Hi poobah,
I wouldn't have know had you not said anything. You posted twice and David once as I was composing. 🙄
I was at the place where you have so far posted 6 times or so. I haven't opened my yap yet. 😉
-Chris
I wouldn't have know had you not said anything. You posted twice and David once as I was composing. 🙄
I was at the place where you have so far posted 6 times or so. I haven't opened my yap yet. 😉
-Chris
EC8010,
I think there is something wrong in you post. It says I should click the thumbnail to see the original image, but it seems I should rather click the toe nail. 🙂
As for the original question, what is the point of scaling if it gives the wrong reading? The meters are usually designed to have a max display value of 199mV, so if your max value is 50 mV, scaling by a factor four will increase the resolution, but will give the wrong reading so you have to rescale the figure manually. That was assuming that the 50 mV corresponded to 50 A. Still, as some have pointed out, you might want a shunt with lower value, in which case you might need to rescale the value, but I think you should still rescale it to get the digits right an loose some resolution.
I think there is something wrong in you post. It says I should click the thumbnail to see the original image, but it seems I should rather click the toe nail. 🙂
As for the original question, what is the point of scaling if it gives the wrong reading? The meters are usually designed to have a max display value of 199mV, so if your max value is 50 mV, scaling by a factor four will increase the resolution, but will give the wrong reading so you have to rescale the figure manually. That was assuming that the 50 mV corresponded to 50 A. Still, as some have pointed out, you might want a shunt with lower value, in which case you might need to rescale the value, but I think you should still rescale it to get the digits right an loose some resolution.
Wow, a lot happened while running my show.
So, it would be best to match a meter to the shunt maybe? Btw, I haven't bought either a meter or shunt. Still in the planning mode.
I'm ok with simply using a resistor to measure current, and a hot one is ok as everything can be mounted to the heat exchanger.
I'll keep looking for a meter with 100 or 50mV fs, haven't seen one yet. Thanks!
So, it would be best to match a meter to the shunt maybe? Btw, I haven't bought either a meter or shunt. Still in the planning mode.
I'm ok with simply using a resistor to measure current, and a hot one is ok as everything can be mounted to the heat exchanger.
I'll keep looking for a meter with 100 or 50mV fs, haven't seen one yet. Thanks!
imix500 said:
I don't think they make those, and if, they are probably uncommon and expensive. Does it matter in your application if you lose two bits of resolution by not using the full scale?
BTW, the decimal point can often be set explicitly so you can scale the reading in factors of 10, if required.
Hi Christer, it was mentioned earlier about 100mV meters but I had not seen too many either at a diy price. Equate the 2 bits into analog for me.
As long as I have 100mA resolution I'm a happy camper.
As long as I have 100mA resolution I'm a happy camper.
Ok, I think I've been overthinking this.
I want .1 A resolution. With a 31/2 digit meter with 200mV fs, I could use a 50A 50mV shunt to give a max reading of 49.9A right?
I want .1 A resolution. With a 31/2 digit meter with 200mV fs, I could use a 50A 50mV shunt to give a max reading of 49.9A right?
imix500 said:
Each bit is a factor two, so two bits is a factor four. That is, if you use a Voltmeter with a range of 200 mV but only use it up to 50 mV, you lose a factor four in resolution, since the minimum resolution is still the same in absolute figures (1 mV or whatever it is). That is, a 3-digit meter with a range of 200 mV will give you a resolution of 200 steps (if we ignore negative values). If your max input is 50 mV, you get a resolution of only 50 steps. You could scale your input by a factor 4 to get 200 steps resolution, but then again, your reading will be wrong.
I should have written "a factor four" instead of "two bits". I guess it is because I am computer scientist and these meters anyway use AD converters so we get a binary value, which is then converted into decimal digits. 🙂
imix500 said:
Yes, or rather you even get a max reading of 50.0 A (ie. 50.0 mV) since you don't hit the limit of the meter, as you do if you try to go up to 200 mV.
Hi,
I note the power and current figures being quoted in the threads.
But, what limits a 50mV shunt to 50A?
Is the manufacturer rating the device to limit the power that can be dissipated?
Presumably to keep some semblance of accuracy both in the short term (while taking the hot measurement) and in the long term when repeated high temperature excursions cause a cumulative error.
It seems that a 1mohm shunt running at 50A and dissipating 2W5 is quite onerous.
Can a bridge be used to measure the connecting cable resistance?
Then if a suitable length were tapped off, this becomes the shunt.
Is accurate measurement in the decimal parts of a milliohm too difficult for our limited amateur resources?
Finally, a 3.5 digit display with a 1mohm (0r001) will read 50.0 at 50A and could go on to read up to 199.9 A without overloading the display.
I note the power and current figures being quoted in the threads.
But, what limits a 50mV shunt to 50A?
Is the manufacturer rating the device to limit the power that can be dissipated?
Presumably to keep some semblance of accuracy both in the short term (while taking the hot measurement) and in the long term when repeated high temperature excursions cause a cumulative error.
It seems that a 1mohm shunt running at 50A and dissipating 2W5 is quite onerous.
Can a bridge be used to measure the connecting cable resistance?
Then if a suitable length were tapped off, this becomes the shunt.
Is accurate measurement in the decimal parts of a milliohm too difficult for our limited amateur resources?
Finally, a 3.5 digit display with a 1mohm (0r001) will read 50.0 at 50A and could go on to read up to 199.9 A without overloading the display.
Hi imix500,
Watch out for the resistance of the wire changing. You may need to have a correction applied for the temperature. With a diode you may be able to mess with your reference voltage to have the meter automatically correct for the change in temperature in the sense element. This can be done with an inexpensive 3 1/2 digit meter. A dynamic meter becomes more difficult. A hall element may be more accurate once temperature has been factored in.
Either way, you will need a reference to get actual readings over some time to correct for temperature while you adjust the readings.
-Chris
Yup.
Watch out for the resistance of the wire changing. You may need to have a correction applied for the temperature. With a diode you may be able to mess with your reference voltage to have the meter automatically correct for the change in temperature in the sense element. This can be done with an inexpensive 3 1/2 digit meter. A dynamic meter becomes more difficult. A hall element may be more accurate once temperature has been factored in.
Either way, you will need a reference to get actual readings over some time to correct for temperature while you adjust the readings.
-Chris
I hadn't given the connecting wire that much thought. Considering the shunt is roughly .01 ohms it probably isn't too far from reality for the internal wiring to reach that, certainly with a heavy load. The power resistors are already wired with #8, but perhaps I should go with a copper buss bar for as many connections as possible.
On these hall affect sensors, do they require a plc with a lookup table, or can they drive a meter dirrectly?
On these hall affect sensors, do they require a plc with a lookup table, or can they drive a meter dirrectly?
Hi imix500,
The linear hall sensors operate on the magnetic field generated around the buss / cable. There would need to be some signal conditioning, but in theory you could simply mount the part close to your cable leading to the test load and read the current.
I don't remember how linear they are across the range. I do know you can increase the sensitivity by using a ring of metal around the cable with an air gap. The hall sensor goes in the gap. You can find commercial assemblies in your current range already made up. Have a look at how they are put together. It may be less expensive to simply buy one and interface it to your meter. The sensor is always the most expensive part.
Have a look at Allegro for one source.
-Chris
The linear hall sensors operate on the magnetic field generated around the buss / cable. There would need to be some signal conditioning, but in theory you could simply mount the part close to your cable leading to the test load and read the current.
I don't remember how linear they are across the range. I do know you can increase the sensitivity by using a ring of metal around the cable with an air gap. The hall sensor goes in the gap. You can find commercial assemblies in your current range already made up. Have a look at how they are put together. It may be less expensive to simply buy one and interface it to your meter. The sensor is always the most expensive part.
So, they may be able to drive your digital meter directly. You would only use a lookup to increase the accuracy. I don't think you would need to do that.
Have a look at Allegro for one source.
-Chris
Hi Imix,
Can your circuit afford to lose that extra 50mV?
How would a current transformer do?
You know, that one turn through a transformer core and measure the voltage/current in the secondary.
Voltage drop almost zero and no dissipation problems, nor tapping points or any of the other paraphernalia.
The measurement transducer may be more complicated.
you really meant 0.001ohms (0r001) which gives 50mV @ 50A and 2W5
Can your circuit afford to lose that extra 50mV?
How would a current transformer do?
You know, that one turn through a transformer core and measure the voltage/current in the secondary.
Voltage drop almost zero and no dissipation problems, nor tapping points or any of the other paraphernalia.
The measurement transducer may be more complicated.
Hi Andrew,
Your math is good. The only problem I can think of is that imix500 is measuring a DC parameter. The hall current reading assemblies are good to 50 KHz if I remember correctly. That's close enough to "DC to light" for power supplies. 😉 Although it will make the residuals from switching power supplies look better than they really are.
-Chris
Your math is good. The only problem I can think of is that imix500 is measuring a DC parameter. The hall current reading assemblies are good to 50 KHz if I remember correctly. That's close enough to "DC to light" for power supplies. 😉 Although it will make the residuals from switching power supplies look better than they really are.
-Chris
Halls kinda suck... they drift all over with temp and they need regular degaussing.
Use the regular old current-shunt... they use special metal with a low tempco and Kelvin connections (4 contacts) come standard.
Keep in mind, the regular style shunts are often designed for continuous use at 67% of their nameplate rating... not a hard and fast rule, but be sure to read the fine print.
🙂
Use the regular old current-shunt... they use special metal with a low tempco and Kelvin connections (4 contacts) come standard.
Keep in mind, the regular style shunts are often designed for continuous use at 67% of their nameplate rating... not a hard and fast rule, but be sure to read the fine print.
🙂
Hi,
sounds just like Bourdon tube pressure gauges, use only between 25% and 75% of full scale reading.
sounds just like Bourdon tube pressure gauges, use only between 25% and 75% of full scale reading.
I didn't know that the hall element needed to be degaussed. Possibly any iron would, as expected in a DC field. Good point. There are commercial assemblies that are already temperature compensated.
Still, temperature compensation should be investigated. Definitely a Kelvin connection.
-Chris
Still, temperature compensation should be investigated. Definitely a Kelvin connection.
Oh yeah! I couldn't agree more with you on that!
-Chris
Hmm, the hall sensors look really cool, but the assemblies are kinda pricey for what this is. Still, I might order a couple raw sensors to play. AndrewT, oops! I was off, I did mean .001 ohms. Good point about derating the shunts. Maybe I'll pick up a 100A shunt for $2 more to be safe.
So would buss bars be a good idea? The only issue is that I'm using relays to step the resistors in or out of the load, interconnects to those are still #8 wire. Maybe I could use a buss bar for the common side?
Thanks guys!!
So would buss bars be a good idea? The only issue is that I'm using relays to step the resistors in or out of the load, interconnects to those are still #8 wire. Maybe I could use a buss bar for the common side?
Thanks guys!!
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