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I would like to distinguish in the easiest and safest way possible between the following kinds of components attached to my circuit:

  • resistor
  • diode
  • zener diode
  • capacitor
  • polarized capacitor (electrolytic)
  • inductor

Safe because I do not want to e.g. exceed the voltage rating of components. Also the electrolytics polarity should be detected properly. Basically you should be able to put the component on the device and it tells you what you put on there.

Since resistors have an inductance, and inductors have a resistance I would be fine with restrictions for useful values; that is something with 10Ω and 500nH should probably be qualified as a resistor and not a tiny inductor.

The circuit should not need any extremely sophisticated parts that e.g. amount to an internal high class oscilloscope. It should probably use some average PIC.

Note that I am not asking for a complete circuit but some rough ideas, if graphical maybe as blocks.

The main points that I am having problems with is:

  • detecting polarity of a capacitor without applying reverse voltage
  • First idea for a zener was (after detecting it is a diode) that I detect the breakdown voltage, but what if it is just a normal diode that gets destroyed this way because it has a rather low breakdown? That would limit the breakdown voltage that I could detect in zeners.
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    \$\begingroup\$ The time and effort to design a box that can determine the polarity of an unmarked electrolytic versus just buying a new cap seems extreme? \$\endgroup\$
    – Andy aka
    Commented Mar 5, 2015 at 11:50
  • \$\begingroup\$ @Andyaka: This thing won't be for sorting my scrap part box ;) This is mostly for a fun project and maybe later as part of some educational "box". Having young people doing their first steps accidentally plug in an elco the wrong way around and spraying in their face might not be the nicest thing. \$\endgroup\$
    – PlasmaHH
    Commented Mar 5, 2015 at 11:55
  • \$\begingroup\$ There are many devices for sale on Ebay that do this function at a cost of under $20. You can buy one and try to reverse engineer it. \$\endgroup\$
    – Barry
    Commented Mar 5, 2015 at 14:00
  • \$\begingroup\$ @Barry: I have one of these. It blindly applies voltage to the elco and can not detect zeners and has no idea that an inductor is not a resistor. Most of them just put 5V on that thing to test, regardless of if it might make it go boom. \$\endgroup\$
    – PlasmaHH
    Commented Mar 5, 2015 at 14:16

1 Answer 1

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As long as you limit the power you apply to the DUT, you won't destroy it.

detecting polarity of a capacitor without applying reverse voltage

You can't. The only way to determine the polarity is to look for the increased leakage current under reverse bias. As long as you keep the current low (microamps), this won't destroy the capacitor.

First idea for a zener was (after detecting it is a diode) that I detect the breakdown voltage, but what if it is just a normal diode that gets destroyed this way because it has a rather low breakdown? That would limit the breakdown voltage that I could detect in zeners.

Again, if you limit the current to a suitable value, you won't destroy a diode, even under reverse breakdown. For example, suppose you want to limit the total power to 100 mW. If your diode is breaking down at 40 V, you need to limit the current to 100 mW / 40 V = 2.5 mA.

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