Power Supplies Without Transformers

September 29, 2023 by Bryan Cockfield 71 Comments

For one-off projects or prototyping, it’s not too hard to find a wall wart or power supply to send a few joules of energy from the wall outlet to your circuit. Most of these power supplies use a transformer to step down the voltage to a more usable level and also to provide some galvanic isolation to the low voltage circuit. But for circuits where weight, volume, or cost are a major concern, a transformer may be omitted in the circuit design in favor of some sort of transformerless power supply.

While power supplies with this design do have many advantages, some care needs to be taken with regard to safety. The guide outlines four designs of increasing complexity which first puts out a basic transformerless power supply, using a series capacitor to limit current. To bring the voltage to an acceptable level, a recognizable bridge rectifier is paired with a capacitor as well as a zener diode. The second circuit presented adds voltage stabilization using a transistor and 78XX regulator. From there, zero-crossing detection is added to limit inrush surge currents, and the final design uses the venerable 555 timer to build a switching power supply.

Although it is noted several times throughout the guide, we’ll still point out here that transformerless designs like these introduce several safety issues since a mistake or fault can lead to the circuit being exposed to the mains voltage. However, with proper care and design it’s possible to make use of these designs to build more effective power supplies that can be safe to use for powering whatever circuit might energy but might not require the cost or weight of a transformer. For more on the theory of these interesting circuits and a few examples of where they are often found, check out the shocking truth about transformerless power supplies.

Thanks to [Stephen] for the tip!

The Simplest Curve Tracer Ever

August 2, 2023 by Dan Maloney 15 Comments

To a lot of us, curve tracing seems to be one of those black magic things that only the true wizards understand. But as [DiodeGoneWild] explains, curve tracing really isn’t all that complicated, and it doesn’t even require specialized test instruments — just a transformer, a couple of resistors, and pretty much whatever oscilloscope you can lay your hands on.

True to his handle, [DiodeGoneWild] concentrates on the current-voltage curves of Zener diodes in the video below, mainly as a follow-up to his recent simple linear power supply project, where he took a careful look at thermal drift to select the best Zener for the job. His curve tracer is super simple — just the device under test in series with a bunch of 10-ohm resistors and the secondary winding of a 12-volt transformer. The probes of his oscilloscope — a no-frills analog model — go across the DUT and the resistor, and with the scope in X-Y mode, the familiar current-voltage curve appears. Sure, the trace is reversed, but it still provides a good visualization of what’s going on. The technique also works on digital scopes; just be ready for a lot of twiddling to get into X-Y mode and to get the trace aligned.

Of course it’s not just diodes that can be tested with a curve tracer, and [DiodeGoneWild] showed a bunch of other two-lead components on his setup. But for our money, the neatest trick here was using a shorted bridge rectifier to generate a bright spot on the curve to mark the zero crossing point. Clever indeed, and pretty useful on a scope with no graticule.

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[Mr. Carlson] Fixes A Fridge

September 21, 2019 by Dan Maloney 28 Comments

A dead refrigerator is an occurrence determined to frustrate any homeowner. First there’s the discovery of hundreds of dollars in spoiled food, and then the cost of a repair call and the delay of the inevitable wait for parts. It’s clear to see why a hacker like [Mr. Carlson] would seek another way.

Now, normally a fridge repair video would by unlikely fodder for a Hackaday article. After all, there’s generally not much to a fridge, and even with the newer microprocessor-controlled units, diagnosis and repair are usually at the board-level. But [Mr. Carlson] has had this fridge since 2007, and he’s got some history with it. An earlier failure was caused by the incandescent interior lights welding relay contacts closed thanks to huge inrush currents when starting the cold filaments. That left the light on all the time, heating the interior. His fix was a custom solid-state relay using zero-crossing opto-isolators to turn the bulbs on or off only when the AC power was at a minimum.

That repair kept things going for years, but when the latest issue occurred, [Mr. Carlson] took a different tack. He assumed that a board that has been powered 24-7 for the last twelve years is likely to have a bad capacitor or two. He replaced all the caps, threw in a few new relays to be on the safe side, and powered the fridge back up. It whirred back to life, ready for another decade or so of service.

Kudos to [Mr. Carlson] for his great repair tips and his refusal to surrender. The same thing happened when his solder sucker started to give up the ghost and he fixed it by adding a variable-frequency drive.

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FM Signal Detection The Pulse-Counting Way

August 28, 2019 by Dan Maloney 7 Comments

Compared to the simple diode needed to demodulate AM radio signals, the detector circuits used for FM are slightly more complicated. Wrapping your head around phase detectors, ratio detectors, discriminators, and quadrature detectors can be quite an exercise. There’s another demodulation method that’s not so common, but thankfully it’s also pretty easy to understand: the pulse counting detector.

As [Allan (W2AEW)] notes in the video below, pulse counting is a bit of a misnomer. Pulse counting works by generating a narrow, fixed-width square wave pulse at a set point in the received FM signal’s waveform, usually at the zero-crossing point. Since the frequency of the modulated carrier changes, the duty cycle of the resulting pulse train varies. That means there will be a fixed number of pulses, but by taking the average voltage of the pulse train, we can tease out the original audio frequency signal.

Simple in theory is often more complicated in practice, and [W2AEW] goes into some detail about those complications, such as needing to use a down-converter to make the peak-to-peak frequency deviation in the pulse train more easily detectable. As is his style, he walks us through a test circuit to prove that the theory works in practice. A simple two-transistor circuit generates the pulses at the zero-crossing point, a low-pass filter cleans up the signal, and a cheap audio amplifier reproduces the original audio. It’s a crude circuit to be sure, relying on the stray capacitance of the breadboard to work, but it proves the point and serves as a jumping-off point for further experiments – perhaps using an Arduino to count the pulses?

We always enjoy [W2AEW]’s videos and learn a lot from them. Not long ago we featured another of his videos talking about the mysteries of RF modulation; SSB, anyone?

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Old Time Traffic Signal Revived With A Raspberry Pi Controller

July 12, 2018 by Dan Maloney 28 Comments

Anyone with even a passing familiarity with the classic animated shorts of the 1940s will recognize the traffic signal in the image above. Yes, such things actually existed in the real world, not just in the Looney world of [Bugs Bunny] et al. As sturdy as such devices were, they don’t last forever, though, which is why a restoration of this classic Acme traffic signal was necessary for a California museum. Yes, that Acme.

When you see a traffic signal from the early days of the automotive age like this one, it becomes quickly apparent how good the modern equivalent has become. Back in the day, with a mix of lights distributed all over the body of the signal, arms that extend out, and bells that ring when the state changes, it’s easy to see how things could get out of hand at an intersection. That complexity made the restoration project by [am1034481] and colleagues at the Southern California Traffic Museum all the more difficult. Each signal has three lights, a motor for the flag, and an annunciator bell, each requiring a relay. What’s more, the motor needs to run in both directions, so a reversing relay is needed, and the arm has a mechanism to keep it in position when motor power is removed, which needs yet another relay. With two signals, everything was doubled, so the new controller used a 16-channel relay board and a Raspberry Pi to run through various demos. To keep induced currents from wreaking havoc, zero-crossing solid state relays were used on the big AC motors and coils in the signal. It looks like a lot of work, but the end results are worth it.

Looking for more information on traffic signal controls? We talked about that a while back.

Light Dimmer Shows How To Steal Power From AC Line

January 8, 2017 by Dan Maloney 33 Comments

We see a lot of traffic on the tips line with projects that cover old ground but do so in an instructive way, giving us insight into the basics of electronics. Sure, commercial versions of this IR-controlled light dimmer have been available for decades. But seeing how one works might just help you design your Next Big Thing.

Like many electronic controls, the previous version of this hack required a connection to a neutral in addition to the hot. This version of the circuit relies on passing a small current through the light bulb the dimmer controls to avoid that extra connection. This design limits application to resistive loads like incandescent bulbs. But it’s still a cool circuit, and [Muris] goes into great detail explaining how it works.

We think the neatest bit is the power supply that actually shorts itself out to turn on the load. A PIC controls a triac connected across the supply by monitoring power line zero-crossing. The PIC controls dimming by delaying the time the triac fires, which trims the peaks off of the AC waveform. The PIC is powered by a large capacitor while the triac is conducting, preventing it from resetting until the circuit can start stealing power again. Pretty clever stuff, and a nice PCB design to boot.

Given the pace of technological and cultural change, it might be that [Muris]’ dimmer is already largely obsolete since it won’t work with CFLs or LEDs. But we can see other applications for non-switched mode transformerless power supplies. And then again, we reported on [Muris]’s original dimmer back in 2009, so the basic design has staying power.

Making A Solid-state Relay With I2C Interface

November 1, 2013 by Mathieu Stephan 34 Comments

As [Mic] often got requests to make high-power switching boards, he recently finally gave in and designed the one shown above based around a solid-state relay. Some of our readers that already play with mains power know that switching should normally occur when the voltage crosses zero volts. The ‘TRIAC BLOC’ is able to do so, which also allows mains frequency measurement. [Mic] then tuned to the internal oscillator of his ATtiny microcontroller with this 50Hz by adjusting its OSCCAL register value, so the switching command can be sent at the ideal moment. Zero crossing detection is implemented by feeding the mains into an AC optocoupler. [Mic] discovered that the optocoupler diodes are not identical, so he had to adjust his firmware to account for the time differences.

All the resources are available on github, we would be interested to hear your detailed analysis of the circuit implemented with the passives R3/C1/L1/R8/C3.