Designing and building equipment for aquaculture means I use controllers. Quite a few controllers, dozens upon dozens of them, little boxes meant to keep some parameter in range. Temperature, pH, water level, whatever, a little box with a display, a few buttons, and a relay in the back to turn something on when needed to control the outcome.
Industrial suppliers will sell you a controller for just about anything, there are catalogs full of them, from inexpensive to thousands of dollars you can buy the solution to your needs. Buy one, wire it in, adjust a few setpoints and you have everything under control.
Enter the STC-1000, a little cheap temperature controller found on eBay, Amazon, everywhere. It comes in a bunch of different versions, need readout in Farenheight or Centigrade? No problem. I have no idea who makes it, some asian factory somewhere. It is available in a hundred different brand names from hundreds of different sellers for somewhere between $12 and $25, all absolutely identical as far I as can tell.
The STC-1000 is cheap. Cheap enough that I am somewhat suspect of their reliability. I do not buy them for production line bioreactors and grow tanks, places where I cannot trust a cheap controller with a few thousand dollars worth of product. For critical uses I buy full industrial rated temperature controllers from a reputable supplier for around $100 each.
But for experimental setups? Temporary research hacks built with more limited budgets? There are a couple dozen of these STC-1000 controllers around the place. They are easy to use with simple configurations, seem to be accurate holding calibration, and I have not had one fail yet.
So how do you sell a device like this for about $15?
Time for a little deconstructive analysis…
The units are quite easy to open, just a couple small plastic clips keep the cover on, slip the tip of some tweezers underneath and the face just slides off. After that the circut board just slides out of the case and all of the unit’s secrets are available for examination.
The very first thing I note is the power supply, or the lack of one. The units run off of 120V, some sort of power supply is required to step that voltage down to the 5V the circuit and display runs on. There is no small transformer as I expected, and no offline switching power supply to convert the 120Vac to 5Vdc for the internal circuitry.
Otherwise there seems to be no real issues. General workmanship is good, the design seems to be well thought out with good PCB layout and design practice. The components seem to be of resonable quality, particularly the connectors at the back, often a weak point.
The main PCB is single sided, no surprise, single sided PCB’s are vastly cheaper than double sided. The small display board is double sided, no other way to cram the needed circuit into the small area. Under the display hides an unmarked SOIC-16 microcontroller or ASIC that makes the controller function. There is clearly a drive to be low cost here, the component count is as low as possible, the PCB has spaces for quite a few components that are not installed, removed in cost cutting?
Back to that power supply…
No input fuse, there is a narrowed section of PCB trace that looks like it may be a makeshift fuse. No EMI filter either, I wonder about radio frequency emissions from the unit. The power supply is extraordinarily simple, a couple diodes, one chip, an inductor and a couple capacitors. Looking at the circuit I guess it is some sort of buck regulator.
This power supply rings a few alarm bells in the back of my engineer’s mind.
A little internet research on the power supply chip shows my guess is correct, the chip is a 600V buck regulator designed for offline power supply applications. The reference circuit in the data sheet looks pretty much like what I see on the circuit board, without some of the suggested components used for regulatory compliance.
There are a few quick and dirty ways to drop 120Vac down to a low voltage, but there are issues when you do this, first and foremost being isolation, or rather lack of isolation. One of the reasons to use a transformer is that there is no actual connection from the high voltage side of the power supply to the low voltage side. This isolation is critical for safety as it prevents electrical shock.
My hand-drawn schematics show the usual isolated AC/DC switcher (top) and the buck converter (bottom) used in the temperature controller.
I left a great many details out in the sketch, and you may not know all of the symbols, but the important point is to note that in the lower circuit, the buck converter, one of the wires runs right from the input to the output.
You will also note that in the top circuit, a typical flyback converter, there are no wires that connect input to output, a transformer provides a barrier that transmits power without using a connection, rather power is transmitted through a magnetic field. This is the isolation barrier.
Without isolation some part of the internal circuitry will be connected directly to one of the incoming wires. Imagine if the line and neutral wires were flipped, this means that the “ground” for the circuitry inside the controller is actually 120Vac line voltage. This does not fry the internal circuit as the components inside the device wil only see the 5 volts, it is just that they ride atop the 120Vac. But, if you touch any part of the controller circuitry and some other ground in the area? You get shocked!
I did this… OK… Not the shocked part. I did flip the AC input wires so that AC hot or line was the wire going through, then used a volt meter to confirm that the entire internal circuit was hot. Meanwhile the controller was running happily, with the display showing temperature. Indeed there is no correct way to connect the AC lines, you can do it either way, accordingly there is no labeling indicating which of the AC input terminals should be line or neutral.
The lack of isolation is not an issue for many devices, in the regulations you can use a non-isolated supply as long as there are no exposed electrical connections on your unit. This STC-1000 does not comply with this. The case is light, has a lot of openings, and is very easily opened exposing the circuit within.
There is also the temperature sensor, a thermistor with a connector that the user is expected to attach. The thermistor wire leads can and will be hot! Fine if you use the supplied thermistor assembly, but the wire is quite light and easily damaged. For many applications it may be necessary to change the wiring arrangement for this sensor. Need to re-connectorize or splice the wire to get the sensor into a chamber or duct?
Yes, this is a wee safety hazard, one that can kill somebody.
I now wonder about the regulatory compliance of these controllers. At least one seller posts a CE certificate for the STC-1000. Is it really? Does it slide through as a non-consumer product? As an industrial component? The older units I have use a screw terminal block for the temp sensor, this was changed to an HX connector on more recent units. I now wonder about the reason for that change.
Given that I can buy a fully regulatory compliant, isolated power supply module for about $2 and change in small quanitity from a major distributor it cannot cost that much to put a proper power supply into these controllers. A supply that would fully meet regulatory requirements and not potentially expose 120 or 240Vac to the user. Open up any reputable industrial controller and you will see a proper power supply.
What do I do now? I have identified a nasty safety issue in a device we use in the facility, and we have a bunch of them, a couple dozen in use around salt water. Worse, those potentially hot temperature sensor leads are exposed at a terminal block where the temperature probes can be disconnected for service and cleaning.
For now I am converting the units we have to run on 12Vdc as there are 12V power supplies in the control cabinets where these temperature controllers are used. My rule is any electrical near salt water is 12Vdc whenever possible. Those supplies are fully regulatory compliant, and isolated. I just have to open the STC-1000, remove a couple components, and stick in a good ‘ole LM7805 voltage regulator. There is even a convenient place to do it, a spot for a through hole capacitor that was not installed. Problem solved in about 10 minutes.
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