Connecting Voltmeter/Ammeter

Certainly a handy site for Ray. They also are thinking in "water comparison". Well it is an easy way to understand a complex topic. At least people can understand pressure, diameter of a pipe, etc and understand easier in this way electricity. Bert

 

Many thanks for your help, gentlemen. Its this handholding that is really useful.

I am going to go through that site for sure.

I will be away for a couple of days over the new year, but I will have a whole heap of questions in the near future.

have a great New Years yourself.

 

Right, back again. Very quiet new year.

For my own reference, I am attaching a link that explains the theory of using shunts in ammeters that finally filled in the conceptual gaps for me.

http://www.allaboutcircuits.com/vol_1/chpt_8/4.html

The main point I was struggling with - Shunts are only to enable the ammeter to measure a larger Amperage than it physically can handle. If you connect them to Amps above their rating - problems. If you reduce the current with a known resistance, then they can be used to represent a proportion of that current.

(It was all there in the previous posts, but didn't 'gel' in my mind )

I will now 'chat' to the RC experts
http://www.modelboatmayhem.co.uk/

on the proper way to wire the thing in, as there are implications for the Electronic Speed Controller and Brushless Motors.

Also, a link to a higher quality, fully featured Ammeter
http://www.rc-electronics-usa.com/ammeters/dc-amp-meter.html


Many thanks for the explanations and concepts guys - it was a big help.

 

Sorry Ray,
I must have still too much champagne in my system from New years Eve. That was a lot of nonsense what I mentioned. It would NOT see 123 Amp. No, you would see 62,5 Ampere and you need to multiply this by 2 given you 123 Ampere. The reason is that with 2 resistors of equal value over the shunt, automatically it is divided by the same ratio. Would you use a different ratio , let say 10 Ohm and 20 Ohm 1:2 You would see 1/3 of 123 Ampere = 41 Ampere. What it all means is , if you cannot get the correct shunt, with resistors you are still able with that lovely digital current/Voltage meter measure currents over the 100 or 200 or 300 Ampere. As long you multiply the reading by the same ratio. Sorry for the error.
Bert

 

No worries Bert - I have ended up making my own questions quite redundant.

I got onto a hobby site, and they informed me that the cheap chinese solution might better be handled by a specialised 'black box' from
http://www.hobbyking.com/hobbyking/store/RC_PRODUCT_SEARCH.asp?strSearch=wattmeter

I never realised there was so much gear available, and not costing much more.

I enjoyed the research though.




One of the things that has go me intrigued, is the logic behind using shunts.

One kind guy sent me a translated version of the wiring diagram.
http://www.modelboatmayhem.co.uk/forum/index.php?topic=45876.new;topicseen#new

I am trying to visualise how the shunt 'interferes' with the Amp calculations. Without it, the full power would go through the black wire, into the meter, and out the yellow wire to drive the motor.

With the shunt in place, it seems that a precise amount of power is 'bled' straight to the yellow wire, so only a fixed proportion goes through the meter.

I may have this totally wrong. Any ideas ?



.http://www.youtube.com/watch?v=xaSt_p0V6w0

 

Without the shunt, the full battery voltage arrives at the display board between yellow and black, where only 0.075 V is expected. You'll hear a soft "pop" and see some smoke coming from a small resistor vaporizing.

 

Hi Ray, good evening
The best is to compare it with a differential pressure meter and a big fat pipe. The pressure meter goes from 0 to 500 millibar (that is the ampere meter) and the big fat pipe is cut in such a way, that when the needle or display stays at 500 millibar, the pipe has a flow of 500 liter per second. (compared with the high current flowing through the shunt). The pipe is calibrated to the differential pressure meter. So is the ampere meter calibrated to the shunt or the shunt calibrated to the ampere meter. If now the flow drops to 250 liter the meter shows 250 millibar.
We are now placing a thinner pipe over the very fat pipe OR drill a hole half way in the fat pipe. The differential pressure meter is now connected over half of that thin pipe or half over the big fat pipe. Logically, if a flow of 500 liter per second flows through the big pipe, the differential over half of the thin pipe shows only 250 millibar. The same if we place the differential meter at the halfway drilled hole. (The needle is now half way and does not exceed the display) If we now increase the flow to 1000 liter per second, the meter is at 500 millibar. The scale of the meter shows 500 liter per second while in reality you pushing 1000 liter through the pipe.
The same applies to the shunt. The shunt is calibrated to the ampere meter. By drilling a hole in the shunt halfway, or make a soldering point halfway or place two resistors over the shunt. The pointer of the meter or the digital display can now go twice the calibrated maximum current the device was sold for. But you have to multiply the result by 2. Due to the fact, that a shunt is calibrated to the maximum display and you are bypassing the ratio with a trick.
Trust that this helps you. I had just a friend coming back from Australia, magnificent pictures he has taken. Like South Africa, Australia is very beautiful.
Bert

 

Hi Ray,
Please allow me to elaborate on the reply from CDK. He is right. An ampere meter is nothing else than a very sensitive Voltmeter. It measures micro or milli Volt. Just that the scale plate has painted numbers on it and state " Ampere" or in your case the digital meter tells you that you are dealing with "Ampere" the fact is the same, you are measuring very low voltages. Thus should the shunt cut or burn through, the full voltage will be placed on the inputs of the ampere meter. Needles to say what happens.

You can at any time create your own shunt. But remember, you don't want to add losses and wasted power to your circuit, thus you take a thick fat piece of copper cable. But now the problem starts, how do you calibrate your piece of cable to your very sensitive Volt meter? Maybe CDK and me, we can calculate the copper thickness and the resistance per 100 meter and look at the temperature coefficient, etc. etc. and come up with our own brewed shunt. Or we have a known current and calibrate to this.
But is better to buy one from Vishay whom are using special materials to keep the losses low. Or stick to the shunt provided with the ampere meter.
Bert

 

Well, no it wont. Its able to handle up to 50 volts. The big question is whether it can handle the Amps (max 100 )

Based on your and Berts reply, my summary seems to hold water. The shunt is a way of reducing the flow into the ammeter by diverting it straight across to the yellow wire.

Some of those specialist Wattmeters from the hobby shops can actually store the peak power measurements.



Just as an aside - when I was looking at the Wattmeter details, they kept mentioning 'balancers'. I did a lot of research ( god bless youtube ) about the options and care of difference batteries. Its a lot more convoluted than I thought.

 

Hi Ray,
Unfortunately your Analogue to Digital input (ADC) from your Ampere meter will NOT be able to handle 50 Volt input. All what a shunt is, nothing else as a calibrated thick copper or special metal piece where over small Voltages are measured. But if you feel comfortable with your own explanation, you are welcome to do so.
Bert

 

If you're that confident Ray, do wire it without the shunt, then tell us what happened!

 

Hi Ray,
You are getting close to it, in principle you are right. However no professional will ever explain it like you do, in view that you measure the very small voltage and yes, as soon there is a resistive connection with a voltage, you will also have an absolute minute current flow. But the purpose is not to measure the tiny current , but the low voltage created over the shunt.
Bert