EEVblog 1482 - Mains Capacitor Zener Regulator Circuit

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.

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#ElectronicsCreators #Tutorial #Zener

[derekloudon]

Hi Dave, these brief 'explanation' videos are a very useful resource for 'faded memory' refreshers (I'm a long retired engineer). Thank you ☺️

[padmasaripalli]

Great explanation of how zener based Derivation of DC from AC mains works. It saved a lot of angst for me. I changed caps and zeners to no avail. now I see the problem is with capacitor on AC input

[Brfff]

I wish we had your YouTube videos back during my university engineering days in the 90s - thanks!

[KeritechElectronics]

The Zener regulator makes perfect sense with the series resistance impedance on the AC side and low current loads.
Good that you give the safety warnings, they're absolutely essential with this type of circuits.

BTW it looks like I'm starting to teach people electronics.
Ttoday at our local hackerspace I had to explain the FULL BRIDGE RECTIFIER's principle of operation to one guy who has a university degree in IT but was not taught the basics of EE.

He had a pair of IN-12B Nixie tubes that he wanted to use in a project, only didn't know how to wire them... poor thing. I breadboarded a simple 200VDC PSU with two power transformers connected back to back for galvanic separation from mains, and taught him how to control these nixies using transistors.

[CaspaB]

Thanks Dave. I've been perusing the comments and realise how much learning can be got from quite simple circuits such as this.

[scottpelletier]

Very cool. I had an LED bulb go out so I smashed the power supply out of it to see how it worked. This pretty much looks exactly like what I saw... Simple rectifier spitting 50 to 60 volts DC out to run a panel of 50 little LEDs. Great explainer!

[petehiggins]

This capacitance degradation in 'Polly put the kettle on' capacitors is a well known phenomena but doesn't seem to be acknowledged by the manufacturers. I've had several small electrical devices like coffee machines and mains timers fail due to this effect. It would be interesting to set up a test to count the number of breakdown events occurring and the rate of capacitance reduction. I suspect it's mainly a problem in 230V countries where many areas, like mine (and BigClive's), still operate at 240V - 250V or maybe it's a function of how clean the mains supply is. It doesn't appear to be a problem on DC supplies, spacecraft use these capacitors to filter the main DC bus and they work happily for a couple of decades. Having said that they are voltage derated by at least 50%. I would do the test myself but I don't have an oscilloscope which would be necessary to establish the size of the current pulses.

[TheCodr]

Love these whiteboard videos Dave. By far the best series for me

[tlhIngan]

While capacitive droppers make poor power factors it really helps the power company. Most load anywhere is generally inductive - think electric motors used for practically everything in industry and residential. Fans used in ventilation, or motors driving compressors in HVAC and refrigeration or motors pumping liquids and such. Thus power companies know the power factor is generally inductive and they have to add capacitive reactance to balance it. There are capacitor banks on distribution lines and at substations to compensate for the inductive loads. You using something with a capacitive dropper helps bring the inductive load down a bit. Though modern LED lights are back to being resistive loads as they are using linear regulation driver ICs. But since the LED array drops about 90% of the line voltage the linear regulator doesn't waste much power. So I wouldn't worry too much about capacitive droppers at all.

[wurl]

Hi Dave. Actually I think that the cuircuit with the two zener-diodes in series is a quite smart solution.
Just imagine the case of a 3.3V linear regulator hanging on the 24V rail; that would mean, in addition to the relay current comes the control logic current: a cheap uC + temp sensor + some LEDs, might sum up to additional 15mA; meaning you need twice the input capacitance and have twice the power drop on the 24V zener.
Compared to that, the shown configuration provides the current for the 3.3V rail almost for free.
I guess, the relay coil version is selected to match the control logic current consumption, in order to get the total component cost down.
BTW: greetings from Germany. I like your channel a lot.

[CaspaB]

Hi Dave.
A trick with this circuit is to select the upper electrolytic value so that when fully charged it will reliably close the relay, but set the quiescent current (set by the 0.22 UF cap) to a value which will still hold the relay in. With the spec sheet you showed, the relay should hold in easily at half the voltage and the cap could be reduced to 0.1 UF, so long as the upper electrolytic cap was sufficiently charged up by the time the "start" signal arrived.

[tomasbergh]

Finally some nerdy electrovideo! By the way, the zener needs to be able to handle the power of 24v 16mA when the relay is not activated. Hope you are doing fine Dave! Thanks! /Tomas

[veganath]

Wow! That I found this in it's entirety intriguing speaks volumes of you terrific delivery, thanks Dave

[TheHuesSciTech]

Worth calling out that the strange-looking stacking of 24V and 3V3 kinda allows the microcontroller to be powered "for free"-ish in some vague sense. I.e., a 240V AC mains + dropper cap = a high compliance, practically-fixed-current AC current source; if you were to run the MCU off a LDO off the 24V, then the current for the MCU and the current for the relay coil would be added up. But with those elements in series, it's only the compliance voltage of the current source that needs to be increased, and since it's already at 240V, it's all good to go.

[billharris]

Hey Dave, I have seen these crude line capacitor dropper power supplies used since the early 1990's. I worked for a company that used these circuits in LED bulbs but, without the surge resistor. I modified the circuit by inserting a 100 ohm pulse rated resistor (US, 120 Vac), which improved reliability and power factor. These circuits are very vulnerable to voltage spikes and outputs from triac dimmers and squarewave/modified power inverters. The circuit you showed has a worst case surge current of 6.6 amps, assuming a clean sinewave. That particular circuit was likely designed by an engineer, then redesigned by management to further lower costs, then redesigned again by manufacturing who would only allow 10 reels of parts to be used on their pick & place machine. If the surge resistor was pulse rated, the redesigns probably changed it to a 1206 thick film chip resistor to reduce cost.

Of course, who cares about ripple on the regulated supply lines or voltage drift over temperature. These zener voltages are horribly unstable over temp. Also, placing a filter capacitor in parallel with a zener feed from a rectifier, can never be free of ripple (I wonder how the microcontroller reacts to that?).

I am assuming this circuit was powering a digital thermostat for a low cost space heater? With this circuit, it no longer replicates the mechanical thermostat it is replacing because, it will no longer run off DC (assuming these was no fan being used).

This circuit has horrible power factor as you mentioned, although would be insignificant when the heating element is energized. The power factor of the circuit is usually overlooked due to the low power draw but, becomes a problem due to the amount of power line harmonics it generates and resulting power loss at the power plant, which is roughly 7 times the power as measured at the device itself.

The power factor problem was recognized in the 1990's and since then, home appliances drawing greater than 50 watts employ some type of power factor correction. The worst offenders now are induction motors but, they have been redesigned as well such that the worst power factor I have witnessed lately is 0.85. Additionally, most all electronics use an off-line-switcher instead of a step-down 50/60 Hz power transformer. So, the mains are far less inductive these days.

If your house is powered from a simple 2 or 4 pole generator, the power factor can quickly consume available generating capacity. Let's assume you are charging a solar battery bank from your generator. The difference in power factor between battery chargers that have 0.5 versus 0.9 is huge, with the 0.5 needing almost twice the generator capacity.

[MrDoneboy]

Great explanation, Dave about the loss of capacitance, not creating enough voltage, which caused the problems further down the circuit. Hope that made sense.

[arvydasn]

Great timing with this content. I was just looking into my failed mains ac power meter. It seems to use this to power dc circuit and voltage is way to low.
So it seems you just pointed me to the culprit (ac capacitor). All other possible sections were testing as good.

[kiltrash]

In some of these types of circuit the zener only limits the maximum supply voltage if the load drops, rather than to regulate the supply voltage. For example, the zener in our fan controller limits the supply rail to +12V whereas the normal running voltage is about 5½V.

[MrDoneboy]

"Come a Gutsa". Gotta love Dave's dielect! Lol.

[MyProjectBoxChannel]

big Clive done a similar failure mode video. There was a capacitor in a cap drop circuit that lost capacitance over time. I think it was the power supply for the clock inside a oven. there was also a relay circuit, the relay wouldn't operate due to a cap dropper losing its capacitance over time.