Current measuring using non-inductive resistors

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  • Last Post 18 September 2019
cd_sharp posted this 11 September 2019

Hey, guys

Let's jump directly into the setup

I took all the suggestions into account and I added a total of 5K gate series resistance and 1M pull-down resistor.

I also reduced the wires length and they are not overlapping any more.

It's worth mentioning that the more gate series resistance I add, the greater the voltage needed from the SG to open the gate of the MOSFET.

Let's see the wave forms:

:

 

The ringing is still there. I stand by my opinion that 15FR250E resistors are not suitable for measuring small currents at frequencies greater than 1KHz. In a few days I'll have another model to investigate.

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Chris posted this 11 September 2019

Hey CD,

Looking good my friend!

Some noise is inevitable, this is just a given with DC Switching, but what you had before was extreme.

I am pleased you have managed to clean up the waveform dramatically.

   Chris

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cd_sharp posted this 11 September 2019

Hey, Chris,

It got cleaned a little, but it's still not usable. This is at 7KHz. I didn't have the time, but I'll add a scope shot at 20KHz.

Without clean current traces I cannot continue my experiments, so this issue is blocking.

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Vidura posted this 11 September 2019

Hey CD

If you think that the current sense resistor is  the cause of the problem just try a standard metalfilm resistor (for small current only).

 in conventional power electronics we would want to keep the rise and fall times on the gate as short as possible, ans thus employ a small gate series resistor of usually 5-30 ohm. Another method to prevent gate ringing would be to put a small ceramic capacitor(1-100nF for high frequency a low value) paralel to the gate resistor.

Vidura

getreal156 posted this 11 September 2019

Hi Vidura,

I'm facing the same type of issues so thanks also from my side

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Chris posted this 11 September 2019

Hey CD,

Vidura is correct, the switching is important. Fast On Time and fast Off time is important.

Also Important is the placement in the Circuit of the Current Sensing Resistor:

Lets ask the question, in the first circuit, what exactly are you measuring?

With the above circuit what are you measuring?

What's the difference between the two?

   Chris

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cd_sharp posted this 11 September 2019

Hey, Chris

The difference is that in the circuit above we measure also the current going back through the internal diode of the MOSFET as the bulb has some inductance that translates in reactance.

I'm not sure this is correct.

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Chris posted this 11 September 2019

Hey CD,

All Electronic Components have a little of the main variables: Inductance, Capacitance and Resistance.

 

If R1 is on the side of the Globe BL1, then we will see added Series Inductance, Capacitance and Resistance from the Globe.

Of course this is not what we want to see, it affects our measurements. We want a clean smooth signal at least to the best of our ability.

You will see, nearly all of the Circuits I share will have the Current Sensing Resistor ( CSR ) as close to the Negative Terminal as possible, with Scope Earth connected to the Negative side.

This way, we only measure the Voltage Drop across the singular Current Sensing Resistor. Thus: I = V / R

With Clean switching on the Mosfet Q1, we will see much less Contact Bounce thus less Noise on the scope.

Vidura's video shows a very nice clean signal, with very little Contact Bounce.

   Chris

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cd_sharp posted this 13 September 2019

Hey, guys

Excellent suggestions and links. I rebuilt the setup with 1K gate series and 1M pulldown.

And the results are the same

These resistors are not good for this. I'll try the other suggestions from Vidura until I receive the resistors of another model.

Thanks, my friends

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raivope posted this 13 September 2019

Hi,

(I am new here)

For better current measurement there are such non-inductive resistors:

https://uk.farnell.com/vishay/lto030fr1000fte3/res-0r1-1-30w-radial-thick-film/dp/1357066

--Raivo

 

 

raivope posted this 13 September 2019

Hi,

For the power measurement, you need to multiply current and voltage over load (or input) and integrate/sum it over time. This multiplication can be done with analog multiplicator components. But you need high skill of doing that electronics in a proper bandwith.

However, I have found the quite exact and reliable way with this remarkable scope:

https://www.siglent.eu/sds1202x-e.html

You take Ch1 as voltage and Ch2 as current on shunt, create MATH multiplication formula and add it as the third beam. Then you add measurement MEAN (average) value on screen. This is exactly the power you are consuming (or generating).

(You have LAN connection for that scope and you can read this data to computer or display on LCD or screen.)

Also, this method of measurement takes into consideration the reactive power, so you can see the real net power.

(sometimes you need to disconnect the scope ground, to measure floating things, but this is another story)

--Raivo

PS! You have the similar feature on Rigol scope, actually, but no LAN.

Fighter posted this 13 September 2019

PS! You have the similar feature on Rigol scope, actually, but no LAN.

Not true, I have the same Rigol DS1054Z oscilloscope, they have LAN and remote control and display from computer, the feature is called UltraScope:

Chris posted this 13 September 2019

Hey, guys

Excellent suggestions and links. I rebuilt the setup with 1K gate series and 1M pulldown.

And the results are the same

 

Hey CD,

The Time that it takes for a Mosfet to turn off, is R x C = t.

So a 1M Ohm resistor from Gate to Source is vary large, with a capacitance of 2200pf, if my math is correct, t = 0.00022 seconds for example.

The Input Capacitance can be read off the datasheet.

So drop your Gate Source Resistor way down to 1K. Some you will see are 220 Ohms in some cases, I have mine at 1 Ohm through a Diode. You can see my setup here: Reliable and Flexible Switching System

The lower the better for very fast turn off of the gate, which is the removal of Charge in the Capacitance.

Its really important!

If you study Vidura's circuits he said the same. Our Mosfet switching thread said the same. Fast turn on and off. The Mosfet's will stay cooler longer if we get the Turn on and off times fast and clean. wink

   Chris

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Chris posted this 13 September 2019

Hello and Welcome Raivo!

Yes the Current Sensing Resistors is one of the better ways to go, with minimal Inductance.

All very good suggestions!

Thank You for sharing!

   Chris

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cd_sharp posted this 18 September 2019

Hey, guys

I did some more work on this following your advices. I have some new and slightly better non-inductive resistors. With a 1K G-S resistor I have this output:

The current (pink) is still not what it should be, the MOSFET does not want to turn off properly.

It turns out 15FR250E non-inductive resistors are not to blame, the driving circuit is. You were right all along.

Now I see why making a clean MOSFET driver circuit is not so simple.

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Chris posted this 18 September 2019

Hey CD,

Very nice! Thanks for sharing your experience! wink

Much better on the scope!

   Chris

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