Computing stuff tied to the physical world

Producing a beefier signal

In Hardware on May 21, 2012 at 00:01

Let’s move on, now that we have a clean sine wave. The goal is to produce a ±10V sine wave to use for constructing a Component Tester. The sine wave produced so far was merely ±65 mV.

I re-used the same circuit as yesterday, but with slightly different settings. First of all, I replaced the op-amp by an LM358, which can handle higher voltages. Next, I halved all the R’s to 5 kΩ and doubled all the C’s to 0.2 µF. This reduces the loading of the feedback loop – it shouldn’t really affect the frequency.

To increase the output voltage, I connected the oscillator output signal to a non-inverting op-amp circuit:

Screen Shot 2012 05 13 at 18 04 33

In a nutshell: this circuit tries to keep Ve as close to zero as possible at all times. IOW, the op-amp will constantly adjust its output so that the tap on the Rf:Rg voltage divider tracks the Vin voltage on the “+” input pin.

Using Rf = 10 kΩ, and Rg = 470 Ω, its gain will be about 22x. The nice property of this circuit is that it has a very high input impedance, so there is virtually no current draw from the oscillator.

And sure enough, the output of this op-amp is a sine wave with many volts of output swing. Now it’s simply a matter of cranking up the supply voltages to ±13.6V and bingo, a ±10V sine wave:

SCR64

Very clean. Better even than the original circuit – the 2nd harmonic is now -49 dB w.r.t. to the base frequency. That’s just 0.35% of harmonic distortion – excellent!

That second op-amp came for free, since an LM358 DIP-8 package has two of them anyway. So the first op-amp is oscillating (at about 470 Hz) and the second op-amp brings the output level to ±10V.

It’s quite a mess on the mini-breadboard I used, but who cares – that’s what prototypes are for:

DSC 3215

One last check is needed to make sure that the LM358 is suitable. A component tester measures the effects of an unknown component in series with a 1 kΩ resistor. So in the worst case, with a simple wire as “unknown component”, the maximum current through that resistor will be ±10 mA. Luckily, according to the specs, an LM358 can supply at least 10 mA, and typically up to 20 mA on its output.

So that’s it: our Component Tester will need a ±13.6V supply, an LM358, and a few R’s and C’s. That supply voltage is not critical, as long as it’s stable – the output level could be adjusted to ±10V via a trimpot.

Welcome to the world of analog electronics!

  1. Nice result. That’s one nice sine wave

    Will ±12v be enough? I’m just thinking that without a bench PSU you’d need to build one, and 12v regulators are rather more common than 13.6v ;-)

  2. With so many op-amps out there, I’m hoping there’s one which can source/sink 10 mA @ 10V while driven from 12V …

  3. Maybe you can find something in this analog devices OpAmp buying guide: http://www.analog.com/static/imported-files/images/overviews/Operational_Amplifiers_Selection_Guide_2011-2012__FINAL.pdf There are some 12V Rail-to-Rail Output opamps in there that will do >10 mA

  4. Kit? :-)

    • One op-amp? Heh – yes, maybe this could become a kit, but it will take a while, this is all far from ready for creating a full kit. Many many different ways this could go – I haven’t been able to decide yet, sorry.

  5. Hi I already have a circuit for this if you would like to take look. I designed because the existing units I had used a transformer to create the sine wave but with all the equipment in the workshop the sine wave was clipped and so not very helpful when trying to check some components, especially capacitors. The sine wave is nice and clean and at 50Hz mimics many of the built in component testers on some oscilloscopes. I even have the pcb layout somewhere (oh and a couple of working units too)

    • I already have a circuit for this if you would like to take look.

      Always interested!

  6. I’ve emailed some details to you…

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