DIY Capacitance Checker. Anyone tried this???

[m_charles]

came across this the other night.

http://www.io.com/~n5fc/capac_ckr.htm

Thought I'd share. I want to try to build this but in the directions, the gentleman who made the page says:

"I suggest a wall-wart for the transformer, to make construction that much simpler and to keep from dealing with 115 VAC wiring. Just be sure you get one with AC output (not DC). Anything from 12 - 18 VAC should work (mine was about 17.5V with no-load)"

I'm not quite sure how to implement a wallwart in this circuit. Don't quite understand how you replace the transformer in the diagram, with a wallwart (basically, where you connect the neg and positive cords you'll have from the wall-wart).
I feel silly not knowing how to do it, and I'm pretty sure I could figure it out, but since it's plugging into a wall-socket,
I thought I'd ask some of you first.
Don't feel like getting shocked...

RG, this seems like the kind of thing you'd apprieciate... comments??

can anyone help me get it goin'? Anyone already built it?

thanks!

chuck

[compuwade]

To replace the transformer with a walwart you just remove the transformer and add a power connector where the transformer would normally connect. Then plug the walwart to the connector. Make sure you get an AC output walwart. You can add a fuse to one of the wires on the power connector if you wanted to.

[WhenBoredomPeaks]

I think it's measuring range is not too practical for guitar effects.

[col]

There is a DIY capacitance meter in the Babani book IC555 projects by E A Parr. I was going to try and build it but I got a DMM with capacitance on from a car boot sale. I'm not going to post it following the previous warnings from Babani on here but the book is still current and costs around £5. The circuit is based on 2 555s and measures from 1n to 10u. Has anyone built it?

[dmaher]

It's already been explained. But to make it easier to understand. Basically T1 and F1 are what's in an AC plug pack, with a mains plug. So essentially in this case you're just shifting where the transformer is/

This is basically how it works, in the red square is the wallwart, then it has the ELV lead to the capacitance checker.

http://img40.imageshack.us/img40/6783/wallwart.jpg

Although that's not really schematically correct. 

Youd typically just draw leads to a connector symbol or an AC supply as the plugpack is off board.

[brett]

Hi
this is terrific.
I've done a quick analysis and it looks like it'll work for much larger devices.

Basically, the meter response tapers off as values get higher.  But only slowly.  If you calibrate 12.6 V AC to 1000pF and 1.0 V (R is approx 250 kohms), the equation relating capacitance to meter response is a perfect quadratic: y = -0.023 + 0.0011 x - .00000003 x^2.  As you can see, the square term is small.   For a case where you are calibrating at 1000 pF, the error at a bit larger larger values isn't too bad (at 0.010 uF the meter reads about 8.0 V, implying 0.008 uF, an error of 20%).

Of course, you can get good results at high capacitance (up to 1uF ?) if you are prepared to draw a curve on a graph from measurements of known capacitors (meter reading on the X axis and capacitance on the Y axis will be convenient).  That's what I'll do.
cheers

[m_charles]

Thank you for all the help guys.
So is this how you'd implement the wallwart?

[rogeryu_ph]

The wallwart adaptor is a DC output. I think you just need the transformer and disregard the inside circuit of the wallwart coz you only need the secondary AC output of the transformer.

[dmaher]

In this case its an ac plugpack, so all its got in it is a transformer and a thermal fuse.