Voltage ratings on components

I have a question about the meaning of voltage ratings on electronic
components. Does the voltage rating mean it is the maximum voltage you can
apply to a device before it melts down? Or does it have a more subtle
meaning?

For instance, if I have an LED with a 5V rating on the package, do the
put that on there to say "If you apply more than 5 volts of pressure on
this component, it will 'snap' inside and all the magic smoke will come
out."

Or, is there another meaning behind it that engineers use?





The Moon is Waxing Crescent (18% of Full)

 

Voltage ratings are in the 'It depends' category.

For most devices, you should read the data sheet for the specific
statement of just what the voltage rating implies. For some parts, the
rating is understood to mean specific things.

The voltage rating of an electrolytic cap, for instance, is usually
taken to mean it's rated 'Working Voltage' - i.e. the voltage it can
withstand continuously in normal operation (although I would never use
a 10V electrolytic in a 9V system...)

For semiconductors, it may mean the normal (highest) rated working
voltage, or (depending on context) it may mean the absolute max rating.


So the answer is - find a voltage rating that you are unsure of, read
the datasheet, and if it's still not clear, ask - someone here should
know what it means.

Your LED rating may refer to either it's maximum forward voltage drop
(probable) or it's reverse voltage rating (possible but unlikely).

Cheers

PeteS

 

For some components (Transistors, Capacitor, resistors...) the voltage
rating is a maximum recommended operating voltage.

For ICs, the data sheets will often list a recommended operating
voltage and a maximum voltage beyond which the device may be damaged.

For light bulbs and relay coils (and probably your "5 volt" LED) the
stated voltage is the recommended operating voltage. (Normally, a bare
LED requires a current limiting resistor in series - an LED spec'd at
5 volts probably has an internal limiting resistor, suitable for
operation on 5 volts.)


--
Peter Bennett, VE7CEI
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Maximum, *recommended*? You sound like you're talking about TOOBS!

That's more like it.. ;-)

Yeah transistors should be lumped here.. I have yet to meet a transistor
that doesn't explode over its limit (or at least act like a zener, i.e.
avalanche effect).

To the OP: for things like resistors, you have three intersecting limit
curves, depending on wattage and value. The straight bounding limits are
current (I don't know what a typical 1/4 or 1/2W resistor is rated for) and
voltage, which depend on the physical size of the device as an absolute
rating. For instance, 1/4W resistors are rated as 250V or so, while 1/2W
resistors are good for 400V and 1Ws are good for 600V. But in almost all
cases, the curved limit of power wins out: for example, a 100k 1/2W resistor
is at maximum dissipation at 224V, well below the voltage limit for that
package.

What you get is, for resistors of yea wattage within whatever range of
resistance, you only have to worry about power. For resistors below some
fractional value, the current limit goes into effect. For values above some
large value (for 1/2W, around 330k), the voltage limit is imposed before you
can reach full power.

For values outside of the dissipation-limited zone, you can still burn out
the resistor by exceeding the limit, BUT, because the resistance is so low
or so high, you can never technically "burn" it out (by which I mean, exceed
dissipation rating).

And all this applies moreso for semiconductors, which are often, in effect,
variable resistors. ;-)

Tim

 

that's pretty much what it means

on a LED that's probably the maximum reverse voltage the manfacurer
guarantees it for, I've seen LEDs witstand as much as 12V (in the reverse
direction) without failing.

it's basically the design limit of the part.
all good parts will exceed the design limit by some extent. those that don't
are often relabled and sold as a less robust part.

Bye.
Jasen