Power Supplies Without Transformers

September 29, 2023 by Bryan Cockfield 66 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!

Car Security System Monitors Tiny Voltage Fluctuations

July 25, 2023 by Bryan Cockfield 42 Comments

As the old saying goes, there’s no such thing as a lock that can’t be picked. However, it seems like there are plenty of examples of car manufacturers that refuse to add these metaphorical locks to their cars at all — especially when it comes to securing the electronic systems of vehicles. Plenty of modern cars are essentially begging to be attacked as a result of such poor practices as unencrypted CAN busses and easily spoofed wireless keyfobs. But even if your car comes from a manufacturer that takes basic security precautions, you still might want to check out this project from the University of Michigan that is attempting to add another layer of security to cars.

The security system works like many others, by waiting for the user to input a code. The main innovation here is that the code is actually a series of voltage fluctuations that are caused by doing things like turning on the headlights or activating the windshield wipers. This is actually the secondary input method, though; there is also a control pad that can mimic these voltage fluctuations as well without having to perform obvious inputs to the vehicle’s electrical system. But, if the control pad isn’t available then turning on switches and lights to input the code is still available for the driver. The control unit for this device is hidden away, and disables things like the starter motor until it sees these voltage fluctuations.

One of the major selling points for a system like this is the fact that it doesn’t require anything more complicated than access to the vehicle’s 12 volt electrical system to function. While there are some flaws with the design, it’s an innovative approach to car security that, when paired with a common-sense approach to securing modern car technology, could add some valuable peace-of-mind to vehicle ownership in areas prone to car theft. It could even alleviate the problem of cars being stolen via their headlights.

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Rapid Charging Supercapacitors

September 16, 2020 by Bryan Cockfield 40 Comments

Battery technology is the talk of the town right now, as it’s the main bottleneck holding up progress on many facets of renewable energy. There are other technologies available for energy storage, though, and while they might seem like drop-in replacements for batteries they can have some peculiar behaviors. Supercapacitors, for example, have a completely different set of requirements for charging compared to batteries, and behave in peculiar ways compared to batteries.

This project from [sciencedude1990] shows off some of the quirks of supercapacitors by showing one method of rapidly charging one. One of the most critical differences between batteries and supercapacitors is that supercapacitors’ charge state can be easily related to voltage, and they will discharge effectively all the way to zero volts without damage. This behavior has to be accounted for in the charging circuit. The charging circuit here uses an ATtiny13A and a MP18021 half-bridge gate driver to charge the capacitor, and also is programmed in a way that allows for three steps for charging the capacitor. This helps mitigate the its peculiar behavior compared to a battery, and also allows the 450 farad capacitor to charge from 0.7V to 2.8V in about three minutes.

If you haven’t used a supercapacitor like this in place of a lithium battery, it’s definitely worth trying out in some situations. Capacitors tolerate temperature extremes better than batteries, and provided you have good DC regulation can often provide power more reliably than batteries in some situations. You can also combine supercapacitors with batteries to get the benefits of both types of energy storage devices.

The Best Voltage And Current Reference This Side Of A Test Lab

March 10, 2020 by Jenny List 16 Comments

When you measure a voltage, how do you know that your measurement is correct? Because your multimeter says so, of course! But how can you trust your multimeter to give the right reading? Calibration of instruments is something we often trust blindly without really thinking about, but it’s not always an impossible task only for a high-end test lab. [Petteri Aimonen] had enough need for a calibrated current source to have designed his own, and he’s shared the resulting project for all to see.

The cost of a reference source goes up with the degree of accuracy required, and can stretch into the many millions of dollars if you are seeking the standards of a national metrology institute, but fortunately [Petteri]’s requirements were considerably more modest. 0.02% accuracy would suffice. An Analog Devices precision voltage reference driving a low-offset op-amp with a driver transistor supplies current to a 0.01% precision resistor, resulting in a reference current source fit for his needs. The reference is available in a range of voltages, his chosen 2.048 volts gave a 2.048 mA current sink with a 100 ohm resistor.

In a way it is a miracle of technology that the cheapest digital multimeter on the market can still have a surprisingly good level of calibration thanks to its on-chip bandgap voltage reference, but it never hurts to have a means to check your instruments. Some of us still rather like analogue multimeters, but beware — calibration at the cheaper end of that market can sometimes be lacking.

A Division In Voltage Standards

November 28, 2019 by Sharon Lin 152 Comments

During my recent trip to Europe, I found out that converters were not as commonly sold as adapters, and for a good reason. The majority of the world receives 220-240 V single phase voltage at 50-60 Hz with the surprisingly small number of exceptions being Canada, Colombia, Japan, Taiwan, the United States, Venezuela, and several other nations in the Caribbean and Central America.

While the majority of countries have one defined plug type, several countries in Latin America, Africa, and Asia use a collection of incompatible plugs for different wall outlets, which requires a number of adapters depending on the region traveled.

Although there is a fair degree of standardization among most countries with regards to the voltage used for domestic appliances, what has caused the rift between the 220-240 V standard and the 100-127 V standards used in the remaining nations?

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If You Need A Measurement Tool Just Build A Measurement Tool

November 23, 2019 by Gerrit Coetzee 14 Comments

[Darlan Johnson] was working on a wearable project and needed a way to measure the change in voltage and current over time.

Most measurement tools are designed to take snapshots of a system’s state in a very small window of time, but there are few common ones designed to observe and log longer periods. It’s an interesting point, for example, many power supply related failures such as resets occur sporadically. Longer timescale measuring devices could pick these up.

[Darlan] had a ton of Feathers and shields lying around, and combined them into the needed instrument. An INA219 current sensor records the measurements. They are then displayed on a TFT and logged to an SD card. Everything is bundled into a neat 3D printed case along with a battery for wireless operation. A set of barrel connectors provide the breakout to split the wires for the current measurement.

It’s a neatly done hack and we can see it as a nice addition to any hacker’s measurement drawer.

Five Channel Monitor Keeps Boat Batteries Shipshape

October 31, 2019 by Tom Nardi 22 Comments

While those of us stuck sailing desks might not be able to truly appreciate the problem, [Timo Birnschein] was tired of finding that some of the batteries aboard his boat had gone flat. He wanted some way to check the voltage on all of the the batteries in the system simultaneously and display the information in a central location, and not liking anything on the commercial market he decided to build it himself.

Even for those who don’t hear the call of the sea, this is a potentially useful project. Any system that has multiple batteries could benefit from a central monitor that can show you voltages at a glance, but [Timo] is actually going one better than that. With the addition of a nRF24 module, the battery monitor will also be able to wireless transmit the status of the batteries to…something. He actually hasn’t implemented that feature yet, but some way of getting the data into the computer so it can be graphed over time seems like a natural application.

The bill of materials is pretty short on this one. Beyond the aforementioned nRF24 module, the current version of the monitor features an Arduino Nano clone, a 128×160 SPI TFT display, and a handful of passives.

Knowing that a perfboard wouldn’t last long on the high seas, [Timo] even routed his own PCB for this project. We suspect there’s some kind of watertight enclosure in this board’s future, but it looks like things are still in the early phases. It will be interesting to follow along with this one and see how it eventually gets integrated in to the boat’s electrical system.

If you’re looking for a way to keep an eye on the voltages aboard your land ship, this battery monitor disguised as an automotive relay is still the high-water mark in our book.