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EEVblog 1482 - Mains Capacitor Zener Regulator Circuit
EEVblog:
A follow up to the previous video on repairing the heater.
A viewer asked how the capacitor diode rectifier gave a 24V output. The key is in the zener regulator, so this vidoe looks at how mains powered zener voltage regulators work, and their limitations.
X class capacitor and self healing.
Zener diode tutorial video:
Kleinstein:
The calculated 16 mA is the RMS current. The average current is a bit (some 20%) smaller.
With the 3.3 V zener in series the current for the µC and LEDs does not add to the current for the relay. It only adds to the voltage. So this is a clever (cost saving) design as with a capacitive dropper supply the current is limited, but a higher voltage come essentially for free. This is why they did choose the 24 V relay and not the 5 V version.
There is an even simpler version of the capacitive droper circuit, using only a zener and 1 extra diode for rectification. This version however uses only half the current and thus needs twice the capacitance. So it's only attractive for very low power. One sometimes still finds this even for a comparable power level - maybe tradition from old times when a diode was more expensive than a capacitor.
The reactive power is usually not a big issue. From mains transformers and induction motors the tendency is to have excess inductive load. So the capacitive load from the dropper supplies does not really hurt and oftzen acts as part of the power factor compensation. Most of the reactive power does not have to travel all the way to the generators. Even if it does the generators don't need much true power to provide the reactive power. Moving the reactive power through the grid adds to the grid loss - though not very much.
Peabody:
So the power used by such a circuit is constant. If the relay coil isn't energized, that current will instead flow through the zener. In fact the maximum zener current is when the load current is zero.
In my limited experience, when these circuits die, it has never been the dropping capacitor that went bad. It's always been the output smoothing capacitor. Probably because it's always an electrolytic. In theory they shouldn't fail - they're never exposed to more than 5VDC. You would think it would always be the cap that's exposed directly to the mains that fails. But in front of me is a Disney coffee mug warmer with this circuit, and the three I've bought have always failed after a couple years, and in all cases it was the electrolytic that failed - actually, very high ESR. No problems at all with the dropping capacitor. So far. Of course the electrolytics aren't exactly name brand parts, so that may be the explanation for my Disney experience. Mickey Mouse capacitors.
SeanB:
Lovely vidoe Dave. Seen a simpler circuit, for industrial logic where the designers decided to go even easier. They used a 555 timer, and use pin 3 to operate the relay, 150R coil, and also have a 180R 2W resistor to the other power rail. Set of pads which are drilled out during manufacture, and a second place for the resistor, so that you can decide if the relay is to be normally energised or normally deenergised on power application. Power on some is via a tiny mains transformer, 110VAC primary, or 24VAC primary on some, to a 12VAC secondary, and a simple bridge rectifier and 220uF 25V capacitor for smoothing. There are also 24VDC versions, where they use another 180R resistor as a dropper from 24VDC, with a single diode in the bridge to provide reverse polarity protection. Same PCB for all the dozen or so timer variants, you just cut tracks for relay operation, and use either transformer and bridge, or resistor and diode, as the pin spacing matches on all those options, and where you connect power.
Modules all date from the late 1970's, it was a job lot I got on auction for around $5, for around 500 modules, so they have been supplying all my relay needs for a long time, provided I can use the 12V or 24V relays, and get to look for whatever is needed, generally DPDT with 5A contact ratings, though some are SPDT as well. Just have to bear with the 150R coils on 12V, and 300R coils on 24V. A lot of vintage 555 and 741 IC's in there, all well aged, but pretty much all do work when tested. Even used some in industrial control as well, as they were around, and I had the right 8 pin or 11 pin bases.
SeanB:
--- Quote from: Peabody on June 24, 2022, 03:46:27 pm ---So the power used by such a circuit is constant. If the relay coil isn't energized, that current will instead flow through the zener. In fact the maximum zener current is when the load current is zero.
In my limited experience, when these circuits die, it has never been the dropping capacitor that went bad. It's always been the output smoothing capacitor. Probably because it's always an electrolytic. In theory they shouldn't fail - they're never exposed to more than 5VDC. You would think it would always be the cap that's exposed directly to the mains that fails. But in front of me is a Disney coffee mug warmer with this circuit, and the three I've bought have always failed after a couple years, and in all cases it was the electrolytic that failed - actually, very high ESR. No problems at all with the dropping capacitor. So far. Of course the electrolytics aren't exactly name brand parts, so that may be the explanation for my Disney experience. Mickey Mouse capacitors.
--- End quote ---
On 115VAC the dropper capacitor is very much under run voltage wise, so does not degrade from spikes much. The same is not true on 230VAC mains, where the capacitor has full working voltage applied, plus the transients are much higher in voltage. Yes that electrolytic does fail, though more often than not because it is overheated and cooked, probably by sitting next to a heater controlled by the circuit, so it will dry out. Better quality capacitor, and higher voltage rating, will help, but really it just needs to run cooler. If you put in say 1000uF 35V into the application Dave has, it will still turn on the relay even with the class X capacitor having self healed down to 50nF, simply because the 1000uF capacitor will charge up to 24VDC, and have enough stored energy to pull in the relay, which will hold in with much lower current, often a 24V relay will still hold closed on 6VDC across it, simply because of the closed magnetic circuit, and this often is exploited to reduce current draw, here because the relay will probably have 20VDC across it when closed, but still have full voltage to pull in before dropping down. The relay would likely have been pulling in with a bit of mains hum for a while before failing, not something you would notice unless you actually listened, but a fair indication it was losing pull in power.
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