Using a shunt resistor for measuring current

1. Nov 21, 2012

F.ono

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.

Last edited: Nov 21, 2012
2. Nov 21, 2012

Staff: Mentor

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. Nov 21, 2012

Staff: Mentor

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. Nov 21, 2012

F.ono

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. Nov 21, 2012

Staff: Mentor

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. Nov 23, 2012

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. Nov 23, 2012

Enthalpy

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. Nov 28, 2012

F.ono

Thank you all and sorry for taking so long to reply.
I used a metal layer resistor and it worked much better.