# Selecting Shunt Resistor 40 ohms and 2.5W

Hi,
I am relatively new to electronic components, I have a project I am working on and I need a shunt resistor to be used to measure a maximum current of 250mA as a maximum of 10V. I know I need a 40 Ohms(V=IR Where I=250mA and V = 10V) resistor with a minimum power rating of 2.5 W (P= IV where I= 250mA and V =10V). I need recommendations in selecting the correct component. I know this sounds so basic I just don’t want to order the wrong component.

Thanks

Hello Advocate and welcome to the forum,

Wattage on a resistor is regarded as a maximum, so if your calculation shows that you need a 40 ohm resistor, that resistor needs to accommodate 2.5W of heat dissipation. Any resistor that meets or exceeds that value would be a valid pick.

Any option in this set should hit your desired spec: 40 Ohm Axial Resistors

If this is a shunt for use in current measurement, you may want a much lower resistance value. This is because a current measuring shunt is in series with the DUT (device under test), so it drops the input voltage based on ohms law.

40 ohms in series with a DUT will drop 10V when the DUT consumes 250mA.

So the voltage available to the DUT will be Vpower_supply - 10V. Most devices won’t power up with a shunt that drops the power supply voltage that much.

Fractional ohms are most common for current measuring shunts, but I have used values as high as 1 ohm in a few low power applications.

Thanks Paul,
So what I want to use it for is to convert the leakage current from my test object (Maximum of 250mA) to 10V so I can connect it to my 10V PLC Analogue input. The idea I have in mind is to measure the voltage drop across the 40Ohms resistor (which is expected to be maximum of 10V for 250mA) through the Analogue input of my PLC and scale it back to current in my PLC.

If your test object (DUT) tolerates a 10V drop in its power supply you’re all set.

Most devices can’t tolerate that much drop in their power supply.

My test Voltage is in the range of 5-10kV and the permitted voltage drop is 5% of the test voltage when loaded with a resistive current of 250mA. which is between 250V - 500V which is much higher than the 10V. This is the analysis I used for my reasoning.

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I might be inclined to go chassis mount for something of this nature, and substantially overboard on the power rating; those are reckoned in most cases as the power that causes maximum permissible element temperature within a room temp environment, so leaving margin allows things to stay much cooler.

Also, I would politely ask what you were thinking of in terms of isolation; a 10kV source capable of 250+mA is not generally the sort of thing a person wants to have galvanically connected to a PLC input.

Hi Rick,
So I intend to use a chassis mount resistor on the HV supply end. The estimated Resistance I calculated on the HV supply end is about 35Kohms and a maximum power of around 2.5 KW( Derived using a test voltage of 8 kV and Load current of 250 mA… R =V/I). I am having trouble deciding on the best high-wattage chassis mount resistor to meet the 2.5kW power rating. (I am open to suggestions)

For the measurement leg connecting to the PLC, I am planning to connect it through a 250mA overcurrent breaker (similar to what is been used for motor breakers) to provide an extra layer of protection for my PLC circuitry.

Am I missing something out here or making some mistakes in assumption and calculations?

It’s not really the calculations so much as the context; you seem to have the Ohm’s law thing down OK, but you’re speaking about a lethal power source (in the might-not-kill-you-but-you’ll wish-it-did category) and connection to a PLC in some fashion; should things go wrong and a source like that gets applied to a PLC input, I’d guess that the shrapnel cloud would be reaching a diameter of 3-10 feet before a circuit breaker had time to open. Not that a typical circuit breaker would do much anyway, because an 8 kV source would likely be capable of initiating an arc across the contacts even in their open state.

I’m missing basically all the context of what you have in mind (schematic/block diagram/detailed description would be helpful) but the nature of the inquiry causes me to question your level of awareness of the more subtle, yet no less important considerations that apply when one enters kV territory.

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If I was doing this I would insist that my employer contract a specialist consulting firm with extensive expertise in kilovolt level testing to ensure there are no fires or explosions.

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