Shunt Resistor Calculator for Ammeters

Ok so here we have one simple calculator which can help us to calculate the shunt resistor value that we need to connect across a normal ammeter when we want to use that ammeter to measure high current, more than its actual range.

So normally, ammeter can measure only small current, like 1 amp or 5 amp. But sometimes we want to measure 10 amp or 20 amp or more. So we cannot just connect the ammeter directly to that big current, it will burn. So we must bypass most of that high current through a shunt resistor, and only allow a small portion to go through ammeter safely.

So here we are making a tool which will calculate that shunt resistor value using simple formula.

Shunt Resistor Calculator for Ammeters

This calculator helps you find the required shunt resistor value to measure high current using a low-range ammeter.

Result:

What Values We Have to Enter

So now, in the calculator we are entering these 3 values:

1. Ammeter Full Scale Current (Ia) – this is the maximum current the ammeter can safely read. Like 1 A or 5 A.
2. Ammeter Full Scale Voltage Drop (Va) – this is the voltage drop across the ammeter when it is reading full scale. Most time it is around 0.05 V or 50 mV or sometimes 100 mV.
3. Total Current We Want to Measure (Itotal) – this is the actual high current we want to measure using our ammeter plus shunt setup. Like 10 A or 20 A.

How It Works, and What Formula It Uses

So now, this is the idea. We know that current always chooses the lower resistance path.

So we connect the shunt resistor in parallel with the ammeter and make it such that only a small current (Ia) will go through ammeter and rest of the big current will flow through shunt resistor (Rs).

We use 2 formulas here:

Rm = Va / Ia
Rs = Va / (Itotal - Ia)

Where:

• Rm is internal resistance of ammeter.
• Rs is the shunt resistor value we want to calculate.
• Va is voltage drop across ammeter (same will appear across Rs because both are in parallel).
• Ia is maximum current allowed through ammeter.
• Itotal is the total current we want to measure.

So basically we are just making sure that when full current (Itotal) flows through both ammeter and Rs together, then the ammeter gets only its safe current (Ia) and rest of the current flows through Rs.

Example to Understand

Let us say:

• Ammeter range is 1 A
• Ammeter voltage drop is 0.05 V
• Total current to measure is 10 A

So from formula:

Rm = 0.05 / 1 = 0.05 ohm
Rs = 0.05 / (10 - 1) = 0.05 / 9 = 0.00556 ohm

So we get:

• Ammeter resistance is 0.05 ohm
• Shunt resistor should be around 0.0056 ohm

So this Rs will allow 9 A to bypass through it and only 1 A will go through meter.

What the Calculator Shows

After clicking "Calculate", it shows two things:

• Rm – internal resistance of the ammeter
• Rs – the calculated shunt resistance which we must use

So with this calculator, we can make any small ammeter work like a big high-current meter, just by putting the correct shunt resistor in parallel.

How to Connect the Shunt Resistor and the Ammeter

Now let us understand in full detail how we have to connect the shunt resistor and the ammeter together in any circuit where we want to measure big high current.

What We Are Trying to Do Here

So let us say we have one load like a motor or heater or inverter which is drawing high current like 10 amp or more.

But we only have a small ammeter, like a 1 amp analog meter or digital one with limited range. So if we connect this ammeter directly in series with the load, it will just blow off or burn. That is why, we use one shunt resistor to safely divide the current, send most of it through the shunt and only little through the ammeter.

What Things We Need

• One ammeter (small range, like 1 A)
• One shunt resistor (calculated using our calculator)
• One load (the thing which draws current)
• One power supply (like DC battery or power adapter)

Step-by-Step Connection Explanation

1. We take the +ve wire from the DC power supply and connect it to the positive side of the load (for example motor or bulb).
2. The negative side of the load we do not connect directly to ground, instead we split that wire into two paths:
   • First path: through the ammeter, positive to negative.
   • Second path: through the shunt resistor.
3. Now we join the negative side of ammeter and one side of shunt resistor together and finally connect them to the –ve terminal of the power supply (GND).

So now the load current splits into two parts after load:

• A small part (like 1 A) goes through ammeter.
• Rest of the big part (like 9 A) goes through the shunt resistor.

Because the ammeter and shunt are in parallel, so both get the same voltage drop and ammeter works perfectly without getting overloaded.

Few Important Points to Remember

• The shunt resistor must be low value like few milliohms.
• It must be thick wire or metal strip or proper shunt block because it has to carry heavy current without heating.
• The ammeter must be connected in correct polarity, or else it will show reverse or damage (for analog types).
• We must mount the shunt close to ground side, not high side because if we put it on high side then the voltage difference may confuse digital meters.

Warning: Never Connect Ammeter Directly Without Shunt

If we make mistake and connect that small ammeter in series with full 10 amp load and it is rated only for 1 amp, then current will exceed and needle will jump and meter will get hot or damaged immediately.

That is why we must always calculate correct shunt value and connect ammeter in parallel with it, so we share the load safely.