shunts
- Thread starter bangbang
- Start date
Many people might call a shunt a resistor, but...I prefer to call it a conductor with a measured and specific amount of resistance.
Voltage will drop a little if it passes through any conductor, even wire. We think of resistors as having resistance/impedance, and we think of copper wire as having conductivity. However, those are their major characteristics. It is also true that resistors conduct electricity, and copper wire has resistance.
How can we measure current through a system? One way is to take a conductor with a measured and very specific amount of resistance, and then measure the voltage just before the shunt, and then just after the shunt. The difference between the two measurements is the voltage drop.
However, since the shunt is actually in the circuit, we don't want high resistance, that would just waste some of the current as heat. Therefore, the shunt has very low resistance.
Once you know how much voltage drop you are getting, the system voltage, and the precise resistance of the shunt...a computer can calculate how much current was passing through the shunt.
Maybe not a precise measurement of the current at that moment, but..."close enough" to be useful.
Voltage will drop a little if it passes through any conductor, even wire. We think of resistors as having resistance/impedance, and we think of copper wire as having conductivity. However, those are their major characteristics. It is also true that resistors conduct electricity, and copper wire has resistance.
How can we measure current through a system? One way is to take a conductor with a measured and very specific amount of resistance, and then measure the voltage just before the shunt, and then just after the shunt. The difference between the two measurements is the voltage drop.
However, since the shunt is actually in the circuit, we don't want high resistance, that would just waste some of the current as heat. Therefore, the shunt has very low resistance.
Once you know how much voltage drop you are getting, the system voltage, and the precise resistance of the shunt...a computer can calculate how much current was passing through the shunt.
Maybe not a precise measurement of the current at that moment, but..."close enough" to be useful.
markz
100 TW
Its hard to measure really low resistance of a shunt.
Low being 0.10ohms or 0.01ohms, you can measure low resistances but the meter is expensive ($150)
You could measure it with other equipment, like a adding a known stable current and measuring voltage, if you have the shunt in your hand, if its in a circuit then its a different story. Then use Ohms law.
If shunts dont work, then it'd be like a fuse so an open, Megaohm reading.
Shunts used to measure.
Low being 0.10ohms or 0.01ohms, you can measure low resistances but the meter is expensive ($150)
You could measure it with other equipment, like a adding a known stable current and measuring voltage, if you have the shunt in your hand, if its in a circuit then its a different story. Then use Ohms law.
If shunts dont work, then it'd be like a fuse so an open, Megaohm reading.
Shunts used to measure.
spinningmagnets said:
If you just measure the potential difference (voltage) across the shunt you don't have to work out any difference.
The voltage across the shunt divided by the resistance of the shunt will exactly equal the current through the shunt at the moment of measurement... no maybe's or close enough's about it.
The system voltage has nothing to do with it, and it doesn't require a computer to calculate. It's a simple example of Ohm's Law.
Thanks!
I did read and i do appreciate all the explanations ! I still dont know how it works on an ebike .
I thought resistance increases with speed was a negative factor
Is resistance needed for powering the bike?
I was riding the bike then a cable that connects throttle, both brake levers and display to a controller broke. on the display plug
The display was a green 5 pin plug and i guess the weather rotted the connector because a huge spark flash occured and it looked like 2 pins completly incinerated .
I missed the cooler season for riding as the bike is a mess and China , Ali baba/express dont have the same cable with small 7circle pins and an 8th pin centered.
Bike fails testing with controller
I thought resistance increases with speed was a negative factor
Is resistance needed for powering the bike?
I was riding the bike then a cable that connects throttle, both brake levers and display to a controller broke. on the display plug
The display was a green 5 pin plug and i guess the weather rotted the connector because a huge spark flash occured and it looked like 2 pins completly incinerated .
I missed the cooler season for riding as the bike is a mess and China , Ali baba/express dont have the same cable with small 7circle pins and an 8th pin centered.
Bike fails testing with controller
miro13car
100 kW
shunt is very, very low resistance resistor in power wire /typically from battery to controller/
, voltage drop across shunt is used for calculating current in this power wire.
example
Cycle Analist so popular with ebikes uses shunt.
, voltage drop across shunt is used for calculating current in this power wire.
example
Cycle Analist so popular with ebikes uses shunt.
MikeEllis
1 mW
bangbang said:
Seems unlikely. As explained in other replies, shunts have very low resistance, e.g. .001 ohm, so as to not waste energy as heat.
For example, you can dump 100 A through a .001 ohm shunt and only dissipate 10W within the shunt (since P = I^2R).
Most circuits have an inline fuse that will blow long before the shunt.
bangbang said:
I wouldn't say mandatory, but a lot of high powered controller are programable. If the functions require determining current, then the controller would need a shunt. You may limit battery current, for instance, to protect your battery, so then the controller needs to know the current in order to limit it.
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