.
Is there some sort of standard for axial resistor sizes?
Hello @mikeharpe,
This is part of the reason for this article. Now there is certainly a common size for many resistors. Take for example 1/4 watt resistors. Of the 51,000 1/4 watt through hole resistors currently in our system you will find that 32,000+ of the options fall between a diameter of 2.29 and 2.5mm and a length of 6 to 7mm. This being said you will also find a about 7,000 that have a diameter of less than 2mm and a length of less than 4 mm.
So no, there is not a standard that says if you order a 1/4w through hole resistor it will be 2.29x6.35mm. This is why it would be important to look a the data sheet for the resistor’s physical size prior to ordering if you are looking to ensure fitment.
This is also important to look at if you are replacing an existing resistor as you will not be able to say that the resistor must be 1/4w because of the size because while 2.29x6.35mm may be a very common 1/4 size you will find options in a 1/2w that will replace it.
If you have any further questions please let me know.
-Robert
