Switching Over To SMPS For Efficiency

[Hesam Moshiri] has built a variable switch-mode power supply over on hackaday.io. When prototyping a new circuit, often the goal is to get a proof-of-concept working as soon as possible to iron out all of the bugs it might have. The power supply can easily be an afterthought, and for smaller projects we might just reach for an adjustable LM317 voltage regulator to dial in the correct voltage and then move on with the meat of the project. These linear regulators are incredibly inefficient though, so if you find yourself prototyping with one of these often enough, it might be worthwhile to switch to something better.

While it’s easy to simply buy a switch-mode power supply (SMPS) that has everything you need, and rated for 90% or higher efficiency at the same time, getting one with an adjustable output isn’t as easy. This one is based on the relatively popular LM2576-Adj chip which handles the switching frequency part of the circuit automatically. You will also need some large capacitors, an inductor (one of the disadvantages of an SMPS circuit) and a small potentiometer to use as the feedback control for the LM2576. This special pin allows the output voltage of the SMPS to be precisely controlled.

Granted, this project might not be breaking any new grounds, but if you’ve never given serious thought to your small breadboard circuit power supplies, it’s definitely worth looking into. An improvement from a linear regulator’s 30% efficiency to 90% efficiency from an SMPS will not only save you a ton of energy but also solve a lot of heat dissipation problems. If you don’t want to build a switch-mode supply 100% from scratch, though, it might also be possible to modify an existing one to suit your needs as well.

Mains Power Supply For ATtiny Project Is Probably A Bad Idea

When designing a mains power supply for a small load DC circuit, there are plenty of considerations. Small size, efficiency, and cost of materials all spring to mind. Potential lethality seems like it would be a bad thing to design in, but that didn’t stop [Great Scott!] from exploring capacitive drop power supplies. You know, for science.

The backstory here is that [Great Scott!] is working on a super-secret ATtiny project that needs to be powered off mains. Switching power supplies are practically de rigueur for such applications, but compared to the intended microcontroller circuit they are actually quite large, and they’ve just been so done before. So in order to learn a thing or two, [Scott!] designed a capacitive dropper supply, where the reactance of the cap acts like a dropping resistor to limit the current. His first try was just a capacitor in series with an LED; this didn’t end well for the LED.

To understand why, he reverse-engineered a few low-current mains devices and found that practical capacitive droppers need a few more components, chiefly a series resistance to prevent inrush current from getting out of hand, but also a bridge rectifier and a zener to clamp things down. Wiring up all that resulted in a working capacitive dropper supply, but a the cost of as much real estate as a small switcher, and with the extra bonus of being potentially lethal if the power supply is plugged in the wrong way. Side note: we thought German line cords were polarized to prevent this, but apparently not? (Ed Note: Nope!)

As always, even when [Great Scott!]’s projects don’t exactly work out, like a suboptimal 3D-printed BLDC or why not to bother building your own DC-AC inverter, we enjoy the learning that results.

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A Switching Power Supply, 1940s-Style

“They don’t build ’em like they used to.” There’s plenty of truth to that old saw, especially when a switch-mode power supply from the 1940s still works with its original parts. But when said power supply is about the size of a smallish toddler and twice as heavy, building them like the old days isn’t everything it’s cracked up to be.

The power supply that [Ken Shirriff] dives into comes from an ongoing restoration of a vintage teletype we covered recently. In that post we noted the “mysterious blue glow” of the tubes in the power supply, which [Ken] decided to look into further. The tubes are Thyratrons, which can’t really be classified as vacuum tubes since they’re filled with various gasses. Thyratrons are tubes that use ionized gas – mercury vapor in this case – to conduct large currents. In this circuit, the Thyratrons are used as half-wave rectifiers that can be rapidly switched on and off by a feedback circuit. That keeps the output voltage fixed at the nominal 140V DC required by the teletype, with a surprisingly small amount of ripple. The video below is from a series on the entire restoration; this one is cued to where the power supply is powered up for the first time. It’s interesting to see the Thyratrons being switched at about 120 Hz when the supply is under load.

Cheers to [Ken] and his retrocomputing colleagues for keeping the old iron running. Whether the target of his ministrations is a 1974 scientific calculator or core memory from an IBM 1401, we always enjoy watching him work.

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Circuit VR: An (Almost) Practical Buck Converter

In the last installment of Circuit VR, we walked around a simplified buck converter. The main simplification was using a constant PWM signal. The result is that the output voltage is a fixed fraction of the input voltage. For a regulator, the pulse width will need to depend on the output voltage so that any changes in the output are self-correcting. So this time, we’ll make a regulator, although we’ll still use a few Spice elements you’d have to replace in a practical design. In particular, we’ll assume you can generate a triangle wave, which is easy enough, and produce a stable 2.5 V reference.

The idea is to take a voltage reference and compare it to the output. We’ll think of the difference between the two as an error voltage, and use a comparator combined with a triangle wave generator to produce a PWM signal that is proportional to the error, and thus works to hold the output voltage constant.

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Circuit VR: Simple Buck Converters

The first thing I ever built without a kit was a 5 V regulated power supply using the old LM309K. That’s a classic linear regulator like a 7805. While they are simple, they waste a lot of energy as heat, especially if the input voltage goes higher. While there are still applications where linear regulators make sense, they are increasingly being replaced by switching power supplies that are much more efficient. How do switchers work? Well, you buy a switching power supply IC, add an inductor and you are done. Class dismissed. Oh wait… while that might be the best way to do it from a cost perspective, you don’t really learn a lot that way.

In this installment of Circuit VR, we’ll look at a simple buck converter — that is a switching regulator that takes a higher voltage and produces a lower voltage. The first one won’t actually regulate, mind you, but we’ll add that in a future installment. As usual for Circuit VR, we’ll be simulating the designs using LT Spice.

Interestingly, LT Spice is made to design power supplies so it has a lot of Linear Technology parts in its library just for that purpose. However, we aren’t going to use anything more sophisticated than an op amp. For the first pass, we won’t even be using those.

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Buck Converter Efficiency

We always appreciate when someone takes the time to build something and then demonstrates what different design choices impact using the real hardware. Sure, you can work out the math and do simulations, but there’s something about having real hardware that makes it tangible. [Julian Ilett] recently posted two videos that fit this description. He built a buck converter and made measurements about its efficiency using different configurations.

The test setup is simple. He monitors the drive PWM with a scope and has power meters on the input and output. That makes it easy to measure the efficiency since it is just the ratio of the power output to input. You can see the two videos, below.

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Hybrid Bench Power Supply Can Also Hit The Road

Everyone needs a bench power supply, and rolling your own has almost become a rite of passage for hackers. For a long time, the platform of choice for such builds seemed to be the ATX power supply from a computer. While we certainly still see those builds, a lot of the action has switched to those cheap eBay programmable DC-DC converters, with their particolored digital displays.

This hybrid bench and portable power supply is a good example of what can be accomplished with these modules, and looks like it might turn out to be a handy tool. [Luke] centered his build around the DPS3003, a constant current and constant voltage buck converter that can take up to 40-VDC input and outputs up to 32 volts at 3 amps. In bench mode, the programmable module is fed from a mains-powered 24-volt switching supply. For portable work, an 18-volt battery from a Makita drill slips into a 3D-printed adapter on the top of the case. The printed part contains a commercial terminal [Luke] scored on eBay, but we’d bet the entire thing could be 3D printed. And no problem if you change power tool brands — just print another adapter.

Those little eBay power supply modules have proven to be an enabling technology, at least judging by the number of clever ways we’ve seen them used lately. From this combination bench PSU and soldering iron supply to a portable PSU perched atop a battery, these things are everywhere. Heck, you can even reflash the firmware and make them do your bidding.

[via Dangerous Prototypes]