Author Topic: 5V Zener Regulated 5V from 240V mains, Why Blow Up?  (Read 6915 times)

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Offline amyk

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #25 on: September 25, 2017, 04:12:07 am »
i've seen china circuit using only one zener succesfully, this is my inspiration of very few components hence cheap china style... see attached of the other board i teared apart few weeks ago, its a drybox machine.. i havent pull it out of the casing so i cant trace the circuit, only the top board 1 zener visible (through hole technology) so i suspect nothing smd or whatsoever is underside... edit: err you are right 2 zeners there, i will consider this, but still i cant explain the blow up...
That picture you posted is a capacitive dropper. See the big blue cap?
 
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Online wraper

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #26 on: September 25, 2017, 04:21:52 pm »
Did the Helping Hand short anything?
its holding the mains cord. The circuit is floating on the air by the cord's stiffness.. if helping hand was to short anything,it will be the cord's core copper strand..

Wow, I'll try again

Mecha, get rid of the ground pour around the 240VAC mains part of the cct
its not a ground pour.. its the floating, or no net pour.. just to save my ferric chloride,and possibly as a place to put additional component if modification is needed. Do you mind sharing your experience?
You have almost no isolation between that pour and mains voltage, no wonder it blew up. It actually shorted even before the fuse. Hint: did you ever see such pours in commercially made mains powered things?
 

Offline Loboscope

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #27 on: September 25, 2017, 08:55:09 pm »
its not a ground pour.. its the floating, or no net pour.. just to save my ferric chloride, ...

Don´t save your ferric chloride, save your health (if not your life!).
There must always be enough space between the traces conducting mains and the other traces/pour. I would not hesitate to add some extra gaps like the safety-gaps found in DMM´s.

But like others told you already I would also prefer to use l little transformer. Tiny, cheap and healthful  :-+.
 

Offline MechatrommerTopic starter

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #28 on: September 26, 2017, 12:00:17 am »
Thanks guys, i'm going as amyk suggested.. googling 'capacitive dropper' led me to interesting info esp one of the old thread over here with link to microchip app note.. it turned out what i'm trying to invent is a transformerless psu resistive dropper.one prominent drawback is excessive power waste in resistive dropper (as i already noted) , so not good. Rather than try to rectify whats wrong and potentially blowing another zener, but still stuck with less efficient circuit.. i'm drawing another pcb for capacitive dropper. My test last night showed the 180kohm resistor i was using is ntc, ie lower resistor when get heated, so it can be the worsening of the earlier problem, beside creepage and clearance that i will also take into account in the new pcb.. i'll post for advice before making pcb when i complete drawing the new circuit..
« Last Edit: September 26, 2017, 12:02:33 am by Mechatrommer »
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline kalel

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #29 on: September 26, 2017, 01:11:17 am »
I heard about capacitive droppers from BigClive's LED videos. Maybe you could take a look at some if you're interested, but... it's not for powering MCUs (maybe in some case).


It seems that some lamps even use a capacitive dropper for power bank-like USB output! So, while charging, the USB is live.


This is the problem of resistive and capacitive droppers (from what I understand, I am by no means experienced or knowledgeable in the field), that they are not isolated from the mains, so there is a safety risk for products that are not fully enclosed. For a lamp in a fully covered case, I guess this is not an issue.



 

Offline MechatrommerTopic starter

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #30 on: September 26, 2017, 01:22:59 am »
Non isolated circuit is a big no no when user interfaces or have access to it.. my project is fully encapsulated,except a potentiometer that user will operate by the plastic knob. The potentiometer exterior metals are not in contact with any node in the circuit,thats why i think its ok for my purpose.. with usb charger, there is possibility of exposure such as on iphone usb cord charger plug..
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline kalel

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #31 on: September 26, 2017, 02:05:36 am »
Quote from: Mechatrommer on Today at 11:22:59 AM
Non isolated circuit is a big no no when user interfaces or have access to it.. my project is fully encapsulated,except a potentiometer that user will operate by the plastic knob. The potentiometer exterior metals are not in contact with any node in the circuit,thats why i think its ok for my purpose.. with usb charger, there is possibility of exposure such as on iphone usb cord charger plug..


Yes, for USB charger it is not acceptable.

Just a thought, not a suggestion, but probably the cheapest solution from China is not (too) much more expensive if you could work with 5v from a cheap USB charger or module (charger is enclosed). Of course I can't really recommend them either - obviously it's not high quality or proven to be safe.
 

Offline MechatrommerTopic starter

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #32 on: September 26, 2017, 02:21:42 am »
anyway transformerless psu only for low power in mA range. modern usb charger can charge up to 1-5A. transformerless psu will become less economic in that range, hence we commonly see smps transformer psu for them, i havent seen non-isolated battery/usb charger.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline thermistor-guy

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #33 on: September 26, 2017, 02:28:09 am »
I have seen capacitive dropper circuits implemented in mains electricity meters, to power a small MCU inside the meter.

The circuit used a self-healing metallized film capacitor. When the capacitor get hits with brief high-voltage transients on the mains, the capacitor can break down for a short time; metal film near the breakdown location vaporizes; but the fault clears. The capacitor keeps working, but its capacitance now is slightly less.

After a few years of exposure to such transients, the capacitor's capacitance value can drop to a point where it no longer provides enough current for the circuit it powers. That's what happened to this particular batch of meters - after a few years of service, the MCU wouldn't operate reliably.

If you feed mains power to the zener using a resistor, you still have to keep these transients in mind, and design accordingly. Some resistors will seem to go short-circuit during kV level transients, due to the insulation on the resistor body breaking down. Under these conditions, the nominal high value of resistance is no longer in effect. Your circuit no longer has a simple resistor.

The approach I have seen in some telecommunications power supplies is to use two long (eg. 1/2 inch) power resistors in series for the resistive feed. The resistors were chosen not for the power rating but for the body length. The increased body length helped the resistors avoid breakdown under commonly occurring transient conditions (and also reduced sneak paths due to stray capacitance). Other posters have mentioned multiple resistors in series.

240 V mains isn't just 240 V. It has all sorts of transients on it, like lightning strikes and near-strikes, and switching transients from heavy machinery located in your area. If you want your circuit to last a few years, think about what voltage and energy spikes you want it to withstand. Keep in mind that a capacitive dropper may have a wear-out mechanism, as described.
« Last Edit: September 26, 2017, 02:30:17 am by thermistor-guy »
 

Offline ludzinc

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #34 on: September 26, 2017, 04:48:34 am »
 But with your home etched PCB I'd bet you have small copper deposits between traces.

At low voltages, it's not an issue.

At Mains voltages, it's conductive enough to be an issue. 

Use more clearance.  Don't want to etch it off?  Peel it off then!
 

Offline MechatrommerTopic starter

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #35 on: September 26, 2017, 05:52:13 am »
this is the capacitive dropper circuit i'm currently working on (attached).. reference: http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf i just swap few components to the top side, easier for me to visualize gnd node mentally...
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Online Brumby

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #36 on: September 26, 2017, 06:06:21 am »
Whatever solution you go for - whether resistive, capacitive or a toroidal transformer --- as others have said ... MAKE SURE you get decent clearances around the mains input!
 

Online wraper

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #37 on: September 26, 2017, 06:18:33 am »
this is the capacitive dropper circuit i'm currently working on (attached).. reference: http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf i just swap few components to the top side, easier for me to visualize gnd node mentally...
That circuit does not have bleeding resistor (likely present somewhere else in the circuit or has other means of discharging). When you remove it from mains socket and touch the plug, it will hit you with electricity stored in capacitor.
 

Offline Ice-Tea

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #38 on: September 26, 2017, 06:28:52 am »
Seems folks missed the problem in the OP? 1/4W resistor blowing something like 300mW?

Offline woody

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #39 on: September 26, 2017, 08:03:49 am »
I used to muck around with this problem a lot. How to get 5V or 3.3V from mains in a simple and safe way. My latest answer to that:

http://nl.farnell.com/vigortronix/vtx-214-001-305/power-supply-ac-dc-5v-0-2a/dp/2775086

Small, simple, safe and cheap. Wide VACin.  No worries if my widget uses 1, 5, 10 or 200 mA. And if I ever design something that needs to be rock-bottom cheap because I need to make a million of 'm I'll pay someone else to design a safe power supply  ;)
 

Offline MechatrommerTopic starter

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #40 on: September 26, 2017, 08:13:32 am »
MAKE SURE you get decent clearances around the mains input!
yup, i heard this since the beginning, i will implement this in the next pcb, thanks.

this is the capacitive dropper circuit i'm currently working on (attached).. reference: http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf i just swap few components to the top side, easier for me to visualize gnd node mentally...
That circuit does not have bleeding resistor (likely present somewhere else in the circuit or has other means of discharging). When you remove it from mains socket and touch the plug, it will hit you with electricity stored in capacitor.
thanks for reminder, i saw it last night on the other website i just forgotten it when drawing schematics, i will add the bleeding resistor.

Seems folks missed the problem in the OP? 1/4W resistor blowing something like 300mW?
yes this is my highly suspicion, as the resistor heated up, it went lower resistance, more and more current flowing some sort of thermal runaway until the things are blown. beside the other possibility of too narrow clearance and the floating copper pour got shorted due to unclean creepage.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline woody

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #41 on: September 26, 2017, 09:18:10 am »
Seems folks missed the problem in the OP? 1/4W resistor blowing something like 300mW?
yes this is my highly suspicion, as the resistor heated up, it went lower resistance, more and more current flowing some sort of thermal runaway until the things are blown. beside the other possibility of too narrow clearance and the floating copper pour got shorted due to unclean creepage.

I might be completely wrong here but then I'll learn something. Is it not so that the .25W resistor behind the diode blows .5* ((240V/220k)^2 *220k) = .13W?

Apart from that, a moment ago I fried a 160k resistor between the leads of my meter, and it went from 159.3k cold to 158.0k when the paint came off so I do not see the lower resistance thing causing this.

Looking at the state of the PCB I would guess that a short is the most likely reason for this to happen. Or else the fact that where most .25W resistors are 250V maybe this Chinese one was a bit lower.
 

Offline LukeW

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #42 on: September 26, 2017, 11:23:02 am »
Read Microchip AN954 on Transformerless Power Supplies.

http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf

You can only get very small load current... you will get a lot of power dissipation in the resistor and it will rapidly become impractical as the load current goes up.

You may also consider an offline buck regulator IC such as Power Integrations LinkSwitch-TN series. This won't give you galvanic isolation, just like a capacitive dropper does not, but you'll get less heat, more efficiency and more output current.

http://www.es.co.th/Schemetic/PDF/LNK304-306.PDF
 

Offline Ice-Tea

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #43 on: September 26, 2017, 11:29:31 am »
You are right and wrong: wrong because it's 180k, not 220k. So, I calculated 0,32W. However, I was wrong also: it's not a full wave rectifier, so only half that. A 250mW resistor should be fine (but quite hot) unless it's a terrible Hung Wong Do resistor ;)

Offline woody

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #44 on: September 26, 2017, 11:43:15 am »
Ah, I see. I used the 220k resistor as shown in C1 but the schematic in the OP uses the 180k  you mentioned. :D
 

Offline orolo

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #45 on: September 26, 2017, 12:39:00 pm »
I know this is slightly off-topic, but after experimenting for a while with capacitive supplies and the such, I really don't find any redeeming value in them except for maybe weight. Consider a tiny epoxy encapsulated transformer like this (first search result). It's dirt cheap (0.9-1.6$), offers much more current than a capacitive dropper, provides galvanic isolation, much lesser ripple and is roughly equivalent in  power waste. Even if the transformer costs more than the dropping capacitor, the safety (no double encapsulation) and lifetime more than offset the additional cost.

Is there any advantage in capacitive droppers excepting some weight, a little volume, and maybe the chance to use the cheapest mains rated capacitors around?

Edit: to clarify, I mean for low voltage regulation. It can be argued that, for long LED strings and such, a capacitive dropper is a sensible option.
« Last Edit: September 26, 2017, 12:41:47 pm by orolo »
 

Offline xani

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Re: 5V Zener Regulated 5V from 240V mains, Why Blow Up?
« Reply #46 on: September 26, 2017, 01:29:29 pm »
The only advantage is slightly lower price (and size I guess) which is why it is so often used in cheap chinese stuff. I see no reason to bother with it for one-off hobbyist projects or small runs

Edit: to clarify, I mean for low voltage regulation. It can be argued that, for long LED strings and such, a capacitive dropper is a sensible option.

IIRC from bigclive videos some even just put resistor in series, but those had enough LEDs in series that resistor only dropped few volts
 


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