Mike Rice wrote:
> When resistors are placed in Series, isn't the string limited
> to the current rating of the least of them?
Well, resistors are not usually rated for current. A resistor converts
electrical energy into heat energy, and it is this heat that must be
limited so the resistor does not melt or burst into flames. Thus, the
limiting factor is the amount of heat that EACH resistor dissipates into
its surroundings.
> When I put (for example)
> several fuses in series, say (3)+(.5)+(2)[I was playing, yes] it
> always blew at 1/2 amp current, couldn't even drive a decent
> "listening" radio. Unless I was playing too hard at the time, is
> there a difference I'm missing?
In a series circuit, the current passing through any one of the components
also passes through all the others, therefore, the amount of current in a
series circuit is the same for each of the components. When you tried to
pass one amp of current through your series array of fuses, the 3 amp fuse
was happy passing 1 amp, the 2 amp fuse was quite comfortable passing 1
amp, but the half amp fuse couldn't take it and opened up, preventing
current from flowing through any of the fuses anymore.
>
> Unclear of the concept, I am.
>
The amount of power dissipated as heat by a resistor is equal to I**2 R.
(That's I times I times R). Since the current through any component in a
series circuit is the same for any of the components, the I**2 part is the
same for all the resistors. On the other hand, the resistance value could
be different, thus the amount of heat created by each resistor can be
different than the others in the series circuit. Since the current through
the resistors is the same, the heat generated is proportional to the
resistance value in ohms, thus, a 100 ohm resistor will always be called
upon to dissipate twice as much heat as the 50 ohm resistor in series with
it.
An example: I have a half-watt 10 ohm resistor and a half-watt 40 ohm
resistor. I wire them in series hoping to make a 50 ohm dummy load. If I
pass 112 mA through the string, the 10 ohm resistor must dissipate 0.112 *
0.112 * 10 watts or 0.125 watts. The 40 ohm resistor must dissipate 0.112
* 0.112 * 40 watts or 0.502 watts. If I try to send more current than 112
mA through the series, I will exceed the power rating of the 40 ohm
resistor, even though the 10 ohm resistor is still cool. What is the power
rating of my resultant dummy load? It is the combination of the power
dissipated by all the resistors, in this case, 0.125 watts plus 0.502 watts
= 0.627 watts.
> C. Mike Rice KB0NND
>
Hope this helps more than it confuses.
Mike K1MG
BTW: Any farmer knows that it is far better to step in it than to fall in
it.
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