Electrical bits - Resistors

defuzz

New Member
I'm pretty new to electronics but am having a blast learning pics and stuff.

So far I've only used picaxe project boards which come with all the bits I need but I'd like to expand a bit and am looking at buying a "multipack" of resistors to have all the right bits for whatever I might do.

this is where I get stuck......

rapid electronics for instance have a number of different "kits" available but I don't understand the terminology:

Option 1: http://www.rapidonline.com/productinfo.aspx?tier1=Electronic+Components&tier2=Resistors+&+Potentiometer&tier3=Metal+Film+Resistors&tier4=MR25+Metal+film+resistor+kit&moduleno=65199#techspec

A pack containing a total of 1000 0.25W 1% 50ppm precision metal film resistors

Option 2:
http://www.rapidonline.com/productinfo.aspx?tier1=Electronic+Components&tier2=Resistors+&+Potentiometer&tier3=Carbon+Film+Resistors&tier4=CR25+Carbon+film+resistor+kit&moduleno=28374#techspec

A pack containing a total of 1000 0.25W 5% carbon film resistors

Option 3:
http://www.rapidonline.com/productinfo.aspx?tier1=Electronic+Components&tier2=Resistors+&+Potentiometer&tier3=Carbon+Film+Resistors&tier4=CR50+Carbon+film+resistor+kit&moduleno=65195


A pack containing a total of 1000 0.5W 5% carbon film resistors.




so my questions.....

- what does the 0.5w or 0.25w refer to?
- what does the % mean?
- whats the difference between metal and carbon film resistors?

- in summary, which, if any would be most useful to me as a pack for various pic based projects??


thanks so much for your help
 

bgrabowski

Senior Member
Go for the 0.25W 5% set. It means one quarter Watt power rated resistors which are guaranteed to be accurate to within 5% of their nominal values.
 

craig008

New Member
i cant help u too much as i am a bit rusty but the w stands for watts, do a quick search on watts and you should find what you need 0.25 is enough for most elcrinocs projects. % refers to the accuracy, so if the resistor is 100 ohms at a 5% tolerence it means the it could be give or take 5% (95 - 105) and if i remember correctly there is three different levels 5% 10% 15%.

as for the meterial i gues it is what is is made out of, perhaps made of different things due to different properties such as heat or durability
________
Ford Essex V6 Engine (Uk)
 
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Tom2000

Senior Member
Of the three options you presented, I'd go for the second. You'll probably get the most use from them.

1/4 and 1/2 watt refer to the amount of power that a resistor can dissipate. That's way overrated. (Or way underrated, depending upon your point of view.) If you try to dissipate anywhere near 1/4 watt in a 1/4 watt resistor, it will get extremely hot. It's best to stay way below a resistor's power dissipation rating.

For the sort of circuitry you'll be messing with when you play with micros, 1/4 watt is the way to go. You don't often need 1/2 watt parts. (When you find yourself needing something larger than 1/4 watt, you usually need something way larger. 2 watt, 5 watt, 10 watt, etc. And if your find yourself needing something that large, you'll probably redesign the circuit to use a transistor instead of a power resistor.)

1% and 5% refer to the tolerance in the resistance values. A 470 ohm 5% part might measure anywhere between 447 and 493 ohms, for instance.

Unless you're going to be designing active op amp filters, you'll seldom need 1% resistors. And I guarantee that when you need 1% resistors, the values you need won't be found in any assortment. Give these a pass.

Carbon film and metal film resistors are made from... drum roll... either metal or carbon film deposited on some substrate. For work with micros, carbon film resistors are just fine for our purposes. If you're working with sensitive low noise and high gain analog circuitry, you might find that metal film resistors don't generate as much noise as carbon film. But, for our work, that's not a concern.

If you find that you enjoy experimenting with electronics, and micros in particular, you might want to buy, in addition to an assortment, 100 piece lots of common resistor values that you'll find yourself using over and over. Some candidates include 330 ohm (for LED current limiting on 5 volt circuits), 470 ohm (same for 12 volt circuits), 1 k (for transistor base current limiting), and 10 k (for pullups/pulldowns).

Hope that helps.

Have fun!

Tom
 

BeanieBots

Moderator
Depends where you see yourself going with this.
The advice given so far is fine but here's a bit that you might also want to consider.
If you are going to stick mainly with digital, then a full selection is probably overkill. You will only ever use 1k,4k7,10k and maybe 330R or 470R. 5% tollerance is fine for those.
If you want to start playing with op-amps and analog electronics, then I would suggest option1. The metal film resistors are closer tollerance but more importantly, they produce less noise and don't drift with age and tempertaure anything like as much as carbon film.
True that for precision work, they will never be exactly the value you want but a custom value resistor will cost mega bucks. You can use multiple resistors to obtain a particular value or use a POT. What you don't want, is that value to be different the next day due to different atmospheric conditions. (the 50ppm means 50 parts per million drift).

I've been doing electronics (mainly analog) for some 40ish years.
I have an enviable array of resistors to select from covering 0R01 to 3G3.
It occupies and entire 12 foot wall:eek:
Most of the drawers are full and contain resistors over 20 years old and should probably be thrown out:eek: . The 1k, 4k7, 10k and 100k drawers are nearly always empty:mad: .
The 3k3 and 5k6 drawers are half full because they get used when I have no 4k7 or 10ks.
 

Tom2000

Senior Member
And while you're shopping for parts, look for a deal on 0.1 uF/50 volt ceramic capacitors. Get as many as you can lay your hands on. You use them for bypassing, coupling, and just about everything.

Your circuits will work much better if you install one of these as close as possible to the power and ground pins on each IC you use.

Good luck!

Tom
 

skyv

New Member
Off course from a cost point of view, there's hardly any difference between carbon and metal film.
From that point of view I would go for the better quality 1 % metal film resistors.

Skyv
 
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