This is the resistor from my macbook power supply.
It *look* to me to be black, orange, white (or grey), silver, brown.
I thought it would be easy to calculate the value but I think it falls into a special category (5 bands, fourth band silver) so I'm not confident about deciding it.
What is it?
What are the colours? it not sane to have the first band as black=0
it *ought* to be 39 ohms 5% 100ppm/°k
But I would have expected that to be orange white black silver brownm.
“A leader is best When people barely know he exists. Of a good leader,
who talks little,When his work is done, his aim fulfilled,They will say,
On Tuesday, 8 October 2019 08:04:57 UTC+1, Brian Gaff wrote:
Well it is from an Apple supply ;-)
I too think it's 0.39 or 0R39
and the black band is zero and probably there to itentify which end you use to start from 039 x 0.01 at 100ppm
rather than 1s930
The only resistors I have with a first band as black are some zero ohm resistors which just have one black band.
A lecturer has asked me if I can buy some 10% (silver band) tolernace resistors
Why does he want resistors with such a wide tolerance?
I haven't been responsible for ordering components regularly
sine 1969 (!) and I didn't have a resistor with a tolerance
worse than 5% in my entire stock even then.
Taking a qick look on-line to see what is available today:
I see lots of 0.25W 1% resistors for just over a penny each
and 5% values for half that so it isn't going to be an economy
L looked for 10% tolerance and only found 9 results - and they
are all 2W at 8p ea!
Interestingly, in 1969 I wasc paying 16/- (80p) for 100 0.5W
5% resistors so a pretty comparable price despite all the
rampant inflation in the intervening years!
Given that all my resistors were carbon film and the modern
ones are superior metal film, todays prices are a bargain!
On Tuesday, 8 October 2019 18:46:08 UTC+1, Terry Casey wrote:
She asked me because part of a lab is to test and understand tolorencies
and also the series of E values that are made for resistors although that w
as my idea. We also have to teach them the intricacies of power rating and
what happens when these are exceeded. So in a lab it'd be nice to have thes
e to test.
I actually remmeber 20% which IIRC had a pink band, they were 2W and had a
sticky waxy feel to them.
Obviously the problem is how do you really test a 5% resistor with meters t
hat are only accurate to 1-2%. I've tested a few 5% and it is suprising how
accurate they are I don't think I've found one that is more than 3% out, a
nd there's no way I'm going through my boxes of 1000 per value :-<>
But don't forget this is a lecturer not someone that actually deals in such
things in the real world. ;-)
Today we are running a lab titled Embeded Systems.
Which uses a freescale development board the first part shows them how to c
onnect a push switch to the GPIO and then an LED off that.
The LED is shown in the diagram as connected to a 300 Ohm resistor the lect
urer has even bothered to draw the resistor in place showing how to connect
it up to a protoboard and the resistor with the colour code LtoR ! of Red
So I have 40 odd (very odd) students coming to me asking why I don't have a
ny 300 ohm resistors.
One student was very proud of the fact of working out he could use 2 150R t
o make a 300R resistor, I did say he could use a 270R or a 330R and all tha
t would do is change the brightness of the LED, I also said if he used the
330 then that would reduce the amount of current and help the world cut dow
n on global warming and perhaps save trees too.
I've stopped a couple of them walking off with 330K resistors, as they were
n't sure of the relivance of the R & K after the digits
But don't forget I'm not allowed to teach students, but I am allowed to and
expected to impart my knowledge, which strangley enough doesn't warrent a
salery scale increase.
This is what I buy
I have bought some 1% t 100R 1K 10K 100K a few 0.1% 1K , 10K and a 50R
and a 1K of 0.01% for £15 each for demonstrating to any inquisitive s
tudent or academic.
We do have a LCR meter that can measure resitors to 0.1% accuracy but as it
was £1,400 we only have one making it difficult for up to 50 students
per session to use.
Yes and I'd need at least 50 of a few values and that is if students after
using them didn't lose them, break them or eat them, which is why I've told
the lecturer I don't keep 10% as they aren't really used/made in this mill
I heard that in 1979 a blue LED retailed at £30 !
I leave them out in the lab for students to help themselves same as the red
, yellow, green and white versions in both 5mm and 3mm sizes.
yes I agree and why I don't really want students putting them back in draws
if the leads are damaged as puting them back into our protoboards can and
does damage the contacts in the protoboards which are £10 each !
we orered 100 protoboard for this year, hopefully most will survive and can
be reused next year.
20% resistors were a bit before my time but the chart demonstrates what
I always thought about the apparently strange value choices.
Make a batch of carbon resistors...... test for value and designate a
fit in one of the available bands. No wastage:-)
On 07/10/2019 17:45, The Natural Philosopher wrote:
I reckon silver is the multiplier x0.01 and it is actually 0.39 ohm.
E12 value shifted along with a leading zero> Not seen one before.
Red or brown is probably the tolerance.
Most of my small power resistors have the value in small type face.
Yes but a resistor of that value is normally a more high wattage device.
What would one have such a low value for in such a low wattage, since the
ruddy power cable is a fair percentage of that. One would not use a fusable
resistor these days.
This newsgroup posting comes to you directly from...
It will be whatever wattage is needed for the circuit. Presumably
something has drawn a lot more current through it than it was rated for
so replacing it will merely kill another unfortunate 0.39R component.
Resistors in properly designed circuits seldom burn out unless something
else catastrophic has happened like a crowbar circuit across the PSU.
Its probably a current sense resistor.
Something will be reading the voltage across it to control the feedback.
1A current will give 0.39V and about 400mW power dissipation.
It looks like a half watt resistor so I expect it to be something around
Without a circuit that's about all I can say.
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