[Test-Equipment] Question regarding antique LCR bridge

[Richard Knoppow]

----- Original Message ----- 
From: "Barry" <n4buq at knology.net>
To: <n4buq at knology.net>; "Discussion of Electronic Test 
Equipment" <test-equipment at mailman.qth.net>; 
<rbethman at comcast.net>
Sent: Tuesday, April 10, 2012 12:20 PM
Subject: Re: [Test-Equipment] Question regarding antique LCR 
bridge


> Not quite.  Are you saying that I should be able to 
> measure wirewounds more effectively with the bridge than 
> other types of resistors?  I haven't tried a wirewound but 
> will do that and see.  Carbon comps were common at that 
> time so if it wasn't designed to work with those, wouldn't 
> it have stated so in the manual?
>
> Thanks,
> Barry - N4BUQ


     I am a little puzzled by this. Most wire wound 
resistors have some reactance. A DC measurement does not 
show the reactance. It is often useful to know the DC 
resistance of reactive components like coils, chokes, 
leakage of capacitors, etc.  I don't know of any impedance 
bridges that do not measure DC resistance.  It sounds like 
your bridge uses somewhat raw DC, that is DC with a ripple 
component, to measure resistance but uses a chopper on the 
input of the amplifier so that an AC amplifier can be used 
as a null detector. I can't figure out any other reason for 
doing this since an AC null detector would work fine for 60 
Hz. If no DC is needed why bother with rectifying the AC and 
then chopping it back to AC?  Chopper stabilized amplifiers 
for DC were very common at one time, one can find them in 
many DC meters. Its a method of avoiding drift which is 
difficult to eliminate from a direct coupled amplifier. Its 
also an economical way to provide a high-gain null detector 
for DC where an AC null detector is already there.
     I don't think there is really much difference between 
resistors made now and those of the 1960s other than film 
resistors of various sorts have pretty much replaced 
composition types. While its commonly thought that 
composition resistors are less reactive than film types 
measurements show that for low or medium values they are 
not.  A very old example can be found in the instruction 
book for the Boonton 250 RX-Meter, which is intended for 
making just such measurements. A sample chart shows the 
relative reactance of several types of resistors, the carbon 
film one having less reactance than the composition one.  I 
have confirmed this using my RX meter.
     I think your bridge is making conventional DC 
resistance measurements.
     BTW, while thermoelectric currents do very definitely 
affect resistance measurements they are not significant for 
values above perhaps an ohm.  As the other poster stated 
very low resistance (perhaps lower than around ten ohms) 
should be measured using a Kelvin Double Bridge or other 
four-terminal arrangement. Where thermal currents are of 
concern as in extremely accurate comparison with standard 
resistors special methods of connection are used to reduce 
them or balance them out.  There is information on this from 
several sources including the old NBS handbook on electrical 
measurements. Such precautions are necessary only under very 
special conditions and are of no significance when measuring 
normal electronic components.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com