surge protection built into ordinary household electronics

[Matt]

I'm building a little microcontroller system to be powered by household
current. I'm thinking it might be good to put in some kind of surge
protection. I wonder whether and how such protection is built into
ordinary devices such as televisions, microwave ovens, washing machines,
stereos, etc.

My system only has a couple of chips (costing $3 altogether), so my
other approach is to put the chips in DIP sockets. I don't know whether
to assume that the discrete passive components would be safe under a surge.

How should I do this, or should I provide any protection at all?

 

[Marra]

I'm building a little microcontroller system to be powered by household
current. I'm thinking it might be good to put in some kind of surge
protection. I wonder whether and how such protection is built into
ordinary devices such as televisions, microwave ovens, washing machines,
stereos, etc.

My system only has a couple of chips (costing $3 altogether), so my
other approach is to put the chips in DIP sockets. I don't know whether
to assume that the discrete passive components would be safe under a surge.

How should I do this, or should I provide any protection at all?

All household goods have filters in now any way to pass EMC regs.
They obviously use just enough to get through the regs as cost is
important.

 

[Marra]

I'm building a little microcontroller system to be powered by household
current. I'm thinking it might be good to put in some kind of surge
protection. I wonder whether and how such protection is built into
ordinary devices such as televisions, microwave ovens, washing machines,
stereos, etc.

My system only has a couple of chips (costing $3 altogether), so my
other approach is to put the chips in DIP sockets. I don't know whether
to assume that the discrete passive components would be safe under a surge.

How should I do this, or should I provide any protection at all?

All household goods have filters in now any way to pass EMC regs.
They obviously use just enough to get through the regs as cost is
important.

 

[D from BC]

I'm building a little microcontroller system to be powered by household
current. I'm thinking it might be good to put in some kind of surge
protection. I wonder whether and how such protection is built into
ordinary devices such as televisions, microwave ovens, washing machines,
stereos, etc.

My system only has a couple of chips (costing $3 altogether), so my
other approach is to put the chips in DIP sockets. I don't know whether
to assume that the discrete passive components would be safe under a surge.

How should I do this, or should I provide any protection at all?

I include surge suppression because it's a product selling feature..
It's zap proof! (To a degree..see fine print..)

I've been using these lately...
http://www.panasonic.com/industrial/components/pdf/awa0000ce2.pdf
D from BC

 

[mpm]

I'm building a little microcontroller system to be powered by household

Wall wart?

Otherwise: Think fuse and MOV.

 

[Matt]

Wall wart?

Otherwise: Think fuse and MOV.

Yep, one fuse, 3 MOVs should do the trick.

 

[Matt]

Wall wart?

I'm planning to put a little transformer, a FWBR, a cap, and a 5V
regulator on the PCB. There will be a relay to switch 20A 120VAC, so I
need to run 120VAC to the board anyway. What would a wall wart have in it?

 

[Matt]

I include surge suppression because it's a product selling feature..
It's zap proof! (To a degree..see fine print..)

I've been using these lately...
http://www.panasonic.com/industrial/components/pdf/awa0000ce2.pdf
D from BC

The design calls for a transformer, a full-wave bridge rectifier, and a
capacitor, then a regulator. Should I just put that ZNR surge absorber
across the capacitor? How do I select the size?

 

[Eeyore]

I'm building a little microcontroller system to be powered by household
current. I'm thinking it might be good to put in some kind of surge
protection. I wonder whether and how such protection is built into
ordinary devices such as televisions, microwave ovens, washing machines,
stereos, etc.

My system only has a couple of chips (costing $3 altogether), so my
other approach is to put the chips in DIP sockets. I don't know whether
to assume that the discrete passive components would be safe under a surge.

How should I do this, or should I provide any protection at all?

What 'surge' do you mean ? A microprocessor circxuit will likely be powered by a
voltage regulator which will remove any line voltage fluctuations.

Graham

 

[Eeyore]

All household goods have filters in now any way to pass EMC regs.

No they don't.

Graham

 

[Eeyore]

The design calls for a transformer, a full-wave bridge rectifier, and a
capacitor, then a regulator.

Then you almost certainly don't need any further 'protection' do you. What do you
think the voltage regulator does ?

Should I just put that ZNR surge absorber across the capacitor?

What would be the point of that ?

How do I select the size?

Get real.

Graham

 

[Eeyore]

Yep, one fuse, 3 MOVs should do the trick.

LMFAO !!!

I hope your designs don't have to sell at a profit !

Graham

 

[Eeyore]

What would a wall wart have in it?

Typically, a transfomer, rectifier, storage cap and maybe a regulator.

Graham

 

[mpm]

Looks like Matt's just buiding some sort of 120V logic-controlled
switch.
(They actually have relays that do this sort of thing already, or
PLC's, or even the stuff from www.pulizzi.com). Because he's got to
run 120 to the box anyway, a wallwart isn't ideal.

But to help Matt out, any surge would have to get through his
regulator (xfmr, bridge, caps, and 7805?) to do any real damage. A
single MOV across the AC input is a nice touch, but it's not
essential. Matt, MOV's have issues too, mostly relating to design
lifetime.

If you're going to sell these things, you need to keep prices low or
competition will eat you alive. At the same time, you'll have to make
sure meet all the regulatory requirements (for example, conducted and
radiated emissions if your project uses any high frequency stuff -
even the clock crystal on a microcontroller.) And depending on where
you want to sell them, you may need an independent safety
certification like UL, TUV, CE, etc... Which costs money.
And don't forget RoHS restrictions.

Is there some special environment that these devices will be placed in
that you're especially worried about surge protection?

 

[Matt]

What 'surge' do you mean ?

Lightning, the kick from shutting off a sump-pump motor, stuff like that.

A microprocessor circxuit will likely be powered by a
voltage regulator which will remove any line voltage fluctuations.

Okay, obviously I am not any kind of expert, but ...

http://www.national.com/ds.cgi/LM/LM2937.pdf

LM2937
500 mA Low Dropout Regulator

Absolute Maximum Ratings (Note 1)
Input Voltage
Continuous 26V
Transient (t <= 100 ms) 60V

I thought that meant that the regulator might be ruined if it gets more
than 60V for more than 100 ms.

 

[mpm]

Lightning, the kick from shutting off a sump-pump motor, stuff like that.
http://www.national.com/ds.cgi/LM/LM2937.pdf

I thought that meant that the regulator might be ruined if it gets more
than 60V for more than 100 ms.

Well, "Yes", that is what it means, but think about what has to happen
to get there.

You have a lot of stuff in front of this spec. Your transfomer,
filter caps, the bridge rectifier, the fuse. The 60V/ 100mS has to
get through all that to do any damage.

It would be a good exercise for you to look at just the transformer.
Imagine 60V on the secondary. How much would that be on the Primary?
Do you think this is a reasonable value you would ever expect to see
coming out of the wall socket?

Now of course, that's still AC at that point, but I think it's
instructive none the less.
Your bridge will lower the voltage even further, and the caps will
tend to smooth out longer term events (100mS). That's why they are
called filter caps.

Lightning is very impulsive, and contains a lot of VHF frequencies,
which can be somewhat difficult to avoid completely. You could spend
a lot of money making it lightning-proof, and never sell a single
product... A lot of people I know just like to tie a knot in the
power cord (inside or outside the enclosure), as a lightning choke of
sorts.

Personally, I would use an MOV, or a Sidactor if it were a modem line
or something...
Good luck.

 

[ehsjr]

Look at the datasheet and select the device based on your line
voltage..
Similar devices can be found inside suppressor computer power bars
It goes across the line..but more can be added..like across the ground
to neutral and another from hot to ground.

Alone, a little linear supply (xformer/bridge/cap/reg) can probably
handle huge line spikes before something blows..
For example...AFAIK ...MOV's or TVS's are not popular inside little
wallwarts..

The inductance of the transformer, maybe core saturation,the step down
ratio..
Also I think the reverse breakdown of the bridge .
...the capacitive reactance of the filter cap..
All these things attenuate spikes anyways..
And then...
Some regulators are tough to kill..
So Mr. Spike has to beat up lots of parts to smoke your project.

While on the topic..Has anyone had a linear wallwart die under normal
use?
D from BC

Yes. A wall wart charger for an NiCd powered vacuum cleaner
attachment. The charge rate was "set" by transformer sag.
The circuit was: secondary---diode---NiCd
Really "extensive" engineering. :-( Cheap piece of crap.
Two shorted cells in the pack took out the wallwart. My guess
is that there are lot of similar "circuits" for wallwarts.

Ed

Ed

 

[D from BC]

Yes. A wall wart charger for an NiCd powered vacuum cleaner
attachment. The charge rate was "set" by transformer sag.
The circuit was: secondary---diode---NiCd
Really "extensive" engineering. :-( Cheap piece of crap.
Two shorted cells in the pack took out the wallwart. My guess
is that there are lot of similar "circuits" for wallwarts.

Ed

Ed

Actually, I've tried to cheapen some designs and ...it can get
difficult and complicated!
It's tough to figure out a sneaky amount of components..

D from BC

 

[Eeyore]

Lightning,

If lightning gets seriously near it - it'll kill it, 'protection' or no, so forget
that one.

the kick from shutting off a sump-pump motor, stuff like that.

A tiny glitch on the mains ? It may not even get through a simple PSU. Have you
checked ?

In any event, the norm for dealing with that is filtering, not 'surge protection'
since, a glitch isn't a surge !

Okay, obviously I am not any kind of expert, but ...

http://www.national.com/ds.cgi/LM/LM2937.pdf


I thought that meant that the regulator might be ruined if it gets more
than 60V for more than 100 ms.

Well it won't will it !!!

This is a 5V microprocessor circuit ? You may have 10 V on the input pin of the
regulator. Lower possibly since it's an LDO. Do you seriously think the ac line is
going to jump to around 5 times its normal value for a whole 100ms ?

Have you ever *looked* at the mains ?

Graham

 

[Matt]

Is there some special environment that these devices will be placed in
that you're especially worried about surge protection?

No special environment---I'm just trying to avoid building a piece of
junk. Just trying to conform to something like accepted practice ...