USB Power Isolator Keeps Smoke In

Anyone who’s done an electronics project knows the most important part of any good design is making sure to keep the magic smoke inside of all of the components. There are a lot of ways to make sure the smoke stays in there, but one of the most important is making sure that the power supply is isolated. If you’re using a USB port on a computer as your power source, though, it can be a little more complicated to isolate it from the computer.

The power supply is based around a small transformer with a set of diodes to act as a rectifier. Of course, while a transformer is great at isolating power supplies, it isn’t much good at DC. That’s what the ATtiny microcontroller is for. It handles the high-speed switching of the MOSFETs, which drive the transformer and handle some power regulation. There are two different power supplies created as part of this project as well — the first generates +5V much like a normal USB plug would have, and the other creates both +5V and -5V. It will be important not to mix these two up, or that tricky blue smoke may escape.

The project page goes into extensive details on the operation of the device, so if electrical theory is of interest, this will definitely be worth a read. Isolating a valuable computer from a prototype circuit is certainly important, but if you’re looking for a way to isolate a complete USB connection, look at this build which includes isolation for a USB to FTDI adapter.

17 thoughts on “USB Power Isolator Keeps Smoke In

  1. If it’s just power that’s desired, then there are many modules that acheive the same. Not as neat looking mind.

    For power and data, Google ADUM3160 for many implimentations. Many seem good for 1 or 2 W of power. You won’t charge a smart phone, but fine for powering a DAC or similar and eliminating a ground loop in messy Hi-Fi (or in my case Mid-Fi) spaghetti.

    1. SN6501 (watch for overload!) / SN6505 / SN6507 and WE 750315240 transformer make a good fit for easy 5V 0.5A isolated power supply, up to 5 kV.
      The ADuM3160 has the problem of running at max speed of 12 Mbps which sometimes causes problems with badly programmed devices which do not expect the lower throughput (for example any LabView board). Texas Instruments now has ISOUSB211 which can achieve 480 Mbps.
      Note that most of these isolated supplies do not have feedback of any kind, so don’t expect stable output voltage on the output.

    2. @Phil said: “If it’s just power that’s desired, then there are many modules that acheive the same… For power and data, Google ADUM3160 for many implimentations.”

      Exactly – IMO these are better.[1] They are off-the-shelf for a little more than $20 bucks, plus they isolate both USB data and power.[1] They are based on an Analog Devices Inc. (ADI) ADUM3160 USB (1.5/12Mbps) Galvanic Isolator IC.[2] For power there is a little 5V/5V isolated switching supply brick on board that can usually supply 200-300mA but not a full 500mA. So be careful if you are using an ESP32, try adding a big capacitor on the output to handle the WiFi current spikes. Another nice thing about these is that isolating both power and data lines eliminates ground loops with USB sound dongles, a must have for direct down-conversion software-defined-radios.

      * References:

      1. GeeekPi USB Isolator Module ADUM3160 USB Digital Isolation USB to USB Voltage Isolator Board Protection (5KV ESD MAX) with OC Protection 3.8 out of 5 stars 92 ratings $21.99

      2. ADUM3160 @ ADI

      1. If you need a bit more power (400mA, but still not 500mA continuous, which is impossible) and you are rolling your own USB isolator, here is a nice little 5VDC/5VDC 1KV isolated switcher module that you can actually buy today:

        * Traco Power P/N TMH 0505S Isolated Module DC DC Converter 1 Output 5V 2W 400mA Output 5V – 5.5V Input 7-SIP Module 4 through Hole Leads 2,437 In Stock 23 Weeks Standard Lead Time Qty.-1 $6.98 ea. Qty.-230 $5.99 ea.

  2. And not only to keep the magic smoke in is useful isolation. Some years ago I built an EEG device that worked using USB or BT + battery. Of course when you are plugging devices to people heads, safety is a must, so I had to isolate the entire USB interface (power + data).

  3. This looks to be a bit over-engineered given that you can get ready-made isolated DC/DC converters in THT and SMD packages which will do the same job (starting from something like Aimtec AM1DS series, ~3€ – but 1W only), very often for less money, smaller BOM, less PCB space, not having to program anything etc.

    Kudos for the documentation in the README though! That is really well written!

  4. Small isolated DC-DC converters are ubiquitous in all the “common” voltages 3V3, 5V 12V 15V 24V, and input and output voltages also do not have to be the same. They’re also available with different power levels and isolation classes and can even have a dual / split output voltage. The cheapest versions start around EUR3 and these are not stabilized. They just have a simple built in oscillator. Efficiency is also not very high (some 80% or so). The only mandatory extra parts are external capacitors (both ceramic and bigger electrolytics for buffering).

    “USB Isolators” for both power and data are also quite common and widely available. For my electronics projects I usually cheap out though. I never put stuff I’m tinkering on directly into my PC, but I always use an external hub. The Idea is that if the magic smoke is ever released, it will be from the hub which is easily replaceable, and not from the PC itself.

    1. Fyi, if you depend on a USB hub, even externally powered USB hub, to protect your computer from external circuitry then you are going to end up with a fried USB hub AND a fried computer. I have confirmed this personally.

      Electronics manufacturers rarely ever add any function beyond what is advertised. Isolation is a complex and expensive function. Isolation for USB data requires that two signals (one inverted) be isolated and an the additional of an external power supply, which is a significant increase to the BOM, significant added costs. Unless the devise clearly states that it’s isolated then it is not..

      One thing that can reduce the chances of a USB device frying your laptop is to never have your charger plugged in while connected to the device. It’s better to have your laptop floating from the devices perspective than to have it be the shortest path to ground.

  5. Unfortunately the Sigrok fx2lafw-driver does not support the lower speed either. I tried the cheap 8-channel logic analyzer with a ADUM3160 module and it doesn’t work at any speed. It’d still be useful at lower speeds for me.

  6. I love how so many replies are “this is unnecessary, try this (module that’s also recommended in the Git link). It’s only 250mA, and sags under load, but…”

    Like, it’s cool to have a different need than this solves, but it’s not pointless to have a 1amp, stable, isolated supply for many projects and having a portable +/-5V@1amp is useful for a lot of things an ADuM isn’t.

    1. What’s the matter Chris, no sense of humor? I first heard of the “magic smoke” decades ago. Then there are also the “Darkness Suckers”. When you turn them on they suck out the darkness and let you see. Those fluorescent ones usually turn black when they stop working… because they’re full of darkness.

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