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Old 12th December 2007, 02:59 PM   #11
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Conrad,

I'll try your design. Like you, I was just trying to use the available materials.

I don't have a scope so my biggest challenge might be detecting the null.
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Old 12th December 2007, 03:20 PM   #12
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by satterfi
I don't have a scope so my biggest challenge might be detecting the null.
an analogue voltmeter is ideal.
The more sensitive the scale the better, but it will need an adjustable attenuator (variable resistor) in series to prevent the needle going off the scale, until you can get your fingers used to adjusting the null without overshooting in the opposite direction.
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Old 12th December 2007, 06:04 PM   #13
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I have a Chinese knock off of a Simpson 260. It's a nice analog meter but you can't throw it around like a Simpson.

It should work. I bought it for my cheap ESR meter.

Click the image to open in full size.
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Old 13th December 2007, 02:02 PM   #14
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Since the classic "meter" for a bridge is a tuned null detector like the GR 1232-A, and that's nothing more than a high gain amplifier and narrow band filter...

take that nice protoboard and stick in whatever low grade op-amp you didn't want to use for an audio project, like a 741, and a couple resistors, and put some gain before your meter. If you want to be really fancy, make it a bandpass filter at 60Hz. You'll have no trouble seeing the null that way! BTW, a meter with a needle is far superior to a digital meter for this purpose.

An interesting thing about bridge circuits is that at balance the output voltage is zero. That means that whatever meter or amplifier is attached, doesn't have any current flowing through it. It could look like ten ohms, but since the voltage is zero, no current flows (at balance only). Thus, you can use an inverting amplifier with a low value input resistor, and get tremendous gain without having to worry about circuit loading. Just stay within the legal limits for input voltage if the bridge is supplied with more voltage than the op-amp.
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Old 13th December 2007, 02:48 PM   #15
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Conrad -- Jim Williams has an excellent application note "Bridge Circuits" on the Linear Tech website.

We are getting our first ice storm here in NNJ.

Jack
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Old 13th December 2007, 05:19 PM   #16
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Yes, IMO everyone with analog intentions should read everything Jim Williams has ever written and buy his books as well! Though most of what he has presented doesn't cover new ground, I found his discussions of probes and bypass caps extremely valuable, not to mention the little circuits like the ultra-fast risetime generator and the bypass cap test method. The bridge article covers a lot of ground, including some active bridge arrangements not often seen.

I was in NYC yesterday and hit the window just right. Up here in Rochester we had an inch and a half of snow in an hour this morning, with more coming down, then a big storm predicted for Sundayish. I put a big load of wood next to the wood stove this morning :-)
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Old 13th December 2007, 05:46 PM   #17
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Quote:
Originally posted by Conrad Hoffman
I was in NYC yesterday and hit the window just right. Up here in Rochester we had an inch and a half of snow in an hour this morning, with more coming down, then a big storm predicted for Sundayish. I put a big load of wood next to the wood stove this morning :-)
William's book "Analog Circuit Design" is a gem -- containing as it does chapters written by Pease, Philbrick etc.

We are laying in an extra supply of Jack Daniels.
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Old 29th December 2007, 07:05 PM   #18
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Ok, I finally got ambitious enough to put this in a box. If you use a 10Kohm ten turn pot with a turns counting dial, it will read out in capacitance directly. The ten turn pots are quite good at high frequencies, 500kHz at least, though this bridge is mainly for 50/60 Hz use. Though my goal was a bridge just for high value electrolytics, I tossed in a range switch so it's now a general purpose bridge. The only pot I had for the DF dial was a very non-linear A-B carbon comp, I actually measured the resistance vs rotation and marked the dial to match. Thus the slightly weird spacing. Though the meter isn't shown (an HP3400 on the 3mV scale), the bridge is shown measuring an old 51000uF Powerlytic. It measured almost exactly 51000uF with a DF of about 0.4- a perfectly good cap. The input voltage from a filament transformer and a 40ohm limit resistor was about 1.5VAC. Note the nice heavy leads and lugs to the cap. You're balancing the cap against a one ohm resistor, so the connections should be low resistance. Why build that complicated digital stuff, when a couple resistors will do the job?

Click the image to open in full size.

Schematic is here.
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Old 14th January 2008, 08:07 AM   #19
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That looks excellent.
Can an oscillator can be the voltage source instead ?

I'll build this this week.

(this this )

I have an LCR meter (ESI 410, 1 MHz)...but I haven't a clue on how to use it. I have never seen a manual for sale anywhere, or it's sister the 295 (?). I don't even know what the "1 MHz" is referring to.

btw, fyi (OT)....
http://www.jptronics.org/radios/GR/GR_History.pdf

I love the old equipment. Eventually I'll get around to learning what this real Potentiometer I have does....
http://i5.photobucket.com/albums/y17...t/DSCN2619.jpg

I don't think this would ever be a DIY project....
http://i5.photobucket.com/albums/y17...t/DSCN2612.jpg
http://i5.photobucket.com/albums/y17...t/DSCN2616.jpg


=RR=
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Old 14th January 2008, 12:47 PM   #20
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You can only use an oscillator like the old GR ones that had transformer output. The input to the bridge really needs to float, and that's why the filament transformer works well. You should also limit the current with a 40-100 ohm resistor, since the cap and other side of the bridge are very low resistance at the test frequency.

Nice potentiometer- Guildline is excellent (and expensive) stuff. Without looking in detail, it looks like it's basically a 6-digit voltmeter. The voltage reference would have been a Weston standard cell (mercury/cadmium) at 1.018 or so volts. If you haven't already, go research "Kelvin Varley Dividers" as a first step to understanding the thing ;-)
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