Re: Meter To Measure Current / Calculator Suggestions

From: Ban (bansuri_at_web.de)
Date: 02/08/05 

Date: Tue, 08 Feb 2005 17:51:18 GMT

Joe McElvenney wrote:
>
>> I was thinking that as a hobby builder I would want to measure
>> between 1uA-5A? I am not having alot of luck with finding a meter
>> that will go that low. The ones that do go that low don't go
>> anywhere near what I would want on the high side.
>
> So what you do is to use more than one meter to cover the range
> you need. 50uA (or lower) analogue meters are easily found in
> surplus and at reasonable prices. Alternatively make one using a
> fixed resistor, an op-amp (or FET) voltage amplifier and an
> el-cheapo DVM. Generally, low current circuits are high resistance,
> so the extra series resistance won't matter.
>
>
> Cheers - Joe

Here is a circuit that looks a bit like an Instrumentation Amplifier with
the property of no voltage drop. It is of course good only for small
currents <10mA. With a FET-input opamp really small currents can be measured
with minimum impact on the DUT.
                ___
             +-|___|-+----+
             | R |
        I | |\ |
  o----->----+-|+\ | ___ ___
               | >- +----)-+-|___|-+-|___|-+
  | +-|-/ | | | R1 | R1 |
  | | |/ .-. | | | ===
  | | | | | | | GND
  | | | |R | | |
  | | '-' | | | |\
 Ue=0 | | | | +-|+\ Uout
  | +-------+ | | | >--+----o
  | | | | | +-|-/ |
  | | .-. | | | |/ | Uout=2*R*I
  | | | | | | | |
  | | | |R | | | |
  | | |\ '-' | | | |
  | +-|-\ | | | ___ | ___ |
  V | >- +----+-)-|___|-+-|___|-+
  o-----<----+-|+/ | R1 R1
        I | |/ |
             | ___ |
             +-|___|-+------+
                 R
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
view\fix font 

-- 
ciao Ban
Bordighera, Italy

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