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| How do you measure and use a low value power resistor? |
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| technix:
One of my component suppliers gave me a power resistor for free for some reason. It is one of those ones with integrated aluminum heatsink and mechanical mounting holes. Whatever marking that was on there has long been scrubbed away. When I measure it with the ohms range on my multimeter, the result seemed a bit inconsistent to me since the resistor is almost within the range of the resistance of the leads I am using. How should I measure it then? I don't have a multimeter capable of four-wire measurement, but will a ghetto four-wire measurement solution using a bench power supply, a Uni-T UT61D and a UT61E work? How should I arrange the two different meters for the maximum accuracy? How do I estimate the power ratings of such a resistor? Also what can I do with it? The leads on both ends of the resistor is millimeters thick and very hard to bend, so should I solder it to a PCB directly at all? If not how should I attach it to a PCB? |
| Conrad Hoffman:
Put it in series with a lowish value resistor that you do know, say a 10 ohm 1% or similar. Attach to a power supply and put a volt or two across them. Measure the voltage drop across the 10 ohm to get the current, then measure the voltage drop across the unknown resistor and use ohms law to calculate the value. Done carefully, this will get you any accuracy you want, based on your knowledge of the reference resistor and the accuracy of your meter. Just watch your power dissipation as the resistors may change on heating. |
| technix:
--- Quote from: Conrad Hoffman on December 16, 2018, 03:20:41 pm ---Put it in series with a lowish value resistor that you do know, say a 10 ohm 1% or similar. Attach to a power supply and put a volt or two across them. Measure the voltage drop across the 10 ohm to get the current, then measure the voltage drop across the unknown resistor and use ohms law to calculate the value. Done carefully, this will get you any accuracy you want, based on your knowledge of the reference resistor and the accuracy of your meter. Just watch your power dissipation as the resistors may change on heating. --- End quote --- The unknown resistor is about the 0.2 ohm range. Also I don't have low value reference resistors handy, but I do have two multimeters. |
| Heartbreaker:
--- Quote from: technix on December 16, 2018, 03:22:41 pm ---The unknown resistor is about the 0.2 ohm range. Also I don't have low value reference resistors handy, but I do have two multimeters. --- End quote --- You have a fine power resister inside both of your multimeters. Furthermore they are already hooked up to measure the voltage across this resistor and calculate the current for you, similar to what Conrad Hoffman suggested. When dialing up the voltage, do the multiplication of the current and voltage in your head and assume that all the power i dissipated into your unknown resistor. From your description it sounds as if the resistor is of a 5-10W type. |
| mjs:
Use a lab supply to push 1A (or more) through it and measure voltage over it on the mV scale of your multimeter. At 1A 1mV=1mΩ |
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