Re: understanding resister / cap specs
- From: "Phil S." <psymonds_no_spam@xxxxxxxxxxx>
- Date: Fri, 28 Apr 2006 10:22:51 -0400
<Chief_Billy@xxxxxxxxxxx> wrote in message
news:1146232643.625809.90800@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
guitarPsych wrote:
I have a couple of questions regarding part ratings and using parts that
are not exactly spec (for instance, if the part on the spec cannot be
found to purchase anywhere). The values on these examples were chosen
to help illustrate the concept, not to discuss how common or available
the parts are:
1. If the needed capacitor is .1 uF (400 V min), can you substitute a
part that is .1 uF 500V min acceptable, or is it .1 uF 300V min that is
acceptable?
'min.' means MINIMUM.
300V < 400V < 500V
You can use 400V or greater.
2. If the needed capacitor is .1 uF (400 V min), can you substitute a
part that is .2 uF acceptable, or is it .05 uF that is acceptable?
0.1 uF is the value NEEDED...
3. If the needed resistor is 100 K (1/2 W min), can you substitute a
part that is 100 K 1/4 w min acceptable, or is it 100 K 1 W min that is
acceptable?
Test: What does 'min.' mean ?
0.25 W < 0.5 W < 1.0W.
You can use 0.5 watt or greater.
3. If the needed resistor is 100 K (1/2 W min), can you substitute a
part that is a part that says 75 K acceptable, or is it 125 K that is
acceptable?
100K is the value NEEDED.
What the Chief says, but also, in general, watt and volt specs are minimums.
Typically, nothing will happen if you are over spec, but just don't get
silly about it. You probably won't be able to fit a 10W resistor in the
place provided for a half-watter. Also, at this point, you will encounter
different materials which may or may not have an affect on tone. When you
alter the value of a cap (uf) or resistor (ohms), you change the circuit and
that will have an affect on other parts of the circuit. Sometimes it is
insignificant, sometimes it will tweak the tone, and sometimes it will be
bad for the amp. Only context will suggest the right answer.
Also, as you probably gathered from your earlier questions about filter
caps, when you get into the larger values, the difference between 80uf and
100uf is probably insignificant. There is no "tone" running on that part of
the circuit, although tone is clearly affected when the circuit is not
operating properly. Coupling caps are a whole different matter and
changing from .1uf to .2uf is a real change. Be aware that sometimes common
values from yesteryear and today have changed. Feel free to use .047uf
where .05uf is spec'd or .022uf where .02uf is spec'd. There is really no
difference. I think you get the drift -- you do need to apply some level of
judgement here. Like the Chief said, using .05uf where .2uf (or vice-versa)
is called for isn't the right thing to do, unless, of course, you'd
determined that the change is purposeful.
All of these electronic parts are subject to some level of variation, say
+/-10% or 5% or 1%, so perfection and precision and not always important.
Again, context will tell you what level of precision you need. For example,
if you are installing 1 ohm resistors on power tube cathodes so you can
check and set the bias, to get an accurate read, you really want 1% or
better tolerance. Can't find 'em? Use 5%-er's and recognize that you've
got a wider margin of error and figure a way to deal with it or live with
the result.
Phil
.
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