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It seems that both 49.9Ω and 50Ω resistors are a thing:

https://www.mouser.co.uk/Passive-Components/Resistors//N-5g9n?P=1z0wnltZ1z0wljo https://www.mouser.co.uk/Passive-Components/Resistors//N-5g9n?P=1z0wljoZ1z0x8f8

Given that they're typically 1% tolerance, why have both? In particular, why even bother with 49.9Ω?

NPE
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    It's not the case in this particular instance, but sometimes you'll get odd values meant to be used as a resistor divider. For example, you might see a 9.9kΩ resistor, which when used in conjunction with a 100Ω resistor provides a 100:1 voltage divider. – Hearth Oct 27 '18 at 21:55
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    @community This is a totally different question from the supposedly duplicate one – F.Ahmed Oct 28 '18 at 09:00

2 Answers2

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If you need a 1% resistor, \$50\Omega\$ resistors are not a standard 1% value. The nearest standard 1% value is \$49.9\Omega\$. See http://www.rfcafe.com/references/electrical/resistor-values.htm for standard values and some explanation of how they're derived.

The equation for standard 1% resistors is: $$ R = 10^{d + \frac{i}{96}}$$ where \$d\$ is the (integer) decade number, \$1 <= d < 6\$, and \$0 <= i < 96\$.

It's the same for other precisions, except that instead of 96 for 1%, we use 48 for 2%, 24 for 5% and 12 for 10%.

Edward
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  • D'oh — I never realised that 49.9R was a standard 1% value. What's not completely clear is why 50R is also a thing. – NPE Oct 27 '18 at 20:14
  • I suspect that 50R is a thing because 50 ohms is the standard RF characteristic impedance, and people get hung up on having **exactly** the right resistance -- even though 49.9R is only 0.2% away from 50. (Having written that, I further -- and cynically -- suspect that there are 2% 50R resistors that are just re-labeled 49.9R). – TimWescott Oct 27 '18 at 20:57
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The point of the E96 series (and the other series for varied precision) is to ensure the optimum permutations within the tolerance. Why produce 50.0R when that value falls within the 49.9R tolerance band.

enter image description here https://mightyohm.com/blog/2009/01/eia-resistor-values-explained/

  • That does make me wonder why 50R also exist (for example, I have some 50R 2% parts). – NPE Oct 27 '18 at 20:18
  • Now that is odd... 2% is the E48 series and that value doesn't appear –  Oct 27 '18 at 20:19
  • Mouser lists a whole bunch (155) of 50R parts: https://www.mouser.co.uk/Passive-Components/Resistors/_/N-5g9n?P=1z0wljoZ1z0x8f8 – NPE Oct 27 '18 at 20:22
  • Ahh those are more power resistors (with the odd 50mR). With these the aim is to extract the power thus the use of values spread of across a Renard series isn't as important as someone isn't also going to need 49.5. the actual value available in that range will be limited and thus will be driven more by the power handling and what can fit in the pacakge –  Oct 27 '18 at 20:24
  • 50R is a radio thing, so9methinguing about quarter wave antennas and transmission lines, – Jasen Слава Україні Oct 27 '18 at 20:52