Capacitive voltage dropper for 1.5 V clock

[soldar]

Over the kitchen window I have a round analog clock which uses a 1.5 V AA cell. For years I have been thinking of replacing the AA with a capacitor dropper but just never got around to it. Sometimes it has lost time , not clear why. Now I think it is because with extreme cold by the window the battery drops in voltage and I found myself replacing batteries which were virtually new. Finally I decided to build the capacitive dropper.

With components from my general junk I built the attached circuit. The three 1N4002 forward diodes in series are there to limit the voltage. I was afraid the voltage might go too high but they work perfectly with an output of about 1.7 volts which is perfect.

Every second the clock pulls a jolt of power to advance the seconds' hand. The output capacitor is there for that. I did not really calculate any values. Just grabbed the first one I found. For the input cap you can roughly calculate 15 nF per milliamp at 230 Vac, 50 Hz.

I am thinking that maybe I can find a way to enclose it inside a Schuko plug and have the low voltage wires come out from there.

Interestingly, both capacitors are bout 50 years old and quite large in physical size by today's standards.

Not really a project worth mentioning . The most difficult part was getting off my butt and doing it. Maybe the idea will be useful to someone.

While I had the soldering iron hot I decided to also adapt a desktop LCD clock, calendar, thermomether, etc. which uses button batteries which I hate, adapt it to using using two AA cells in a holder taped to the back.

[PGPG]

Your device should be plugged in exactly when AC voltage is crossing 0. In other way your capacitors and diodes get stressed.

I am thinking that maybe I can find a way to enclose it inside a Schuko plug and have the low voltage wires come out from there.

Such circuit assembled in plug with 2 wires connected to thumbtacks stuck into a wooden clothespin were widely used (in 70/80s) to charge unchargable hand watch batteries (I still have one).
In many countries L and N have not guaranteed order in socket. So your "low voltage" can kill someone.

[Xena E]

Surge limiting resistor, (100Ω 1W say) in series with the input, and a discharge resistor 680k-1M across the dropper capacitor would be good practice.

If no one can touch anything, these little circuits are fine

 

[Gyro]

Make the surge limiting resistor fusible (and first, at the L pin) to guard against your 15nF cap shorting.

[Zero999]

Use two Y1-rated capacitors for the dropper, one for phase and one for neutral and there will be no risk of electrocution on the secondary side.

[soldar]

Use two Y1-rated capacitors for the dropper, one for phase and one for neutral and there will be no risk of electrocution on the secondary side.

Well, yes but I just threw this together with what I had at hand.

Reminds me of "If we had some ham we could have some ham and eggs if we had some eggs".

If I had a couple of Y1-rated caps I probably be needing them for something.

[soldar]

Surge limiting resistor, (100Ω 1W say) in series with the input, and a discharge resistor 680k-1M across the dropper capacitor would be good practice.

If no one can touch anything, these little circuits are fine

 

Make the surge limiting resistor fusible (and first, at the L pin) to guard against your 15nF cap shorting.

Good ideas which I incorporate into my build. A 100Ω series resistor does not need to dissipate any power and the tiniest one will be "fusible" enough.

It is true that in Schuko land L and N are not differentiated but I do identify them when needed and, in any case, there is no risk of anyone touching this other than myself. The clock is hanging out of hand's reach and requires a step-stool to reach it. Put the circuit in a well protected enclosure and it is safe enough for my needs. 

[soldar]

Just for fun I thought I add a neon bulb to indicate polarity. If it glows then L and N should be reversed.

It does not need to be connected to PE as just touching that wire should be enough.

[mikerj]

These cheap clock movements do go bad and start killing batteries, it's happened a couple of times to me with no obvious issues like corrosion or leaky caps.  I just replaced the movements.

[Zero999]

Just for fun I thought I add a neon bulb to indicate polarity. If it glows then L and N should be reversed.

It does not need to be connected to PE as just touching that wire should be enough.

Have you tried connecting the neon lamp via a gimmick capacitor? I've found small neon lamps will flicker dimly, with the slightest capacitive coupling: often holding the wires and touching the conductor with the glass bulb is enough.

[soldar]

These cheap clock movements do go bad and start killing batteries, it's happened a couple of times to me with no obvious issues like corrosion or leaky caps.  I just replaced the movements.

I suspect you are right. I suspect the bearings just wear out as they are just plastic and the clock in question is over 20 years old but it is not worth replacing the movement which would cost more than the entire clock. I am looking at the Ikea website and they have several similar clocks for under 10€ and one for under 5€.

Still, the battery eliminator circuit saves batteries even if I get a new clock. I need to make a dummy AA battery with cables though because I am using alligator (or is it "crocodile") clips for now.

The only problem with the circuit is that when there is a power outage the clock loses time. Which in a way is useful as I can tell for how long the power was gone while I was out.

I am a bit OCD and I like to have clocks and thermometers in every room, including showing outside temperature.

[SteveThackery]

Are you going to put an appropriate capacitor in each leg? Then you can dump that somewhat kludgy neon indicator, and you will know you've made it as safe as reasonably practicable.

[Terry Bites]

See attached on how to avoid a housefire and a lifetime of guilt.
Add a TLV431.
Skimping is a very, very bad idea!

[soldar]

See attached on how to avoid a housefire and a lifetime of guilt.
Add a TLV431.
Skimping is a very, very bad idea!

Wut?

[IanB]

I would say the circuit as presented has a significant lack of fault protection and safety features. There is no current limiting resistor on the input, no fuse, no overvoltage protection. At best I would think a mains spike might blow up your clock, and at worst you may have a fire. I really would not want to have something like this permanently connected to the mains supply.

In the UK at least, there would be a 3 A fuse in the plug, but I don't think fused plugs are common in continental Europe?

[StillTrying]

It doesn't matter what N or PE do, it's the L that takes the -Ve output of the bridge to -330V.
Which is something to remember during testing, and after.

[soldar]

I would say the circuit as presented has a significant lack of fault protection and safety features. There is no current limiting resistor on the input, no fuse, no overvoltage protection. At best I would think a mains spike might blow up your clock, and at worst you may have a fire. I really would not want to have something like this permanently connected to the mains supply.

In the UK at least, there would be a 3 A fuse in the plug, but I don't think fused plugs are common in continental Europe?

This type of capacitor dropper circuit is used in millions of LED light bulbs and other low power devices in America, in Europe, in China and other parts of the world.

I consider it safe enough.

[soldar]

It doesn't matter what N or PE do, it's the L that takes the -Ve output of the bridge to -330V.
Which is something to remember during testing, and after.

I must be missing something because I do not understand what you are saying.  The line side of the capacitor is the only point that sees the higher voltage. The rest of the circuit is never more than 2 or 3 volts from neutral.

[soldar]

Have you tried connecting the neon lamp via a gimmick capacitor? I've found small neon lamps will flicker dimly, with the slightest capacitive coupling: often holding the wires and touching the conductor with the glass bulb is enough.

I am not sure what you mean by "gimmick capacitor" but I think it may be like putting some metal on one side of plastic and touching the other?

I might try playing around with that. I like the old NE-2 bulbs and have quite a stock and like to use them whenever I get a chance.

[IanB]

This type of capacitor dropper circuit is used in millions of LED light bulbs and other low power devices in America, in Europe, in China and other parts of the world.

I consider it safe enough.

I beg to differ. LED light bulbs have safety features your circuit doesn't have. LED bulbs have to pass UL or CE certification. Your circuit would not pass those.

[soldar]

This type of capacitor dropper circuit is used in millions of LED light bulbs and other low power devices in America, in Europe, in China and other parts of the world.

I consider it safe enough.

I beg to differ. LED light bulbs have safety features your circuit doesn't have. LED bulbs have to pass UL or CE certification. Your circuit would not pass those.

What safety features? They have a fusible resistor and my circuit has one, in fact taken from a commercial light bulb that failed. What else is missing?

[Grandchuck]

I would use a capacitor with a higher voltage rating.

[soldar]

 

I would use a capacitor with a higher voltage rating.

You can do that. I consider 400 V to provide a reasonable enough margin. I am using a polyester film capacitor and in my experience if and when they fail they tend to lose capacity, not to short out. And, in any case, if it shorts out then there is a fuse.

I consider the circuit to be reasonably safe and similar to commercial circuits which also use 400V dropper caps. Don't get me wrong. If I had to order the capacitor I would probably order 630V but, as I said, this is something a put together with what I had at hand and I consider to be good enough and most commercial circuits I see also use 400 Vdc or 250 Vac capacitors.

And there is even a video about these capacitors ...




ETA: In this thread some years ago I dealt with a panel meter using a capacitor dropper supply and the film capacitor had lost capacitance.

https://www.eevblog.com/forum/repair/repairing-panel-volt-amp-meter/

[IanB]

What safety features? They have a fusible resistor and my circuit has one, in fact taken from a commercial light bulb that failed. What else is missing?

Fair enough. I was looking at your circuit from post #1. I see you made some revisions since.

[langwadt]

Add a TLV431.

what's that going to do that the three diodes doesn't do?