From: Roger Johansson
Subject: Re: How to make/solder a circuit fast?
Date: Sun, 29 Sep 2002 08:56:33 +0200
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firstname.lastname@example.org (N. Thornton) wrote:
>The little lost angel and Roger Johansson wrote:
First of all, let me admit that I thought we were still talking about
the current measuring project of lost angel. So I replied in a
suitable way for her.
Now I realize that it is your current measuring needs we are talking
>>>Wouldn't it be easier to use a voltmeter to measure the DC and use
>>>soundcard for the AC content of the same signal?
>Bear in mind your sound card wont go much below 20Hz, and your DVM
>sure wont reach 20Hz... so you're left with an unmeasured gap. And 5
>or 10Hz fluctuations are entirely possible.
Yes, I thought about that and wrote a little about how to fill that
gap, but I deleted that part again to not become too complicated.
You will see 0-4 Hz on your voltmeter, if it is analogue, 0-2 if it's
There shouldn't be much signals in the 4-20Hz range, but one could
couple it to an amplified loudspeaker and feel the vibration with a
hand towards the loudspeaker. But if you need precision in that range
it would be appropriate to use a better system.
>>It doesn't sound like you have any problems with the DC part of the
>>signal, so why measure it.
>I thought that was an issue needing monitoring. Thermal drift is no
>rarity after all.
The question was about spikes, if more features are needed we need a
better method, for example a simple adc at the printer port.
>>Also, the other problem is I haven't managed to find a cheap DVM that
>>can do 1~100mV good enough for measuring current.
>It sounds easy to use an opamp to amplify that upto easily measured
Yes, of course, if you can build such an amplifier.
>>It is maybe not the fault of the voltmeter, if you use cables
>Well, it IS. Because my Fluke measures it correctly. A 5A current
>reads out 5.x mV on the Fluke. I get anything from 20mV to 40mV on
>cheapo ones. I need like 3~4 of them for current readings, and 4
>Flukes would really kill my budget.
An opamp to amplify and buffer the signal should work.
>4 opamps would come in at around 10 bucks for the ICs.
A quad opamp contains 4 amps, so you need only one chip.
But you can never place so small orders, you must buy more things at
the same time if you want to send for it.
You could talk to the service technician at your local TV shop, and
buy only one quad opamp.
>>>If you had used a piece of thick cable, as I told you months ago,
>>>which gives you 10 times or a hundred times higher voltages, this
>>>would be much less of a problem.
>>heehee, I couldn't find thick cables here. When I asked for AWG10,
Buy battery cables at the gas station, and cut it up and use in
parallel if needed.