A wall-wart alternative

[[email protected]]

I finally got tired of replacing the 2 C-cells in a clock with a moving
pendulum, so I measured the current (3 volts at 340 microamps) and built
a charge pump like this, viewed in a fixed font:

0.047 uF @400 V
|| |
-----------||--------------------->|------------>
|| | | |
| | 3V
120 VAC --- --- to clock
^ _
0.047 uF @400 V | |
|| | |
-----------||----------------------------------->
||

I used two caps in case one shorts and for some ground isolation. The diodes
are 5.1 V zeners ($1.29 for 2 at Radio Shack) to limit the clock voltage if
someone unplugs the batteries. This circuit supplies 390 microamps. I hope
to avoid replacing the batteries until their shelf life runs out in 2012

This could be useful for lots of low-power stuff, eg smoke detectors.

Nick

 

[[email protected]]

I hope you are using rechargable batteries!
NO!

Regulart alkaline or others may dry out, overheat and start a fire!

How much will they overheat if overchaged at 3Vx50uA = 150 microwatts?

USE ONLY RECHARGABLE BATTERIES IN THIS APPLICATION!

NO!

Happy new year,

Nick

 

[William P.N. Smith]

Looks like a shock hazard to me.

It is, but it's a current-limited shock hazard. Never pass UL, but it
works. Well, until the wrong cap shorts, then someone dies. 8*|

 

[Jeff Wisnia]

Looks like a shock hazard to me.

And at that low a current, why not ferget the caps and just use a couple
of 150K resistors? The additional power loss will hardly spin your
meter off the wall and you'll avoid the possible cap failure problem.

Jeff (Ducking and slinking off...)

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."

 

[[email protected]]

It is, but it's a current-limited shock hazard. Never pass UL, but it
works. Well, until the wrong cap shorts, then someone dies. 8*|

It might pass UL, in its "double-insulated grandfather clock"

Nick

 

[Don K]

I finally got tired of replacing the 2 C-cells in a clock with a moving
pendulum, so I measured the current (3 volts at 340 microamps) and built
a charge pump like this, viewed in a fixed font:

0.047 uF @400 V
|| |
-----------||--------------------->|------------>
|| | | |
| | 3V
120 VAC --- --- to clock
^ _
0.047 uF @400 V | |
|| | |
-----------||----------------------------------->
||

I used two caps in case one shorts and for some ground isolation. The diodes
are 5.1 V zeners ($1.29 for 2 at Radio Shack) to limit the clock voltage if
someone unplugs the batteries. This circuit supplies 390 microamps. I hope
to avoid replacing the batteries until their shelf life runs out in 2012

This could be useful for lots of low-power stuff, eg smoke detectors.

Nick

It seems counter-productive to build booby-traps into smoke detectors
and the like.

Unlike a wall-wart, it will require special precautions not to accidentally
touch anything when you go to change the battery or maybe even set the time
on a clock. You will never know whether a capacitor has shorted and whether
you might be killed the next time you provide a path to earth ground.

There's nothing to limit the current when capacitors fail, so that failure
will cause a fire. There's no transient voltage protection, so the odds
of capacitor over-voltage failure is pretty high.

Don

 

[Vaughn]

Unlike a wall-wart, it will require special precautions not to accidentally
touch anything when you go to change the battery or maybe even set the time
on a clock. You will never know whether a capacitor has shorted and whether
you might be killed the next time you provide a path to earth ground.

There's nothing to limit the current when capacitors fail, so that failure
will cause a fire. There's no transient voltage protection, so the odds
of capacitor over-voltage failure is pretty high.

A few points of data:

1) Even transformers can (and do) short out.

2) Switching power supplies do not necessarily have transformers, (more and
more wall warts are switchers, they are far more efficient)

3) Transient voltage protection can be added to virtually any circuit.

4) With proper circuit design, there can be something to limit current when a
capacitor fails.

5) With proper circuit design and component selection, the danger of a capacitor
failing can be infinitesimal.

Vaughn

 

[RoughRider]

See response point by point....
"Vaughn" wrote in message A few points of data:

1) Even transformers can (and do) short out.

They fail and usually the primary blows open. But short from primary to
secondary is rare.... I've never seen it with wall-warts.... I have seen
this with pole top distribution cans after a lightning strike.

2) Switching power supplies do not necessarily have transformers, (more
and more wall warts are switchers, they are far more efficient).

You are thinking about buck or boost designs that are used with DC-DC
voltage conversion. While many of these also use a transformer for
isolation, cheaper ones where the ground doesn't have to be inverted or
isolation isn't required don't have a transformer, just an inductor.
Anything connected to a wall outlet will have a transformer for isolation
and to provide a ratiometric voltage reduction for the output side. And
since it is switching at 100kHz or higher, the transformer has a ferrite
core and is very small (unlike an iron core operating at 60 Hz).

3) Transient voltage protection can be added to virtually any circuit.

Sure. But that will require a fuse as well to open and protect the
conducting MOV from self destruction. For the low current application
described in this thread, a (diode) or other semiconductor based OVP would
be more appropriate to be used with a resistor.

4) With proper circuit design, there can be something to limit current
when a capacitor fails.

True. This entire concept is a throw back to the
transformerless/batteryless Rogers tube radio. Remember even old TV's had a
HOT chassis. Every home with a TV or tube radio had one of these. I used
to play a game with my sister by holding the metal TV tuner dial and an
adjacent speaker lead from a radio. The harder one pressed, the more
current would flow. The winner was the one that could make their muscles
visibly twitch! But standards, liability, insurance, experience being what
it is, we are a lot smarter these days not to make products this way.

5) With proper circuit design and component selection, the danger of a
capacitor failing can be infinitesimal.

Of course. But many of us get components from a surplus store, so who knows
why the caps are there in the first place. ??

Bottom line is that the concept is electrcially sound. Is it safe? No. Is
anything likely to go wrong? No. Remember, it takes a collection of
mistakes to create a catastrophy.

Bye!

 

[George Ghio]

See response point by point....
"Vaughn" wrote in message A few points of data:


They fail and usually the primary blows open. But short from primary to
secondary is rare.... I've never seen it with wall-warts.... I have seen
this with pole top distribution cans after a lightning strike.


You are thinking about buck or boost designs that are used with DC-DC
voltage conversion. While many of these also use a transformer for
isolation, cheaper ones where the ground doesn't have to be inverted or
isolation isn't required don't have a transformer, just an inductor.
Anything connected to a wall outlet will have a transformer for isolation
and to provide a ratiometric voltage reduction for the output side. And
since it is switching at 100kHz or higher, the transformer has a ferrite
core and is very small (unlike an iron core operating at 60 Hz).


Sure. But that will require a fuse as well to open and protect the
conducting MOV from self destruction. For the low current application
described in this thread, a (diode) or other semiconductor based OVP would
be more appropriate to be used with a resistor.


True. This entire concept is a throw back to the
transformerless/batteryless Rogers tube radio. Remember even old TV's had a
HOT chassis. Every home with a TV or tube radio had one of these. I used
to play a game with my sister by holding the metal TV tuner dial and an
adjacent speaker lead from a radio. The harder one pressed, the more
current would flow. The winner was the one that could make their muscles
visibly twitch! But standards, liability, insurance, experience being what
it is, we are a lot smarter these days not to make products this way.


Of course. But many of us get components from a surplus store, so who knows
why the caps are there in the first place. ??

Bottom line is that the concept is electrcially sound. Is it safe? No. Is
anything likely to go wrong? No. Remember, it takes a collection of
mistakes to create a catastrophy.

Bye!

Nick Pine + Tools = Danger.

This is just another example.

 

[Vaughn]

See response point by point....
Bottom line is that the concept is electrcially sound. Is it safe? No. Is
anything likely to go wrong? No.

You made a collection of pretty good points, which collectively fail to
logically lead to your conclusion that the circuit is inherently unsafe.

Bye yourself
Vaughn

 

[[email protected]]

Unlike a wall-wart, it will require special precautions not to accidentally
touch anything when you go to change the battery...

And it has battery backup

There's nothing to limit the current when capacitors fail, so that failure
will cause a fire.

I doubt that. The diodes will probably open, and I used really skinny wire.

There's no transient voltage protection, so the odds of capacitor over-voltage
failure is pretty high.

Or diode damage. So... I added a 1K series resistor. This would also work:

0.022 uF @400 V*
|| |
-----------||--------------------->|------------>
|| | | |
| | 3V
120 VAC --- --- to clock
^ _
| |
10K | |
-----------www---------------------------------->

*Digikey's $29.95 M400-KIT-ND 400 V capacitor assortment contains
10 of these, along with 10 of 14 other values from 0.001 to 0.1 uF.

Nick

 

[Clark]

Looks like a shock hazard to me.

Caps do have a failure mode, so a series safety R is normally included
with these type circuits.

If your ac supply is polarised, you'd be a bit safer putting one cap in
the live than one in each pole, as the output will then be at apx earth
potential, though not isolated. As it stands its semi-live.


NT

 

[[email protected]]

The rest of this trickle charger is cheap...

$13.65 for 100 1N5231BD1CTND 5.1V zeners,
$15.86 for 1000 P10KBACTND 10K resistors,
and a line cord cut off an old appliance.

My smoke detector draws 7 uA = 60x370C, which makes C = 315 puffs, eg 2
0.001 uF 400 V caps in series, Digikey's P1058-ND, $11.43/100. They also
sell 12 V zeners suitable for 9 V batteries, 1N5242BDICT-ND, $13.65/100.

Nick

 

[Bill_M]

My smoke detector draws 7 uA = 60x370C, which makes C = 315 puffs, eg 2
0.001 uF 400 V caps in series, Digikey's P1058-ND, $11.43/100. They also
sell 12 V zeners suitable for 9 V batteries, 1N5242BDICT-ND, $13.65/100.

Nick

Of course you don't have enough current for the audible device so your smoke
detector should sit quietly by while you toast!

 

[[email protected]]

0.001 0.001 uF @400 V*
|| || |
----||-----||--------------------->|------------>
|| || | | |
| | 9V
120 VAC --- --- to smoke detector
^ _
| |
10K | |
-----------www---------------------------------->

My smoke detector draws 7 uA = 60Hzx2xsqrt(2)x120VxC, which makes C = 344 pF,
eg 2 0.001 uF 400 V caps in series, Digikey's P1058-ND, $11.43/100. They also
sell 12 V zeners suitable for 9 V batteries, 1N5242BDICT-ND, $13.65/100.

Of course you don't have enough current for the audible device so your smoke
detector should sit quietly by while you toast!

No. That current would come from the 9 V trickle-charged battery.

We might replace it every 6 years or so, if the alarm
fails to shriek with the test button.

Nick

 

[Don K]

X rated capacitors use fusible coatings, so a short causes a burn out
of the conductor around the short. The overall capacitance is not
noticeably affectde, IOW its self healing.



Very fine wire sounds like effective fusing to me.

You only need 20mA of AC current to cause ventricular fibrillation.
If the capacitor fails, someone could get killed touching whatever
your circuit connects to.

Don

 

[[email protected]]

0.0047 uF @400 V*
|| |
-----------||--------------------->|------------>
|| | | |
| | 1.5V
120 VAC --- --- to clock
^ _
| |
10K | |
-----------www---------------------------------->

can trickle-charge a Seth Thomas quartz clock with hands and a AA battery
("Replace every 12 months or in case of suddenly inaccurate timekeeping.")
The trickle-charged battery might last 10 years.

The battery current jumps when the second hand ticks, but a 1K resistor
and a 10K microfarad smoothing capacitor reveal an average 95 microamps
= 60Hzx2xsqrt(2)x120VxC, which makes C = 0.0047 uF, one of the 150 caps
in Digikey's $29.95 M400-KIT-ND. Their P10KBACT-ND 10K resistors cost
$15.86/1000... 1N5231BD1CTND 5.1V zener diodes cost $13.65/100. Their
number is 1-800-344-4539. Most orders are shipped in 12-14 minutes.

This circuit uses about 1.5Vx95uA = 142 microwatts, ie 1.2 Wh/year worth
$0.000125 at 10 cents/kWh. It's probably illegal. Don't try this at home.

Nick

 

[[email protected]]

You have to supply bleeping power, relying on the battery would be
unsatisfactory. In which case it'll draw bleeping power al the time,

Only with constant house fires.

Nick

 

[phatty mo]

I finally got tired of replacing the 2 C-cells in a clock with a moving
pendulum, so I measured the current (3 volts at 340 microamps) and built
a charge pump like this, viewed in a fixed font:

0.047 uF @400 V
|| |
-----------||--------------------->|------------>
|| | | |
| | 3V
120 VAC --- --- to clock
^ _
0.047 uF @400 V | |
|| | |
-----------||----------------------------------->
||

I used two caps in case one shorts and for some ground isolation. The diodes
are 5.1 V zeners ($1.29 for 2 at Radio Shack) to limit the clock voltage if
someone unplugs the batteries. This circuit supplies 390 microamps. I hope
to avoid replacing the batteries until their shelf life runs out in 2012

This could be useful for lots of low-power stuff, eg smoke detectors.

Nick

Why not a small dc-dc converter,from your main 12/24/48V battery bank..
Safer,and probably more efficient. (takes the inverter out of the
loop,atleast.)

 

[[email protected]]

Why not a small dc-dc converter,from your main 12/24/48V battery bank..

I don't have a battery bank.

Safer,and probably more efficient. (takes the inverter out of the
loop,atleast.)

If your main inverter doesn't run 100% of the time, you might make the cap
larger to charge the battery more when the inverter does run. This circuit
itself is unlikely to make the inverter come out of hibernation.

Nick