Author Topic: Why are most DC resistor shunts rated at 75mV ?  (Read 2329 times)

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Offline ez24

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Why are most DC resistor shunts rated at 75mV ?
« on: January 16, 2018, 02:01:06 am »
I am trying to come up with a way of stop blowing fuses so I am looking at using shunts.  A search on Aliexpress brings up all shunts rated at 75mV.  I found some 100mV ones by typing in 100mV.  Does this have something to do with analog meter movements.  It makes no sense to me.

thanks
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Offline neo

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #1 on: January 16, 2018, 02:35:59 am »
Yes, it has to do with analog meters. The vast majority use 75 mV for reasons beyond my understanding, the older ones used 50 mV.

But also of note as far as shunts go you could even use a .1 ohm resistor as a shunt. I've done it that way, if you have a 10 watt .1 ohm resistor you can measure up to 10 amps.

EDIT:
To add, accurate to the nearest volt ohm meter.
« Last Edit: January 16, 2018, 02:42:51 am by neo »
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Offline BravoV

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #2 on: January 16, 2018, 02:36:22 am »
I'm curious too.

Example analog current analog meters, and as these are Japanese, wonder why 60mV ?  :-//






Shunts salvaged from old industrial power panel



 
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Offline neo

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #3 on: January 16, 2018, 02:38:09 am »
With 60 mV, and really with the original 50 VS 75mV question it just comes down to picking a value and running with it.
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Offline ez24

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #4 on: January 16, 2018, 03:21:09 am »
But also of note as far as shunts go you could even use a .1 ohm resistor as a shunt. I've done it that way, if you have a 10 watt .1 ohm resistor you can measure up to 10 amps.

This reminded me that a few days ago I came across a .1 100w resistor and thought what the heck could I do with it.  Now I know  :-+ :-+ :-+  I hooked up it at 4.3a and it works good enough for me.

Thanks
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Offline paulca

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #5 on: January 16, 2018, 11:44:57 am »
Have you considered hall effect current sensors instead?

https://uk.rs-online.com/web/p/hall-effect-sensor-ics/6807135/
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Online Brumby

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #6 on: January 16, 2018, 12:51:45 pm »
I'm curious too.

Example analog current analog meters, and as these are Japanese, wonder why 60mV ?  :-//


It's no secret - just basic ohms law.

 * A meter movement will have FSD when a magnetic field of a particular magnitude is generated by the coil.
 * This occurs when a specific current is passing through the coil
 * The coil has resistance
 * .......  :D
 
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Offline HighVoltage

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #7 on: January 16, 2018, 01:06:43 pm »
A few month ago I was after this very same question with my low resistance measurement project.
https://www.eevblog.com/forum/metrology/low-ohm-precision-resistor-standard-and-testing/msg1270671/#msg1270671

I even called the German manufacturer WEIGEL, who makes really nice shunts.
The explanation was the analog display.
The analog meters became a standard for Weigel at 60mV or 75 mV full scale in those days.
Why the standard was at 60mV or 75mV nobody at Weigel could say and those who knew had been retired for several years already.


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Offline neo

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #8 on: January 16, 2018, 01:19:54 pm »
A few month ago I was after this very same question with my low resistance measurement project.
https://www.eevblog.com/forum/metrology/low-ohm-precision-resistor-standard-and-testing/msg1270671/#msg1270671

I even called the German manufacturer WEIGEL, who makes really nice shunts.
The explanation was the analog display.
The analog meters became a standard for Weigel at 60mV or 75 mV full scale in those days.
Why the standard was at 60mV or 75mV nobody at Weigel could say and those who knew had been retired for several years already.

As i said, picking a (reasonable) value and running with it. That is how standards became standard.
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Offline hayatepilot

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #9 on: January 16, 2018, 03:24:51 pm »
Why not make your owm shunt?

 
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Offline Gregg

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #10 on: January 16, 2018, 09:48:21 pm »
I once made my own shunt for a wheeled automotive battery charger when replaced the diodes.  I used a length of #2 welding cable and after finding the correct length put a small brass screw through the insulation for a connection point and covered it with heat shrink.  Manufactured shunts have a little better heat dissipation and are actually calibrated properly, but it all depends on the tolerance you can tolerate. 
I used to evaluate telco 48V DC plants for additional loads and battery requirements and would hardly ever trust the installed metering, but the shunts were always dead on. 
Enclosed are a couple of pictures of a 5000 amp shunt which was rated for a 50 millivolt drop at the full 5000 amps.  The bus bars are 1/4 inch thick by 4 inches wide and the thick cables are 600MCM. 
A good shunt tends to be more stable and accurate than a hall effect clamp meter.
 

Offline graynomad

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #11 on: June 22, 2021, 10:05:26 am »
So the short answer is that nobody knows, it's just that way because it is. Probably a 100 years ago somebody wound a coil to fit something they had in the parts drawer, it measured at 75mV with their current and we've (mostly) used that ever since  :)
« Last Edit: June 22, 2021, 08:56:10 pm by graynomad »
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Offline Vovk_Z

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #12 on: June 22, 2021, 05:16:10 pm »
We may use 75 mV shunt as 50 mV or 100 mV shunt (or any other value). It'll have  higher or lower accuracy respectively.
« Last Edit: June 25, 2021, 11:51:07 pm by Vovk_Z »
 

Offline Doctorandus_P

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #13 on: June 22, 2021, 05:43:02 pm »
Sure you can push less current through a shunt, and get less voltage drop, but pushing more current through it may overheat the shunt.
For example, with the 5kA shunt above, it dissipates 375W at nominal load, but 666Watt if there is 100mV over the shunt.

>>> U=.075
>>> I=5000
>>> R=U/I
>>> R
1.4999999999999999e-05
>>> P=I*I*R
>>> P
374.99999999999994
>>> U=.1
>>> P=U*U/R
>>> P
666.6666666666669
 

Online Renate

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #14 on: June 22, 2021, 06:28:30 pm »
Here's my shunt: a fuse!
Maybe it's not the most accurate but you can't beat the voltage drop.
The resistance is 0.34 milliohm.
150 Amperes would be 51 millivolt.
Except for starting the generator, 20 Amperes is the more usual peak.
That's only 7 millivolt.
 

Offline Doctorandus_P

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #15 on: June 22, 2021, 08:02:48 pm »
Using a fuse as a shunt resistor is a terrible Idea.

Fuses are metal wires designed to melt at a certain current, and the element is very hot at that point.
I won't be surprised if the resistance would have doubled or tripled around the maximum current.

In the picture shown you would be much better of by using a section of the red power cable on the left as a shunt resistor. Copper has a higher tempco as regular resistor materials, but at least the temperature difference will be much less than that of the fuse.
« Last Edit: June 22, 2021, 08:09:24 pm by Doctorandus_P »
 

Online Renate

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Re: Why are most DC resistor shunts rated at 75mV ?
« Reply #16 on: June 22, 2021, 10:22:25 pm »
Using a fuse as a shunt resistor is a terrible Idea.
Oh, I've had worse.

Fuses are metal wires designed to melt at a certain current, and the element is very hot at that point.
Well, let's consider that:
(150 Amperes)^2 * 0.34 milliohms = 7.65 Watt
Not an inconsiderable amount of power.
But as I said, that current only happens when I start the generator or jump the truck engine.
20 Amperes is a more normal peak.
(20 Amperes)^2 * 0.34 milliohm = 0.136 Watt
I think that I can live with that heating effect.

You would be much better of by using a section of the red power cable ...
Well, a foot of 2 AWG only gives me 0.14 milliohms and the geometry is not as convenient.
 


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