Testing caps with a DIY ESR meter

There’s a problem with collecting old tube amps and vintage electronics – eventually the capacitors in these machines will die. It’s not an issue of a capacitor plague that causes new electronics to die after a few years; with time, just about every capacitor will dry out, rendering antique electronics defective. The solution to getting old gear up and running is replacing the capacitors, but how do you know which ones are good and which are bad? With [Paulo]’s DIY ESR meter, of course.

An ideal capacitor has a zero equivalent series resistance, and failure of a capacitor can be seen as an increase in its ESR. Commercial ESR meters are relatively cheap, but [Paulo] was able to build one out of a 555 chip, a small transformer, and a few other miscellaneous components.

The entire circuit is built on stripboard, and if you’re lucky enough to find the right parts in your random parts bin, you should be able to build this ESR meter with components just laying around.

Solar clock uses capacitors for style and function


This solar clock was built using a lot of salvaged parts. We find it interesting that [Nereus] combined a ring of storage capacitors with a power cell (translated) to create a hybrid energy storage setup.

The machine translation makes it a bit rough to understand how this works, but the schematic helps quite a bit. The pair of solar cells, which were pulled from some cheap solar cellphone chargers, feed the bank of capacitors encircling the clock face. If placed in a room that gets plenty of sunlight the cells will top off the capacitors which then feed an ICL7663 regulator. We’d love to hear comments on this part choice, as it’s our experience that linear regulators are rather inefficient. But anyway, the regulated power feeds both the energy cell as well as the clock motor. When output from the regulator dips the battery picks up the slack. The project also includes a voltometer and thermometer which can be displayed on the tiny LCD screen just about the six o’clock tick mark.

Now if you want something completely battery-free you’ll have to check out [Jack Buffington’s] take on solar clock.

[Thanks Manekinen]

Which way do non-polarized film capacitors go? There is an answer.


If you, like us, thought that capacitor orientation only matters for polarized varieties like electrolytic capacitors you should read through this article. [Bruce Trump] looks at why some film capacitors have a stripe printed on one end and why their orientation can matter.

He has an image rolled into his post showing both axial and dipped capacitors with a black stripe printed on one end of the package. This is an indicator of what is going on inside of the component. The end with the line has a conductive foil layer which acts as a shield. But it seems that this shield will do its job better if you do a better job of designing for the capacitor.

The diagram above shows two op-amp circuits, both using a non-polarized capacitor that will affect the circuit if it receives external interference. [Bruce] discusses various aspects of this phenomenon, mentioning that although these careful layouts can be tested in your designs to prove which has more benefits, simulated applications (using SPICE) will perform exactly the same.

Repairing a junked signal generator

We must be walking past the wrong dumpsters because we certainly haven’t encountered equipment like this just waiting to be salvaged. [Shahriar] found an HP 8648C Synthesized Signal Generator while he was ‘dumpster diving’ and set out to fix the malfunctioning lab equipment. He posted a 1-hour video on the project, which you can find embedded after the break. The actual fix happens in the first half, the rest of the video is spent testing the resurrected device.

The back corner of the case has been dented, which may be the reason this has been thrown out. When it is first powered it emits an unpleasant screeching noise and the user interface doesn’t do anything. [Shahriar] says he recognizes the sound as a malfunctioning switch-mode power supply. Sure enough, when disconnected from the main board it still makes the noise. It turns out there’s a huge electrolytic capacitor the size of a stack of poker chips which has come loose from the PSU board. When it’s resoldered the device fires up as expected.

Now how are we going to find a digital capture oscilloscope that just needs to have its PSU reassembled?

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How a quarter shrinker works

This machine is capable of shrinking coins. What you’re looking at is actually a 3D model of the Geek Groups impulse generator, which is called Project Stomper. The model is used to explain how induction shrinks a quarter to the size of a dime.

The grey chamber to the left is a reinforced containment device. It’s a safety feature to keep people in the same room as the Stomper safe from flying particles which may result from the forces this thing can put out. You see, it uses a mountain of magnetic energy to compress the edges of a coin in on itself.

As the video after the break illustrates, the main part of the machine on the right starts off by boosting mains voltage using a microwave oven transformer. This gets the AC to 2000V, which is then rectified and boosted further to get to 6000V DC. This charges three huge parallel capacitors which are then able to source 100,000A at 6 kV. When it comes time to fire, the charge is dumped into a coil which has the coin at its center. The result is the crushing magnetic field we mentioned earlier.

This isn’t a new concept, we featured a different coin crusher build in the early years of Hackaday’s existence.

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Simple concepts behind complex coilguns

Coil guns use electromagnetic coils to propel a metal projectile. On the surface they may look rather complicated. But when you break down the concepts it’s pretty easy to learn. If you’ve ever thought of dabbling in this field this lengthy coilgun primer will be a great help.

The basic concept of a coilgun comes in three parts: the coil, the voltage source, and the switch that combines the two. In the build above you can see two spools of wire on the clear barrel of the gun. These make up a pair of accelerators which connect to those huge black capacitors supplying the voltage. The switch they used can’t really be seen but from the article we know it’s a Thyristor; a Silicon Controlled Rectifier (2N6504).

In the video after the break you can see these three parts coming together for a test firing. This is the first step in a longer journey. To achieve higher projectile velocities you must add coils, as in the image above. But spacing and timing quickly complicate the simple concept. But if you can work out all the kinks you end up with some pretty great hardware.

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[Quinn] resurrects an amplifier that experienced death-by-capacitor

[Quinn Dunki] is adding wireless audio to all of the rooms in her home. She’s going with Airplay, snatching up used or refurbished Airport Express units because of their ability to work with both her existing WiFi and the Airplay protocol. The last piece in the puzzle is to get an Amp and she chose the small unit seen above. The problem is that it was dead on arrival and she couldn’t get the company to respond to her issue. So she cracked it open and fixed it right up.

The offenders are the three electrolytic capacitors at the top of the picture. She took some close-up images of each and you can’t miss the fact that they’re blown out. These are often among the higher price-per-unit parts and manufactures try to pinch the penny as much as possible. Add to it the heat in a small enclosure like this one and you’ve got a failure. [Quinn] dug through her junk bin but the size of the replacement had to be a perfect match so she ended up putting in a parts order. The new caps fit and work perfectly as you can hear in the clip after the break.

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