[Diy_efi] Re: looking for help with a prom N82S181

[Ludis Langens]

> David Cooley wrote:
> 
> never seen an Ohmmeter that delivered unlimited current...
> Most modern DVM's have a standard ohms setting which supplies minimal
> current (micro-amps) and a "diode" setting that supplies 3V or so to
> ensure diode junctions are biased on for testing...  I have been
> playing with TTL, CMOS and discrete devices for 30 years and never
> has anything been damaged by measuring in a circuit with an
> ohmmeter...
> 
> -------Original Message-------
> 
> From: Phil Hunter
> Date: 11/10/04 12:12:20
> To: diy_efi at diy-efi.org
> Subject: [Diy_efi] Re: looking for help with a prom N82S181
> 
> NO!!! This is a good way to destroy ICs. Ohm meters have a voltage
> across
> the leads, this voltage can turn on internal diode junctions and the
> resulting "unlimited" current can destory devices anywhere along the
> trace.

My ohmmeter uses voltages in the range 0 to 0.2 volts for measuring
resistance.**  This is low enough that any silicon (or germanium!)
junctions are never forward biased.  This means that any such junction
is an open circuit to the ohmmeter.  It also means that the circuit
cannot be damaged.

** It applies a constant current of the correct amount to generate that
much voltage drop across a resistor in the currently selected range. 
This means that the actual display part of the meter is simply running
in voltage readout mode.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/
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