Performing safer AC line voltage measurements using isolated amplifiers

DISCLAIMER: AC line (mains) voltage is not something to be taken lightly! Attempting to safely handle line voltages while minimizing the risk of harmful or fatal electric shock is the main motivator for me to design and build this circuit. However, I am no electronics engineer and I definitely have no formal training on international standards pertaining to high-voltage safety. I accept no responsibility, direct or indirect, for any damages that may occur if you attempt to make this circuit yourself, including personal harm or property damage. Additionally, there is no warranty or guarantee, express or implied, on any content pertaining to this blog post (or any other posts).

UPDATE (November 19, 2018): Added isolation voltage ratings for the amplifier and DC-DC converter.

As seen on Hackaday!

Back in mid-2017 I won a Keysight DSOX1102G digital storage oscilloscope (DSO), a piece of equipment long on my wish list but never acquired until then. One thing I’ve wanted to be able to measure with an oscilloscope for a long time was the waveform of the AC utility (in other words, the wall outlet). However, doing so presents a very real risk of blowing equipment up or shocking yourself (and possibly other people). In order to prevent this, I needed a way to perform measurements on the AC line without being directly connected to it; in other words, I need galvanic isolation.

Isolation Methods

There are many different ways to achieve galvanic isolation. Common methods are the use of transformers and optocouplers, but they each have their own disadvantages.

Optocouplers (aka optoisolators) are a common component used for isolation, but they require a fair bit of external circuitry to work correctly – not to mention its current transfer ratio (CTR) varies with temperature and age, resulting in drifting measurements over time if a feedback circuit isn’t used. They also aren’t very fast; the common Sharp PC123 optocoupler has a cutoff frequency of only 80 kHz and a response time of 3-18 µs (but newer ones can be much faster).

Transformers don’t require active circuitry and would make stepping down voltages simple. However, their inductive nature causes issues when measuring waveforms with low-frequency content and sharp edges (like the output from modified sine wave inverters), resulting in inaccurate measurements due to the ringing and other distortion that the transformer creates. Additionally, common iron-core transformers aren’t very good at capturing frequencies above 20 kHz.

Solution: Isolation Amplifiers!

I settled on using an isolation amplifier to provide the necessary protection from the AC line and the oscilloscope. Several years ago TI provided sample kits for electronic motor drives, with one component being the AMC1200 isolation amplifier; this is the IC that I used in my AC waveform viewer – however, note that there are some limitations that I will address later in this blog post.

The AMC1200 uses TI’s digital capacitive isolator technology, using high-voltage SiO2 (silicon dioxide) dielectric capacitors on the chip itself for high voltage protection. The amplifier’s input is essentially digitized using a sigma-delta modulator, whose output is then sent digitally across the isolation barrier before being demodulated back into an analog output. It is rated for a working isolation voltage of 1200 Vpeak (848 Vrms), and a maximum isolation voltage of 4000 Vpeak (2828 Vrms), well above the typical voltages experienced on a 120V line.

AC Waveform Viewer Construction

 

As with most of my projects, my AC waveform viewer is built on FR4 fiberglass perfboard. The isolation components used are the AMC1200 isolation amplifier by Texas Instruments, and its corresponding power supply is the NXE1S0505MC isolated DC-DC module by Murata. It is rated for reinforced insulation up to 125 Vrms and basic insulation up to 250 Vrms, with a production-tested Hi-Pot rating of 3 kVDC. It does provide reinforced insulation at the voltages used in North America, but is still the weaker link in terms of maximum isolation voltage.

The AMC1200 features differential inputs and outputs, with a maximum input voltage of +/- 200 mV intended for use with low-resistance current shunt resistors.

One potential problem with perfboard is that the through-holes compromise the high-voltage isolation of the circuit (reducing the creepage and clearance distances), acting as multiple series spark gaps. The solution to this is similar to how isolation slots are used on commercial PCBs; that is, drill out the holes! This greatly increases the distance between each side of the circuit and improves the safety of the circuit.

The AC voltage input is scaled down to a manageable level via a resistive voltage divider. I used four high-precision 300kOhm resistors in series, plus a 1kOhm resistor placed across the AMC1200’s input terminals. Since the input is floating thanks to galvanic isolation, I decided to place the amplifier’s input in the middle of the voltage divider (that is, 600kOhm of resistance is present from the neutral and line terminals) to provide some extra protection from harmful electric shock; 120 Vrms / 600 kOhm = 0.2 mA to ground is the maximum amount of current that could possibly flow if I were to contact this floating node on the amplifier (this calculation assumes that my body has zero resistance, but human skin resistance is generally much higher than this). The voltage divider and the AC input terminals of my waveform viewer are further insulated with a layer of clear epoxy for even more protection.

The power supply terminals are fused with a 500 mA fuse before being protected by an SMAJ5.0A TVS (transient voltage suppression) diode and filtered with a 22 uF tantalum capacitor. The AMC1200’s output terminals are protected with 5.1 V Zener diodes at the terminal blocks for ESD and overvoltage protection.

Due to the floating nature of the waveform viewer, this essentially is a differential probe for my oscilloscope (and most high-voltage differential probes actually aren’t isolated!).

Circuit Limitations

No circuit is perfect, and mine is no exception. Here’s a few issues with my circuit that I’d like to address:

Isolation limitations

The AMC1200 only provides “basic insulation“; that is, it will provide protection from electric shock as long as its insulation barrier is not damaged (in other words, there is no redundancy). Circuits that have terminals that can be directly touched by humans needs “reinforced” or “double” insulation to be compliant with international regulations.

The NXE11S0505MC isolated DC-DC converter has a maximum working voltage of 125 Vrms for reinforced insulation and 250 Vrms for basic insulation, with a Hi-Pot test at 3 kVDC. This is lower than the AMC1200’s maximum voltage of 4000 volts, but these should still have enough headroom to keep me safe in the event of a mild voltage spike. It might prove useful to add some sort of surge suppression with a MOV (metal oxide varistor) or similar device.

The perfboard layout is also sub-optimal for the sake of isolation. Despite drilling out a row of holes to increase the creepage and clearance distances, it isn’t quite enough to meet regulations, as the clearance is only 3 mm and the the creepage isn’t much better, around 4 mm. This is still more than enough to withstand normal AC line voltages, but there is always a chance that higher-voltage transients will make their way onto the line and the isolation barrier needs to take this into account.

Output limitations

The AMC1200 provides a differential output that is centred (common-mode voltage) at 2.5 V, which can be an issue with single-ended inputs like that on an oscilloscope. I’ve worked around this by using a floating power supply, like a USB power bank, and connecting the oscilloscope’s ground terminal to the AMC1200’s Vout- pin. Also, the AMC1200 has a limited bandwidth of 60-100 kHz, but for the purposes of waveform monitoring it is sufficient; however, the amplifier’s noise and offset also negatively impacts performance as the high attenuation ratio essentially amplifies these values to the point where the AC waveform looks like a 2 Vdc offset and the noise level is so high that I need to use the averaging or high-resolution acquisition modes on an oscilloscope to get a clean waveform.

Power supply limitations

The NXE1 isn’t quite suited for such a low-power task as operating a single amplifier input. According to the datasheet, the output voltage can rise to twice the rated voltage if it is loaded with less than 20 mA. To combat this, I placed a 5.1 volt Zener diode across the output to provide regulation, which unnecessarily wastes power. Another regulated module like the NXF1 series would be a better choice, and the unit cost at one-off quantities isn’t a huge deal either.

Room for improvement

With this circuit working properly, I had plenty of ideas to make the second iteration even better:

Simultaneous voltage/current inputs

With the ability to measure current, I can perform measurements on the current draw of a device, allowing me to determine the power factor of a device.

True single-ended outputs

Most ground-referenced devices like oscilloscopes are not meant to handle differential inputs directly. Multimeters, especially battery-powered ones, are an exception.

Reinforced insulation rating on amplifier

The AMC1200 is only rated for basic insulation, so having an amplifier rated for reinforced insulation would provide greater electric shock protection. Alternatives like the Silicon Labs Si8920 could be a viable solution.

Waveform captures

 

 

Conclusion

Despite its ubiquity, AC power is a force that must not be taken lightly. Performing measurements on it, especially when viewing its waveform on a non-isolated oscilloscope, requires extreme caution as line voltage (especially in countries where 230 V is common) can easily injure or kill.

Using a voltage divider and isolation amplifier allows for safer measurements of the AC line without introducing distortion, especially compared to transformer-based implementations; this is critical when measuring the waveforms of modified sine wave inverters.

My implementation of an isolated differential probe helps protect me from electric shock when making measurements, while costing much less than a commercial high-voltage differential probe (for example, the CT2593-1, costs almost $330 USD on DigiKey).

But… which one would you trust more?

eMMC Adventures, Episode 3: Building a custom adapter to use cheap eMMC-based 32GB SSD modules

As seen on Hackaday!

While on my quest for more eMMC-based storage devices, I stumbled upon a few devices that piqued my interest: eMMC-based SATA SSDs! I found two models of particular interest: Dell had M.2 modules with a 2.5″ adapter, and HP had custom boards intended for use in cheap laptops (for example, the HP 14-an012nr). Although the former was easier for me to use (but not acquire), I will be focusing on the latter in this blog post.

Overview of HP 14-am/14-an Series SSD Module

Unlike Dell’s convenient M.2 modules, the cheaper boards from HP (costing about $12 USD when I purchased them) had a physical interface intended for use only with its intended host; despite using a SATA interface, physically it used a 10-pin FFC (Flat Flexible Connector, aka “ribbon cable”) since it was designed to work only with HP’s 14-am/14-an series of  low-cost laptops. The boards are labeled “DINERAMD-6050A2862201-DB-A01” and have a copyright date of 2016 in my case.

The BayHub OZ788WR2 Bridge Chip

These eMMC-based SSDs use a curious little chip, the BayHub OZ788WR2 (labeled 788WR2A on the chip itself). It is an SD/MMC-to-SATA adapter, with an SD UHS-II/MMCplus HS200 device interface and SATA II 3Gbps host interface. Apart from the brief description from the manufacturer, no other data is available for the chip (and even finding the chip online is basically impossible).

It’s a shame that so little is known about this chip (and that it’s so rare to find in actual devices), especially since high-performance SD-to-SATA adapters otherwise do not exist, as they use outdated SD-to-CompactFlash adapter chips that are limited to 25 MB/s speeds. If I had the engineering expertise, time, money, and ability to acquire these chips, I’d totally try to make an SD-to-SATA adapter with this chip… but alas, that will still remain a fantasy.

Step 1: Pinout Discovery

The single connector on the eMMC SSD is a ZIF FFC (Zero Insertion Force, Flat Flexible Connector), with no publicly available pinout or any other information. Perhaps this was why I got them for so cheap – apart from holding only 32 GB, nobody could even use them in their own computer even if they wanted to!

When trying to reverse engineer an unknown connector pinout, one needs to first look for ground pins. This is easily accomplished by using a multimeter with a continuity or diode test function, with the multimeter’s positive lead on a known ground point on the DUT (Device Under Test) – screw holes are often good candidates to look for. Ground pins will read as a short, but active IC and power pins will look like a forward-biased diode – appropriately 0.5 to 1 volt. I found 3 power pins (these are often grouped together on connectors for greater current capacity), 3 ground pins, and 4 SATA data pins. The data pins don’t show up on the multimeter test since they have series AC coupling capacitors, but they are easily located next to the connector and have clearly visible differential pairs leading to them.

The issue now is trying to find what order the SATA data pins are in, and how they relate to a regular SATA interface. As it turns out, the pinout is very simple: it matches the pinout of the 7-pin regular SATA interface! This makes sense as the SSD module and the laptop itself are designed to be cheap to manufacture.

Step 2: Building the Adapter (Take 1)

With the pinout known, the harder part is wiring up the connector. However, without a matching connector for the ribbon cable, I have no choice but to solder to it.

As I soon learned, not all flex cables are made of heat-resistant polyimide (aka Kapton) – this one melted before I could even tin the exposed leads. No matter, I’ll just use my trusty magnet wire and hook up the data and power lines! With the help of a salvaged SATA connector from a dead laptop drive, I was able to cobble together a crude adapter for the eMMC SSD board.

Although I didn’t end up taking a picture of the adapter, it wasn’t pretty. It also wasn’t very functional either – although the eMMC SSD board was able to identify itself (on my PC it showed up as a “BHT WR202HH032G E70211F5”), I couldn’t actually perform any data transfer without causing the OZ788WR2 to log hundreds of interface checksum failures (but hey, it supports S.M.A.R.T. data reporting!).

After some tweaking of the wire spacing, I was able to get the adapter stable enough to work, and encased it in hot glue for protection. It lasted a few weeks but eventually stopped working because one of the data wires broke off inside the blob of hot glue. Additionally, the outer contacts on the ribbon cable connector were peeling away from its plastic substrate. It was time for a rebuild.

Step 3: Building a Dedicated eMMC SSD (the teaser!)

Since I had multiple eMMC SSD boards, I took one, replaced the eMMC with a 128GB one from Samsung (the KLMDG8JENB-B041) and removed the ribbon cable connector. In its place, I used some very thin twinaxial cable from a dead MacBook and used a gutted CFast-to-SATA adapter for a shell. Stay tuned for that in another blog post!

Step 4: Building the Adapter (again!)

Much like my previous attempt, I used a salvaged PCB from a dead laptop drive, but left a lot of it instead of chopping it off directly at the connector. This particular one was a dead Samsung HDD, and it had one particular feature that I could use to make a stronger adapter: it had a TSOP footprint for the DRAM cache, which was just the right pitch for me to solder the ribbon cable to!

With a little help of my hot air rework station, I removed the DRAM cache and DC-DC converter, leaving the SATA AC coupling capacitors and the power input components (filtering choke and capacitors, and input overvoltage protection) behind.

After scraping off some solder mask, I soldered the SATA data wires and the ground wires surrounding them with very thin magnet wire, trying to keep the data pairs as close to each other as possible to minimize the chance of interference causing problems. The power wire was soldered to the power input components, right next to the input capacitor for better power delivery.

After checking with the multimeter that no short circuits were present, I hooked up an eMMC board and plugged it into my PC. It enumerated without issues, and running several tests including CrystalDiskMark, h2testw, and Hard Disk Sentinel’s random read test, amassing several hundreds of gigabytes in reads and writes with zero CRC errors logged in the S.M.A.R.T. data.

With everything checked out, I cleaned the circuit with isopropyl alcohol and covered the exposed end of the ribbon cable and the magnet wires with clear epoxy for protection. I also used a bit of epoxy on the flex connector to re-secure the lifted contacts to the substrate.

Conclusion

With a bit of wire and a circuit board from a dead HDD, I was able to reuse cheap eMMC-based SATA SSDs on computers that they weren’t meant for (and they even had copies of Windows 10 Home with extractable license keys! 🙂 ). Although not as fast as a modern full-fledged SSD, its relatively high 4K IOPS performance means it works well enough as a quick boot drive for running quick tests of OS installation without needing to sacrifice a bigger drive just for testing – and they consume less than a watt even when fully active!