Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Actually, I am trying to build a current monitor for a wireless sensor network. I am going to use a current sense resistor as the current will pass through the resistor. By measuring the value of the voltage, the current value can be calculated.

My application is very sensitive as the current that will be measured is in the range of mA, 30 mA maximum.

When it came to buying the resistors I got confused. I found a type of resistor called "precision resistor" that has a very small error, and another one called "current sense resistor".

Here is a picture of a current sense resistor:

current sense resistor, 25 ohm, 1%

So my questions are:

  1. What is the difference between these two types current sense and precision resistors?
  2. How does this current sense resistor differ from the ones we normally use, such as as these 5.6k Ohm, 5% resistors?

4x 5.6k Ohm 5% resistors, image source wiseGEEK

share|improve this question
    
Good question but writing looks a little sloppy because of lack of capital letters at start of sentences. Fix that for a +1. – transistor Feb 2 at 19:17
2  
They're both resistors but one has high precision and the other is good for several amps. In some cases there are resistors good for several amps AND have high precision. – Andy aka Feb 2 at 19:27
up vote 14 down vote accepted

Resistors are resistors. They only see the current thru them and the voltage across them.

However, specific models can be targeted to specific applications. What you show as a current sense resistor looks like it's designed to dissipate significant power. However, it would work for lots of purposes within its power and voltage limits.

Current sense resistors tend to:

  1. Be run with little voltage across them.

  2. Be fairly accurate, since being used for measurement.

  3. Drift little due to temperature.

  4. Have low values.

  5. Sometimes have 4 leads so that you can use a Kelvin connection. Two leads carry the current, then the voltage is measured across the two other leads. This keeps the voltage drop due to current in the leads off the measurement.

share|improve this answer

There is no absolute difference between the two.

Current sense resistors are generally low-value, high-power resistors. They are intended to develop a small voltage for a given current, so they are low in value. When used in power supplies they may well carry significant current and dissipate significant power, so they tend to be high-power units. For many applications they are not used to measure current with high precision, but this is not guaranteed.

Precision resistors are simply resistors which have stable, well-defined resistance. The existence of non-zero temperature coefficients means that, for a similar size a very high precision resistor will be limited to lower power than a non-precision resistor, although this does not apply for precisions of 1 % or less.

Your picture of a current sense resistor shows a unit which (if you look at it closely) has 1 % precision. The other picture shows resistors with 5% precision (the gold band at left), so they are lower-precision than the current-sense resistor which you show.

In your case, you need to measure 30 mA. What voltage are you willing to drop across the resistor at this current? Let's assume that you want 0.1 volts. Then the resistor should have a value of $$R = \frac{V}{i} = \frac{0.1}{0.03} = 3.33 \text{ ohns} $$ and at maximum power it will dissipate $$P = i^2 R = (.03)^2 \times 3.33 = 3\text{ mW} $$

As a result, if 0.1 volts is acceptable, you can use a standard 1/10 watt, 1% resistor.

share|improve this answer

That current sense resistor is possible to mount on a cooling surface (PCB or heatsink) so it does not overheat. If this is needed is entirely up to your application. The power loss in the resistor will be: P = I^2 * R

Some current sense resistors also has two more terminals to be able to measure the voltage over the resistor even more accurately. These usually connects directly at the middle of the resistor.

share|improve this answer

Normal resistor, precision resistor, and current sensing resistor, they are all resistors, but they are all different. Precision resistor has high accurate value than normal resistor. Current sensing resistor has higher accurate value when giving in a load because heat will alter the resistance of a resistor, even a precision resistor, but not a current sensing resistor.

share|improve this answer

A high precision resistor, refers to the accuracy used by the manufacturer to produce the resistor. For example, a manufacturer tries to make 100 ohm resistors, but the variability of the process makes resistors that are between 99 and 101 ohms, then he has 100 ohms with +/- 1% tolerance.

A current sense resistor, refers to a "special type" of resistor used for a particular application.

There is no reason why you could not have any of the four possible combinations: 1)low precision regular; 2)low precision current sense; 3)high precision regular; 4)high precision current sense. You can also have any of these with a "low temp coefficient." It's all a matter of more or less cost.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.