I have a back-burner project to take an old bench power supply and retro-fit it with two 4 digit LED displays for current and voltage. The power supply is 3A/30V, so four digits means 3000 count and 12-bit conversions (4096 = 2^12).

I've been trying to design an analogue current measuring block that just has a pair of current shunt resistors with a relay and some range detection circuitry etc. But I've bumped into a weird question about which resistors to use.

Ignoring the range selection, let's assume that I choose resistor and current sense amplifier so that 3A => 3V. Then, in order to get 1mA accuracy, I have a maximum error of 333ppm. Round that down to 200ppm to give the later ADC some headroom. Now, it seems that a reasonable temperature range is maybe 10°C - 30°C, which basically corresponds to "rooms I can function in". The 15°C range from the midpoint then says that, assuming I null out errors correctly, I can have a maximum TC of 13.3ppm/°C.

Even ignoring any errors from the current sense amplifier, I don't think that one can buy such resistors! (Or not easily). Looking hard at Farnell/Digikey/Mouser, I can find a few 20ppm/°C resistors but that's it.

So the question: Have I messed up my calculation horribly? If not, how do people build multimeters...?

  • \$\begingroup\$ Farnell sell 0.1% resistors that are 10ppm. I bought some yesterday. The common ones are 5ppm, 10ppm, 15ppm and 25ppm. \$\endgroup\$
    – Andy aka
    Jul 12 '13 at 10:40
  • \$\begingroup\$ Ah, looking again I see that they do but the SMD ones start at 10 ohm and the through hole ones at 5 ohm, which explains why I didn't spot them. Of couse a 5 ohm resistor is not really suitable for a 3A current shunt... \$\endgroup\$ Jul 12 '13 at 10:49
  • \$\begingroup\$ Have you checked the total error of your ADC? I suspect it is much larger than the resistor error. \$\endgroup\$
    – Joe Hass
    Jul 12 '13 at 10:51
  • \$\begingroup\$ Welcome to the wonderful world of high-precision measurement of anything. Anyways, if you really need that high precision, you should look into adding a temperature sensor, and having calibration tables across the temperature range over which you expect to use the device. \$\endgroup\$ Jul 12 '13 at 11:22
  • \$\begingroup\$ Joe: No, the "cunning plan" was to build the analogue section first (so I could insert it and carry on using the bench supply!). Connor: Hmm, that seems likely. I think I'll probably add a temperature sensor to the board with analogue out and when I design the ADC/uC/display part I can decide what to do with the information! \$\endgroup\$ Jul 12 '13 at 12:08

One option is Vishay's bulk metal foil resistors.

They have many package configurations available, but the VCS series looks good for current sensing applications.

  • 2 ppm / C TCR typical
  • 0.5% initial tolerance
  • 4-terminal configuration
  • Much more stable in value after ESD/EOS events, compared to other types.

Pricing and availability will probably drive you crazy. I found a 1 Ohm VCS1625 part at Digikey for $16 each. For 3 A you probably need to go to 0.1 or 0.05 Ohm due to the resistor power limit.

  • 1
    \$\begingroup\$ Hmm, thanks. Now I've stared at the catalogues for a bit longer, I think I'll use the Welwyn "OAR" series (Open Air Resistor). They have a 20ppm/°C TCR and maybe a twentieth of the price... Thank you (and the other responders) for the help: I really wanted to check that I wasn't just doing something completely wrong since there seemed to be so few viable parts. Now back to the gain trimming... \$\endgroup\$ Jul 12 '13 at 16:58

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