Very handy Gadget-AC Continuity Tester
- From: mebratziujane@xxxxxxxxx
- Date: Sun, 15 Mar 2009 00:51:01 -0700 (PDT)
This lets you phase out transformers faster than with a HP 200CD or
Woo-Woo and a DMM.And it charges itself if you have an appropriate DC
source.
"Ultimate" Continuity Tester Hides Many Tricks Up Its Sleeve
Louis Vlemincq | ED Online ID #19768 | October 2, 2008
I WAS REMINDED RECENTLY of a tester I designed years ago. At the time,
I wanted to build “The Ultimate Continuity Tester,” and I established
a wish list of all the features I required:
• A “real continuity” tester. Too many multimeters and sounders react
at resistances as high as hundreds or even thousands of ohms, which
makes them practically useless in many cases. Within a board or a
system, there are always medium-conductivity paths everywhere, so they
sound most of the time. A connector, a PCB track, or a wire, even a
long one, has a resistance generally below 1 . Having a threshold
much higher than that generates false alarms.
• Speed. Many testers require a contact of tens of milliseconds or
more, which makes the testing of large numbers of connections very
frustrating. It is impossible to swipe quickly across a large number
of pins.
• Cheap to make and use. That meant a very small number of dirt-cheap
components and a power consumption as frugal as possible from a cheap
power source. This ruled out the usual 9-V battery, one of the least
efficient and most expensive sources available.
• No power switch. You invariably forget it’s “on” the afternoon
preceding your holiday leave, and timers aren’t good enough. They tend
to go off unnoticed just when you reach the wanted connection.
• Ruggedness. You can sometimes inadvertently test charged capacitors
or energized circuits, and the tester must survive such situations.
• Safety. Even with the most sensitive electronics, safety means low
voltage and current at the test probes.
At first sight, the circuit in Figure 1 doesn’t seem very impressive,
but it fulfills all of these requirements, and then some. It looks
like some half-cooked multivibrator, but appearances can be
deceptive.
Q1 and Q2 form a two-stage, non-inverting amplifier, whose input and
output are connected via C3 in order to cause oscillations. Each stage
has its gain carefully defined: Q1 by the ratio of R4 to R1, and Q2 by
the ratio of R2 to the sum of R8 and whatever sits between the test
probes. When the product of these gains exceeds unity, oscillation
occurs. With the values shown, the oscillation condition is met when
the contact under test is less than 5 . Other values are possible by
altering the R3/R4 ratio, or both resistors can be replaced by a
trimpot, Aj1.
To maximize the drive with the low supply voltage, the piezo-buzzer
(BZ1) is connected between antiphase outputs of the oscillator. The
resulting sound isn’t very loud with a standard transducer, but it’s
quite sufficient for a lab or office environment.
R8, together with D1 and D2, protect the tester in case it’s
accidentally connected to a live circuit. It’s a 1-W, preferably
fusible, resistor. For an extended and resettable protection, it can
be substituted with a positive temperature coefficient resistor. If
so, the resistor will be able to withstand a prolonged connection
directly across the mains. Optional diodes D3 to D6 can be installed
in parallel with the battery to ensure its protection.
The battery can be a single standard or rechargeable AA cell, since
the circuit operates from 1 to 1.5 V. A power switch is unnecessary
because the test terminals also act as a switch. The only quiescent
current is caused by leakages in the components—typically in the
hundreds of nanoamperes range. When the terminals are shorted, the
current rises to about 10 mA. So in normal operation, the battery will
last for years, and it can be soldered in place.
The buzzer can be any cheap, passive type. As noted, this circuit goes
beyond the requirements of the “ultimate” continuity tester. For
example, it not only checks the resistance, but also the modulus of
the impedance presented to the test probes. In some cases, this can
make a big (and useful) difference.
The secondary of a 50/60-Hz transformer will generally have a dc
resistance below the 5- threshold, but its impedance is mainly
inductive and higher than 5 , so no oscillation will take place.
There will simply be a light “click” at the instant the circuit is
made, because BZ1 receives a pulse via R2 and R4. This will always be
the case when a galvanic contact is made between the probes.
This is a useful feature because you can differentiate between the
wires of a multitap transformer, where a more conventional tester sees
a bunch of shorted wires. Also, when a single diode in a bridge
rectifier is shorted, you can immediately pinpoint the faulty one
without any de-soldering.
But there is more. If the transformer itself is faulty, with one or
more turns shorted, the tester will sound. And if any of the windings
are connected in opposition, it will also sound. This feature lets you
determine the phase of the windings, both primary and secondary.
Figure 2 shows a number of situations and their consequences for a
typical dual-primary/dual-secondary transformer. Therefore, you can
comprehensively test a transformer without ever connecting it to the
mains.
Finally, note that if you use a rechargeable battery, there’s no need
for a specific charge connector. The test terminals can act as an
input to the charger, via protection diode D2." < <
http://electronicdesign.com/Articles/Index.cfm?ArticleID=19768
Schematic: http://electronicdesign.com/files/29/19768/fig_01.gif
.
- Prev by Date: Re: NAT: Coming Soon to
- Next by Date: Re: Quiz for Jenn
- Previous by thread: Finnish Man Jailed in U.S. for "Thought-Crime"
- Next by thread: OT: DC HIV rate hits 3%
- Index(es):
Relevant Pages
|
Loading
