Normally, when you want a low DC voltage from the AC line, you think about using a transformer of some kind. [RCD66] noticed that an AC monitor meter must have some sort of power supply but had no transformers in sight. That led to an exploration of how those work and how you can use them, too. You can watch the work in the video below.
Sensibly, there is a transformer in the test setup — an isolation transformer to make it safe to probe the circuit. But there’s no transformer providing voltage changes. Isolation is important even if you are taking apart something commercial that might be trasformerless.
The circuit is simple enough: it uses a capacitor, a resistor, and a pair of diodes (one of them a zener diode). He uses this basic circuit to drive simple regulators with input and output filter capacitors. We’ve seen many variations on this design over the years.
You can’t draw a lot of power through this arrangement. But sometimes it is all you need. However, this is pretty dangerous, as we’ve discussed before. Be sure you understand exactly what the risks are before you decide to build something like this.
Why does a transformer count as isolation, but not a capacitor? Both are open circuits to DC, and both have the “dangerous” side and the “safe” side directly adjacent to each other in the same package, separated only by a thin layer of insulation. Do transformers tend to fail open while capacitors tend to fail closed, or does the big iron just provide an illusion of safety?
The capacitor only acts as a current limiting impedance. The current flowing through the capacitor is still ground referenced – i.e. the current to the source can return through any grounded conductor – including the human body. This is particularly a problem if the plug gets reversed and the capacitor ends up on the neutral instead of the HOT.
Whereas, the transformer removes the reference to ground and prevents the return current flowing through your body. Note: “Not all transformers”, an “auto” transformer (like nearly all variacs) does not provide ground isolation.
To be honest, I can’t remember the last time I saw a reversible plug. All the ones I’ve seen in the past 10+ years only fit one way.
The main problem is that there’s no shortage of outlets that are wired wrong. I moved into a recently-renovated home and found one. Moved again a couple of years later and found one again. Sometimes, it’s a contractor who gets sloppy doing the hundredth outlet in the home. Other times, it’s the homeowner DIYing a replacement.
It’s especially silly when you get a 120V two-wire line cord with a C1 or C7 (reversible, non-polarized) two-prong connector on one end, but still has the polarized blades on the wall plug end.
On appliances where polarization doesn’t matter (i.e., most of them) I often grind off the fatter blade on two-wire cords just to avoid the aggravation.
Anything with switch or fuse only in the hot wire keeps the polarization, of course. As does anything with an edison socket.
Because without isolation the return side of the circuit is the ground you are standing on.
But that’s what I’m saying: the capacitor is the isolation.
The main reason is transformer output is not ground referenced.
You could lick a single output from the secondary of a transformer standing in a bath tub. You really shouldn’t of course.
But the capacitor means current is flowing through you cause it’s AC.
If the capacitor is the isolation how do you get current flowing on the other side of it in this circuit? A capcitor only blocks DC, and as PaulB points out in this AC mains circuit “The capacitor only acts as a current limiting impedance”. If this circuit is good for 25mA (through the capactitor) then that’s up to 120V, 25mA that could happily include you in the circuit which is more than enough to be dangerous (I know this is a gross over simplification before anyone points it out).
I recognise I might have just restated the issue. Consider this.
Let’s make a capacitive dropper to output 5Vrms from 230Vrms.
We wanted 23mA, so we need 10k ohm impedence. That meals the capacitors reactance must be 220Ohm.
If the circuit fails with the capacitor lead out you might think I’m isolated it’s fine.
But definitely do not lick that capacitor lead standing in a bathtub!
Would two capacitors, one on each side, create isolation then? Like in a balanced arrangement.
What if the return side has a capacitor in the path too?
One of the stupidest things i’ve done when experimenting with electricity was playing with transformer-less supply design found on internet. It was that time that i had couple of first successful electronic designs nad prototypes behind me and i was geting little bit too confident. With my fingers i was touching 12V rail provided by similar capacitive supply, completely oblivious to the risks involved. Only reason i haven’t received any shock was that i was working at the lab bench that had its mains outlets protected using isolation transformer (effectively making NOT-transformer-less circuit). I’ve only realized my mistake couple days later. Lucky enough, the isolation transformer was installed like week before i’ve been doing the experiment.
Later i’ve started noticing these kinds of supplies being used in cheap chinese LED lightbulbs with exposed pads of individual LEDs and similar cheap stuff… The fact some circuit is providing “safe” voltage across LED does not mean it does not mean there is not an unsafe voltage between that very same LED and something else (lets say your laptop, radiator pipe, or any other circuit…)
Think thrice with this stuff guys.
Yeah I recently needed to disassemble my very old plug-in electric shaver and found a buck converter taking 120/240v to 6v dc to drive the motor. I looked up the part but for the life of me can’t find the number again. The circuit was almost exactly taken from the data sheet. I think it was rated 750ma. No heat sink. Probably 20+ years old and still working fine.
Most LED bulbs nowadays use buck converters or capacitive droppers for power due to cost. Very few still use switch mode supplies with transformers.
A lot of LED bulbs now use a linear regulator with a lot of LEDs in series so the regulator doesn’t have to drop much voltage. They are designed to have a good power factor, which is something you can’t get with a capacitive dropper.
Switching supplies are not necessarily isolated either.
As long as they’re are files and three prong adapters polarity is a crapshoot.