What are suggestions for resistor and capacitor substitutions?

Question

When thinking about substituting components, I know that resistors are easily substituted within their 10%/5%/1% tolerances. But what about wattage on a resistor? Is it safe to do substitutions, say 1/4W when the schematic calls for 1/6W. What about tolerances? I assume that I can substitute a 10ohm/1% if the schematic calls for a 10ohm/5%, but not the other way around.

For capacitors, the only substitutions I've done to date are installing higher voltages than specs...e.g., if my schematic calls for a 470uf/15V, I've subbed a 470uf/50V that I had lying about. Is that a safe practice? or should cap substitution be avoided?

Answer 1

You can always replace a resistor with a higher power rated one. Like you say a 1/6 W (does that exist? I know there's 1/8 W and 1/10 W) can be replaced with a 1/4 W, if it otherwise uses the same technology. While it may be safe to replace a metal film by a carbon film resistor there may be specific reasons why the designer chose the metal film, so you would have to check that.

Same with tolerance: you can replace a 10 Ω/5 % resistor with a 1 % type. If the designer did his job properly he will have checked that the design works for at least all values between 9.5 Ω and 10.5 Ω, so the range of a 1 % resistor, 9.9 Ω to 10.1 Ω falls within that range, and should be OK.
Sometimes the other way around may be OK, though. 1 % resistors are often chosen because they give a design a higher level of reproducibility: with larger tolerances different devices may have very slightly different behavior. But if you want to do that you're on your own; there are no guarantees.

For capacitors higher voltage versions are often OK, but may have other parameters which may also be important in the application, like ESR. Don't assume that all 470 µF are created equal. Check the schematic to see what parameters may be relevant (like ESR), and compare datasheets.