Is there any issue with this configuration?

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I want to use two polar capacitors in series to power some load. Is it safe to run them for long time?( more than 1000 hrs) is there any issues?

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I think the capacitors might get upset. What current is expected to flow?

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Won't reach 1000 hours unless your load is huge (drawing nA). Both capacitors will fail within 1000 hours.

Both capacitors will be reversed for 50% of the time. This will eventually kill them. How long it takes depends on the current drawn.

Why do you want to use those capacitors in series with the bridge?

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I want to have a high voltage non-polar and cheap capacitor(around 3-4uf 400v). they will act like a resistor and they so I can use a secondary zener and low enough resistor to have some regulation.my final load will get 5v @ 200ma

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And i have read in some books before that we can make non-polar capacitors this way...bit unfortunately i don't recall the book.

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might work with a diode in parallel with each capacitor. this will guard the capacitor for it's negative polarity.

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The power factor will be ghastly. And the capacitors must be rated to handle the amount of current, they will heat up. Above a certain wattage (25W IIRC) the power factor must be corrected, if you want to keep things legal.

There are special high capacity mains rated film capacitors, mainly to compensate for bad power factor in lighting ballasts or motor run capacitors. They are absolutely huge! Think of a big can of 30mmx150mm or something like that.

For audio application there are special unpolarized electrolytic caps but you cannot use them.

Not a viable circuit IMO.

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I have heard too that you can use regular electrolytics this way, unfortunately I can't tell you anything about life expectancy since it was something I was told maybe 10 years ago in a brief conversation!
If you want to be safe, get another capacitor!

Anyway, do you have a constant or variable load? If your load varies (ie. microcontroller circuit) your voltage will vary a lot too! So if it is for powering a micro controller you will probably end up killing it in a spectacular way!
(Or you need some kind of regulator before your load)

For a variable load a small transformer is probably your best bet, since we are talking <2VA you could probably find a very small transformer at a pretty good price!

- Brian

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I have used that kind of power supply in a couple of small boards in the past, not my favourite.

You can get mains rated metallised polypropylene capacitor X2 rated for that knid of work, however be warned that frequencies higher than the 50Hz or 60Hz will go straight through the cap.

Therefore you should have high frequency filters and a MOV, maybe a zener to protect things, in my last circuit where I needed a small current from mains I ended up using a small transformer.

The one I used (maybe 1,000 off) http://www.digikey.com/product-d... seems to have been discountinued now but similar types are available from other suppliers.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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And don't forget your circuit is galvanically connected to mains with this topology.

That means you cannot hook up your JTAG/ISP/Serial/whatever to it without blowing up your computer and anything connected to it; unless you use an isolation transformer or isolate all the external interfaces.

And it's easy to forget that transformer when you are in hurry.

I have seen it happen first hand.

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Ok, if it's not a good option,how can i have 5v @ 200ma around 1USD?

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How many do you need? You might consider importing those USB charger wall warts from China.

http://www.alibaba.com/product-gs/307046433/USB_charger_power_adapters_with_CE.html?s=p

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You want to use a divider circuit and this means that you will dissipate 43W in your divider (215V x 0.2A). It doesn't matter what you use (capacitor, resistor or coil): 43W in heat must be dissipated.

The only way to go is using a transformer circuit (using 2 coils, switching logic,...).

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Ali_dehbidi wrote:
Ok, if it's not a good option,how can i have 5v @ 200ma around 1USD?
1. LinkSwitch-TN - may not meet the price requirement.
2. FSAR001B - does not quite meet the current requirement.
3. 2-transistor Black regulator - "High voltage use", needs some thought.
jayjay1974's idea (wall wart) - a lot of us have these already for our mobile phone, usually easy to buy at a local retailer, and meet safety standards (CE, CSA, etc.).

"Dare to be naïve." - Buckminster Fuller

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jan_dc wrote:
The only way to go is using a transformer circuit (using 2 coils, switching logic,...).
Some off-the-shelf (OTS) ICs for this from On Semiconductor and Power Integrations; the transformer for these is also OTS.

"Dare to be naïve." - Buckminster Fuller

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I would think you might find non-polar motor start capacitors of the value you need. They are often used on air conditioning units.

It all starts with a mental vision.

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At $1.00 dream on

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I saw complete USB phone chargers for $0.54 on Alibaba.

I'd not be surprised if you ask that supplier nicely it is possible to send just the PCBs without the enclosure and shave a $0.10 off the price.

It all depends on the required quantity.

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I'm not sure why you are all saying this is a bad configuration, almost all electrolytic or tantalum capacitor manufacturers have a note in their datasheet about non-polar application.

Here's a paragraph in a Cornell application note:

Quote:
If two, same-value, aluminum electrolytic capacitors are
connected in series, back-to-back with the positive termi-
nals or the negative terminals connected, the resulting sin-
gle capacitor is a non-polar capacitor with half the capaci-
tance to either of the original pair. The two capacitors recti-
fy the applied voltage and act as if they had been bypassed
by diodes. When voltage is applied, the correct-polarity
capacitor gets the full voltage.
In non-polar aluminum electrolytic capacitors and motor-
start aluminum electrolytic capacitors a second anode foil
substitutes for the cathode foil to achieve a non-polar
capacitor in a single case. While non-polar aluminum elec-
trolytics are available for momentary-duty AC applications
like motor starting and voltage-reversing applications, the
high DF of aluminum electrolytic capacitors – from 2% to
150% – causes excess heating and short life in most AC
applications.

Here is one from Matsuo Electronic:

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BTW I'm not saying this is the best way to go about this. If you need non-polarized capacitors, use non-polarized capacitors. Ceramic caps come in ever increasing capacitance values and voltage ratings. If you need big capacitance, just use a normal polarized cap and protect it with a parallel diode.

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hugoboss wrote:
I'm not sure why you are all saying this is a bad configuration, almost all electrolytic or tantalum capacitor manufacturers have a note in their datasheet about non-polar application.

Here's a paragraph in a Cornell application note:

Quote:
If two, same-value, aluminum electrolytic capacitors are
connected in series, back-to-back with the positive termi-
nals or the negative terminals connected, the resulting sin-
gle capacitor is a non-polar capacitor with half the capaci-
tance to either of the original pair. The two capacitors recti-
fy the applied voltage and act as if they had been bypassed
by diodes. When voltage is applied, the correct-polarity
capacitor gets the full voltage.
In non-polar aluminum electrolytic capacitors and motor-
start aluminum electrolytic capacitors a second anode foil
substitutes for the cathode foil to achieve a non-polar
capacitor in a single case. While non-polar aluminum elec-
trolytics are available for momentary-duty AC applications
like motor starting and voltage-reversing applications, the
high DF of aluminum electrolytic capacitors – from 2% to
150% – causes excess heating and short life in most AC
applications.

Here is one from Matsuo Electronic:

It's also a bad configuration due to the excessive heat loss in the capacitors. The bad configuration is not in the use of capacitors but in the circuit: divider type on a high voltage drop. This means generate a lot of heat to get that 215V drop.

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Ali,

This configuration is used for low current applications. For example, remote controled fans, even some energy meters. But not at the current level you are asking for. If you want to learn further on this check on the energy meter application notes at Atmel and Analog Devices site.

Alternatively, you can try AC/DC convertors from Rohm or others. Like this at digikey http://www.digikey.sg/product-detail/en/BP5041A5/BP5041A5-ND/658559

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thanks for your replies. I think going to buy USB chargers from alibaba is the only option!

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I love Digital
and you who involved in it!