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I'm reverse engineering a circuit board for a 180V dc motor driver and one of the components which is presumably a power resistor (white rectangular with two leads) is marked "N5W0Ω03J" is there a standard or at least best guess for interpreting the value for this component? Google is no help in this circumstance due to the Ω character and any attempt to find a resistor code key just comes back with the color codes.

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    \$\begingroup\$ It's not google or the 'Ω' symbol that is the problem, but what is there available for it to find online. Googling 'N5W0Ω03J' brings up this question as the only hit. \$\endgroup\$ – Neil_UK May 4 '18 at 6:06
  • \$\begingroup\$ Notice you'll have to get the heat out of that resistor. If no moving air flow, nor vertical-style convection, then the PCB traces and planes are the mechanism. Thus you need to reverse engineer the Heat Extraction paths as well. \$\endgroup\$ – analogsystemsrf Sep 1 '18 at 4:33
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I would not bet the bank on this, but that code looks a lot like 5 watts, 0.03 ohms, which would be a good in-rush current limiter for a motor. A sign (sometimes the letter 'R') is placed between the ohms value to indicate it is 1 ohm or less, or a fractional value. The 'sign' is a virtual decimal point.

A white rectangle means it is wire-wound ceramic, which are very durable resistors and are designed to run too 'hot' to touch as a normal operating condition.

If the resistor gets too hot for long periods the code marked on it will burn away. A huge overload can make them crack open.

30 milliohms is difficult to measure without an expensive DVM. An option is a 1.00 amp constant current source, which would give you a 30 millivolt drop across the resistor. Little difference, as it is probably only 10% tolerance.

NOTE: Based on the chart I found it is a 5% tolerance resistor. About as good as it gets for off-the-shelf wire-wound.

I found this Resistor Tolerance Chart on the web.

(Reference IEC60062):

$$ \begin{array}{ccccccc} Colour& Value 1& Value 2& Value 3& Multiplier& Tolerance& Letter Code\\ Silver &&&&x 0.01& 10\% & K\\ Gold& &&&x 0.1 &5\% &J\\ Black& 0& 0& 0& x1\\ Brown& 1& 1& 1& x10 &1\% &F\\ Red &2 &2 &2 &x 100 &2\% &G\\ Orange& 3 &3 &3 &x 1K&\\ Yellow& 4& 4& 4& x 10K\\ Green& 5& 5& 5& x 100K &0.5\% &D\\ Blue& 6& 6 &6 &x 1M &0.25\%& C\\ Violet& 7 &7& 7 &x 10M &0.1\%& B\\ Grey& 8 &8 &8 &x 100M &0.05\% &A\\ White &9 &9 &9 &x 1G\\ Blank&&&&& 20\%& M \end{array} $$

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  • \$\begingroup\$ 0.03 ohms is probably a current measuring shunt rather than an inrush current limiter but otherwise I think this is spot on. \$\endgroup\$ – Brian Drummond May 4 '18 at 8:29
  • \$\begingroup\$ Do you know what the J stands for? \$\endgroup\$ – Sanchises May 4 '18 at 9:30
  • \$\begingroup\$ Either 5% or 10% tolerance. Not so important with resistors that are wire wound. \$\endgroup\$ – Sparky256 May 4 '18 at 9:36

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