ppm

[Simon]

so a resitor has a temperature coefficient of 50 ppm/C what does this mean ? a rise in resistance of 0.005 % for every C ?

[Kiriakos-GR]

Do you plan to build an decade ?   

[alm]

Correct. ppm is parts per million, so 1 ppm = .0001%. The correct term would be ppm/?C = ppm/K, since ppm/C would be ppm per Coulomb.

[Simon]

Do you plan to build an decade ?   

no, I was just trying to understand the thermal specs of resistors as I plan putting considerable current through a few

[Kiriakos-GR]

Well I have another question puzzling me , I think its a stupid one ,
but I will say it any way ...

Does the 1/4W  film metal , can take more abuse than the carbon ones 1/4W .

In some resistor decade boxes , I found predefined  Max amperes ratting for its selector = team of resistor values.

1) I do not know how they predefine those ratings ..
2) At 12 volts  at about the 680 Ohm and under ,  the resistor specs  1/4W , comes at the none critical conditions ( low amperes that makes some sense with the ohms law )      

I like also to know the max non - dangerous current for my own decade box .
but only the 0.10 ohms are at 5W ,  all the rest are at 1/4W ( 1 - 10 - 100 ohm  1K 10K 100K )

But as I said and above , If I was aware of the standard (reference)  voltage , I would be able to calculate it,
but with out an predefined voltage , there is  no standard value for Amperes ...

So, how they do it ?    

[alm]

I'm not aware of any difference between metal film and carbon film in this regard, these tend to be pretty similar (except that carbon film is worse in everything related to accuracy). I do know that carbon composition resistors are better at handling peak currents (average power is obviously limited to 1/4W). This is because film resistors are made by cutting a serpent shape in the film, which creates a long, fairly narrow, strip of film. A peak current could heat up just the narrowest part of this strip. Carbon composition are one piece of solid resistive material, so they have much more thermal mass for absorbing peaks. Bob Pease talks about this in his book 'Troubleshooting Analog Circuits'.

If you're going for high accuracy, you might want to derate the power, because you don't want them to get hot. The tempco will decrease the accuracy. 'Real' reference resistors, like those made by Leeds & Northrup, sometimes have an accuracy specification for both low power, and a reduced accuracy specification for higher power. They also tend to be really huge compared to their rated power.

[Kiriakos-GR]

At the above picture it gives rating of 700mA for the 1 Ohm &  200mA for the 10 Ohms ..

There are several types of  decade resistor boxes , my version has as target the high accuracy ,
I assume that others will target more wattage and strength on Amperes.



I = 0,7A   R= 1 Ohm  V= 0.7 Volts ?

 

[Simon]

I = 0,7A   R= 1 Ohm  V= 0.7 Volts ?

 

correct