Using a shunt resistor for measuring current

In summary: The voltage across the resistor was much more stable and I was able to get more reasonable voltages and currents.
  • #1
F.ono
16
0
I was using a signal generator to perform some tests. The circuit was simply the generator in series with a 1 ohm shunt resistor.
Even though I was working with high frequencies (1 to 20 MHz), shouldn't the voltage drop across the resistor be the generator amplitude itself? I was using the maximum voltage (10 Vp), but the voltage across the resistor was around 1 V or so.
I measured its resistance and it was really 1 ohm.
 
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  • #2
F.ono said:
I was using a signal generator to perform some tests. The circuit was simply the generator in series with a 1 ohm shunt resistor.
Even though I was working with high frequencies (1 to 20 MHz), shouldn't the voltage drop across the resistor be the generator amplitude itself? I was using the maximum voltage (10 Vp), but the voltage across the resistor was around 1 V or so.
I measured its resistance and it was really 1 ohm.

You are shorting out the output of your signal generator by connecting it to such a low impedance. The output impedance of your signal generator is probably around 50 Ohms, so you will get a significant voltage divider trying to drive a 1 Ohm load.

Instead, drive a 50 Ohm load witih your 1 Ohm shunt resistor in series (on the low side, connected to the ground of the signal generator. You should then get more reasonable voltages and currents.
 
  • #3
Or better yet, drive a 49 Ohm load resistor in series with the 1 Ohm shunt resistor. That will match the 50 Ohm output impedance of your sig gen.
 
  • #4
Thanks berkeman!
Does this internal impedance varies with frequency? I am asking this because i was getting voltage outputs between 0,8~1,3 V depending on the frequency.
Oh, another thing... the resistor is a wirewound type. For that range of frequency, is the reactance considerably large compared to the resistance?
Which do you think was the main reason for that varying output voltage? I want to know it because if it was mainly caused by the impedance of the generator (implying that it varies with frequency), I can use the values of output voltage as current values.
 
  • #5
F.ono said:
Thanks berkeman!
Does this internal impedance varies with frequency? I am asking this because i was getting voltage outputs between 0,8~1,3 V depending on the frequency.
Oh, another thing... the resistor is a wirewound type. For that range of frequency, is the reactance considerably large compared to the resistance?
Which do you think was the main reason for that varying output voltage? I want to know it because if it was mainly caused by the impedance of the generator (implying that it varies with frequency), I can use the values of output voltage as current values.

The sig gen output impedance should be pretty resistive and constant with frequency. What model sig gen are you using? The better ones have very flat output impedance characteristics over their frequency range (it should be listed in the sig gen datasheet as well).

The wirewould nature of the 1 Ohm resistor probably accounts for the increase in voltage across it with frequency. I wouldn't use a wirewound resistor in a setup that spans your frequency range. Can you find another type of resistor to use?
 
  • #6
Hello F.ono - I am agreeing with the feedback you have received. I just wanted to point out that the 10V Sig Gen and 1 Ohm load ... 10Amps - I am sure well beyond the rating of your signal gen - that is why it seems non-linear in that range ( 1 Ohm).
 
  • #7
Signal generators try hard to have a very precise resistive 50 ohm output impedance, to minimize parasitic reflections.

The wirewound resistor doesn't fit here.
Even a metal layer resistor gets inductive at HF if it's resistance is as low as 1 ohm.

A leaded 1/4 W resistor has about 5 nH inductance and 0.2 pF capacitance (including the effect of its size) so below 150 ohm it gets first inductive and above it gets capacitive.

Even with a metal layer, 1 ohm 1/4W leaded would be essentially inductive above 32 MHz hence not purely resistive at 20 MHz.

Low or high resistances are essentially avoided in HF; when necessary, put several parts in parallel (or series); Smd are better.

This holds for impedances in general, which designers try hard to keep around 30-200 ohm unless the circuit is tuned and narrowband; low impedance resulting from the transistor's limited breakdown voltage makes power amplifiers difficult.
 
  • #8
Thank you all and sorry for taking so long to reply.
I used a metal layer resistor and it worked much better.
 

What is a shunt resistor and how does it work?

A shunt resistor is a small, low resistance device that is connected in parallel with a circuit to measure the current flowing through it. When current flows through the shunt resistor, a small voltage drop is created which can be measured and used to calculate the current.

Why would I need to use a shunt resistor for measuring current?

Shunt resistors are commonly used in situations where the current being measured is too high for a standard ammeter to handle. By using a shunt resistor, the majority of the current is diverted through the resistor, allowing for a smaller, more manageable current to be measured by the ammeter.

What is the accuracy of using a shunt resistor for measuring current?

The accuracy of using a shunt resistor for measuring current depends on the quality of the resistor and the precision of the measuring equipment. Generally, shunt resistors have a tolerance of around 1%, so the measurement may have a margin of error of up to 1%.

What are the safety considerations when using a shunt resistor for measuring current?

Shunt resistors are designed to handle high currents, so it is important to ensure that they are properly rated and installed to avoid the risk of overheating or electrical damage. It is also important to use proper safety precautions when working with high currents.

Are there any alternative methods for measuring current besides using a shunt resistor?

Yes, there are other methods for measuring current such as using a clamp meter, current transformer, or hall effect sensor. These methods may be more suitable for certain applications, so it is important to consider the specific needs and requirements before choosing a measuring method.

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