Analog multimeter + diode bridge - dual polarity current

Discussion in 'General Electronics Chat' started by atferrari, May 10, 2015.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    I am about to start testing some current sources. Output varies between -1000 mA and +1000 mA. Voltages involved: -5V / +5V.

    I will use an analog multimeter. To make it simpler and avoid damages (wrong polarity), I considered using a diode bridge.

    I need this not to be fast nor extremely precise, just to allow appreciating if for equivalent control voltages (measured with a DMM) the absolute value of the currents is in the ballpark.

    Not sure if the use of diodes requires kind of spectial additional precautions.

    Gracias for any suggestions.
     
  2. OBW0549

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    Mar 2, 2015
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    Nope. Should work just fine the way you described it, provided you use diodes that can handle the expected current; for 1000mA max., I figure 1N4001's or equivalent should do the job.
     
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  3. AnalogKid

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    No special precautions. Is the analog meter a true current meter? That is, does it have a low value shunt resistor built-in? If so, and if the current sources you are testing have enough voltage compliance to compensate for the two diode voltage drops and the meter, then your accuracy should still be pretty good.

    You can confirm this by using the dvm to measure the output voltage of the current source with very low load current, and measure the resistance of the current meter. With Ohm's Law you an calculate the voltage compliance needed to drive you test set and compare it to the voltage compliance of the current source.

    ak
     
  4. atferrari

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    Gracias AK for replying.

    I will use the 10A range. I guess it is fitted with a shunt but I will chek by your suggestion.
     
  5. DickCappels

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    Aug 21, 2008
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    Don't worry. If it is a DMM with a 10 amp scale, it has an internal shunt already.
     
  6. ian field

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    Shottky-barrier diodes have about 1/3 the forward volt drop - which is of course doubled in a bridge. Its only a small voltage error that probably won't have much effect on current.

    SB diodes also have significant reverse leakage, but probably not significant for what the TS stated.

    Ordinary diodes are likely just fine, but a little more information can light the way.
     
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  7. OBW0549

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    Good point about the voltage drop; 1N5819's or MBR160's might be a better choice, then.
     
  8. ian field

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    Recently while browsing an archive of old magazines, I found a project with an unusual use for a SB rectifier - as the varicap in a tuned AM loop antenna. SB rectifiers tend to have higher junction capacitance, but they still had to use a TO220 style power rectifier to get enough.
     
  9. AnalogKid

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    If what he is measuring is a true current source and has more than 4 volts of compliance, the diode type won't matter.

    ak
     
  10. ian field

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    I'd probably draw the line at selenium stacks......................
     
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  11. OBW0549

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    Or copper oxide rectifiers... remember them?
     
  12. ian field

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    Even those were better than electrolytic rectifiers.

    I think they were a strip of lead and a strip of aluminium suspended in a solution of borax or something.
     
  13. OBW0549

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    Electrolytic rectifiers... never encountered one of them.

    The Ancient Ones had all kinds of bizarre electrical/electronic devices-- thyratrons, ignitrons, klystrons, tunnel diodes, unijunction transistors, SCS's, et cetera...
     
  14. ian field

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    A while back I found an archive of Wireless World starting from issue 1 in 1913 on americanradiohistory.com - there were descriptions of various weird and wonderful ways of rectifying AC.

    Back then, not many people had mains supply at all - and some of those had DC supply. So rectification (other than signal) was something that only got covered from time to time.

    They probably covered just about every possible permutation of things you could clamp in the holder and prod with a "cats-whisker".
     
  15. AnalogKid

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    I resemble that remark. And on your list, I've worked with 5 out of 6, plus the SBS - silicon bilateral switch

    ak
     
  16. ian field

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    Diacs are still alive and well - they frequently turn up in start up pulse generators for half-bridge drivers.

    Programmable unijunctions are getting hard to come by - but the PUT is basically just a small signal thyristor with the gate at the wrong end.

    Sometimes you can get away with up-ending a PUT circuit and use a small thyristor instead.

    From the complementary-assymetry of the basic 4-layer device, it isn't hard to work out how to re-arrange the circuit.
     
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