5V Zener Regulated 5V from 240V mains, Why Blow Up?

[Mechatrommer]

i assembled the following circuit and blew up the traces, any good reason why? other than possibly my careless mistakes such as short circuited traces, leakage etc, but i checked for short between mains etc, everything were fine. from circuit point of view, i cant see a reason why it should blow up, even if damaged zener. i have 180Kohm resistor to 5V zener, that should limit the current to 330V/180Kohm = 1.8mA tops... although 0.6W flowing through the resistor, the resistor is doing fine (not blown), its the zener that died. simulation indicates everything works the way they should... but, i need confirmation before reassembling the same circuit and potentially blowing another zener...

[IanMacdonald]

Resistors do have voltage limits irrespective of dissipation, and 330v is a bit much for some of the smaller styles.

Also worth noting that you will get a  fair a mount of ripple with this arrangement since C1 can only charge to a 'hard limit' of the zener voltage. If a problem,  the best workaround is to use a higher voltage zener and postregulate.

[Mechatrommer]

Resistors do have voltage limits irrespective of dissipation, and 330v is a bit much for some of the smaller styles.

i'm using jelly bean china 1/4W through hole resistor. is there any issue?

Also worth noting that you will get a  fair a mount of ripple with this arrangement since C1 can only charge to a 'hard limit' of the zener voltage. If a problem,  the best workaround is to use a higher voltage zener and postregulate.

this is 2nd concern after i get this circuit working (not blown), with blown circuit, i dont have enough time to probe for undesirable ripple. in fact i'm not worry too much with ripple, the 5V will be used for powering non-critical non-precisian part, such as just powering a mcu...

[amyk]

I'd recommend using a capacitive dropper instead of a resistive one.

[Benta]

I'd recommend stopping this approach, otherwise you'll earn a Darwin Award.

[janoc]

This is 2nd concern after i get this circuit working (not blown), with blown circuit, i dont have enough time to probe for undesirable ripple. in fact i'm not worry too much with ripple, the 5V will be used for powering non-critical non-precisian part, such as just powering a mcu...

  Good luck debugging that. Excessive ripple will cause all sorts of issues with spurious signals.

[cdev]

This is a question, not an answer..

Since we're on the subject of zener diodes used for voltage regulation..

OTOH it is reasonable (right?) to use this voltage drop across a zener diode approach to get 3.3v or 5v at very low ma off of a regulated 12v or 9v rail, without needing another full blown voltage regulator, right?

How hot will it get? or would it? Would it get hotter than a linear voltage regulator or not? Both would emit heat, which one would emit less into a closed space..

Remember in my circuit, we're just taking about low voltage DC.  dropping 12v or 9v down to 5 or 3.3v

[Karlo_Moharic]

just use a regulator like everybody  else

[max666]

To elaborate on Benta's post. Your circuit is mains referenced, Mechatrommer. One careless swap of the mains leads or a breakdown of the likely underspeced resistor and you could easily kill yourself.

[wraper]

What was connected to this circuit? It should't blow up traces by itself. My guess is there was something connected which have current path to earth. I hope you did not attach an oscilloscope to it.

[wraper]

i'm using jelly bean china 1/4W through hole resistor. is there any issue?

Although it shouldn't blow up normally, they are rated for lower voltage. Higher power resistors usually have higher max voltage rating, should check a datasheet for particular resistor.

[SeanB]

Typically these use 2 zener diodes, the first being a higher voltage ( around 12V) with the mains smoothing capacitor across it, and then a second resistor to the 5V zener, so that it is fed with something approaching DC, which it then regulates down to 5V. Can then be used to supply a low current ( under 1mA with your values, and will still have some residual ripple on it along with a rather long ( as in around 10 mains cycles) start up voltage build up as the capacitor charges.

[Mechatrommer]

What was connected to this circuit? It should't blow up traces by itself. My guess is there was something connected which have current path to earth. I hope you did not attach an oscilloscope to it.

see attached, it was floating in the air held by the helping hand by the mains cord... nothing connected, just the mains cord to the circuit. i was just testing for circuit stability when connected to the mains, about a second later, boom! btw, i know how to probe this safely dont worry people, i got shocks since i'm very little i'm very well aware of the danger to the point of all my kids are too scary when come close to a naked wire sigh ... the circuit was not probed yet, as i said, only stability test...

attached is the circuit i'm working on. nevermind other components they are not populated yet... only fuse, D1, R3, D2 (zener), and C2 and the mains cord (2 wires cord) are populated. do i have with copper pour clearance issue? i dont thing so, since the blast points are along the path of D1,R3,D2 (see 4th picture, i reattached the broken traces ready for the next blast? but waiting for the counsel verdict here). only D2 zener is dead, D1 and R3 still ok... fwiw...

[Mechatrommer]

just use a regulator like everybody  else

with too many components involved if i'm going the normal regulator route... then the china wins.. see below..

Typically these use 2 zener diodes, the first being a higher voltage ( around 12V) with the mains smoothing capacitor across it, and then a second resistor to the 5V zener, so that it is fed with something approaching DC, which it then regulates down to 5V. Can then be used to supply a low current ( under 1mA with your values, and will still have some residual ripple on it along with a rather long ( as in around 10 mains cycles) start up voltage build up as the capacitor charges.

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...

[AlfBaz]

Two words
Creepage and clearance

[Cerebus]

OTOH it is reasonable (right?) to use this voltage drop across a zener diode approach to get 3.3v or 5v at very low ma off of a regulated 12v or 9v rail, without needing another full blown voltage regulator, right?

Yup, nothing wrong with it as long as you don't need a precise voltage rail. It would be quite reasonable to power a single 5V op amp like this in a circuit that's otherwise all 12V.

How hot will it get? or would it? Would it get hotter than a linear voltage regulator or not? Both would emit heat, which one would emit less into a closed space..

Remember in my circuit, we're just taking about low voltage DC.  dropping 12v or 9v down to 5 or 3.3v

It'll get a little hotter than the best choice of regulator, a little less hot than a poorly chosen regulator. But we're talking about maybe chucking 5mA through a zener, 5mA to a load, dropping 10mA from a 12V rail to a 5V one, so (25mW+70mW) =  95 mW dissipation + another 25mW in the load. That'll warm up a 500mW BZX84 series diode by 12.5C above ambient and an RC55 series 1/4 watt resistor by 7.7C.

[rstofer]

Did the Helping Hand short anything?

[AlfBaz]

Wow, I'll try again

Mecha, get rid of the ground pour around the 240VAC mains part of the cct

[cdev]

Thank you. Think I will use some kind of partition to separate my GPSDO from its controller and two voltage regulators.

[32]

OTOH it is reasonable (right?) to use this voltage drop across a zener diode approach to get 3.3v or 5v at very low ma off of a regulated 12v or 9v rail, without needing another full blown voltage regulator, right?

Yup, nothing wrong with it as long as you don't need a precise voltage rail. It would be quite reasonable to power a single 5V op amp like this in a circuit that's otherwise all 12V.

How hot will it get? or would it? Would it get hotter than a linear voltage regulator or not? Both would emit heat, which one would emit less into a closed space..

Remember in my circuit, we're just taking about low voltage DC.  dropping 12v or 9v down to 5 or 3.3v

It'll get a little hotter than the best choice of regulator, a little less hot than a poorly chosen regulator. But we're talking about maybe chucking 5mA through a zener, 5mA to a load, dropping 10mA from a 12V rail to a 5V one, so (25mW+70mW) =  95 mW dissipation + another 25mW in the load. That'll warm up a 500mW BZX84 series diode by 12.5C above ambient and an RC55 series 1/4 watt resistor by 7.7C.
[/quote]

[Brumby]

Two words
Creepage and clearance

Wow, I'll try again

Mecha, get rid of the ground pour around the 240VAC mains part of the cct

My first recommendation also.  Sorry, but this just screamed at me!  It's asking for trouble.

Second is to take that 180K resistor and use at least 4 resistors in series (47K) - but check the datasheet for voltage limits on the resistors to work out the right number.  I couldn't do a single resistor like that.

[AlfBaz]

this just screamed at me!  It's asking for trouble.

One of my first home made pcb's used 240VAC and I did the obligatory ground pour over the whole board. Didn't see anything wrong at all on the screen. It wasn't until I went to solder the mains in I thought to myself WTF! and out came the dremel pcb modifier bit

[AlfBaz]

Second is to take that 180K resistor and use at least 4 resistors in series (47K) - but check the datasheet for voltage limits on the resistors to work out the right number.  I couldn't do a single resistor like that.

Yes, first time I saw that was on an industrial 600kW VVVF drive. If I remember correctly they used what looked like 4 standard 1% 1/4W resistors in series to feed an opamp with its 415VAC output

[wasyoungonce]

Please why are you not using an isolation step down transformer for safety.   Never run directly from mains.

Another fail is mains neutral in Australia is not earth....most house development us MEN (Main Earth Neutral) where earth is linked to neutral at one point in the power box.

You can control mains using a low pwr cct, its much safer so please consider this as direct mains circuits are very particular in components ratings.

[Mechatrommer]

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?

[Brumby]

Whether it's ground or not is basically irrelevant.  It's a lot of metal around mains that doesn't need to be there.

Don't get cheap in addressing the needs of adequate:

Two words
Creepage and clearance