Capacitive Voltage divider

Discussion in 'Power Electronics' started by arishy, Nov 8, 2018.

  1. arishy

    Thread Starter Member

    Apr 26, 2014
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    My AC source is 230 V 50 Cycle.
    I have quite a few voltage adapters that function ONLY on 110 Volt. Rather than use a Transformer 230 V to 110 Volt I thought of using Capacitive
    voltage divider. Using TWO 0.47 Uf X2 capacitors and 1K ohm resistor,( all in series) I manged to get 110 Volt AC on the second capacitor.

    Here is the problem When I use my Electronic Load with an 110 volt / 9 volt dc and 2 AMP Adapter As soon I start increase the load the voltage drop dramatically and get no current.

    But when I use Normal transformer (230/110) and connect the same adapter It works flawlessly and give me the 2 AMP.

    So I concluded that capacitive voltage divider has serious draw backs. Your comments will be greatly appreciated.
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    It is a lot like a resistive voltage divider. The reactance of each of your 0.47 uf capacitors at 50 Hz is 68k ohms. Assuming the 230 VAC source is a low resistance and ignoring the 1k resistor for now, the output of your divider has an impedance of 34k ohms. It will not be able to supply more than about 3 milliamps to a short, 2A is out of the question. This is the same kind of performance you would get by making a voltage divider with two 68k resistors.

    I suggest that future experiments be performed using an isolation transformer so you can take more measurements, such as current measurements, and do so safely.
     
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  3. arishy

    Thread Starter Member

    Apr 26, 2014
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    1. Great response.....That explains the sudden drop of voltage as soon as I start the electronic load.
    2. I appreciate the safely rules you suggested. And I will use isolated Transformer in the future especially I have an osciloscope which can easily be damaged.
    3. My electronic load displays both the voltage and current in the circuit.
    4. Do you have a different approach to reduce the voltage from 230 to 110 apart from using step down transformer.??
     
  4. danadak

    Well-Known Member

    Mar 10, 2018
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  5. ericgibbs

    Moderator

    Jan 29, 2010
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    Hi Dana,
    The TS is asking how to reduce 230Vac to 110Vac when trying to use a 110Vac transformer on a 230Vac supply.??
    No scoping ?
    E
     
  6. arishy

    Thread Starter Member

    Apr 26, 2014
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    I am not trying to use 110 ac transformer.I am trying to use dc power adapter that operate only on 110 ac ( mobile charger etc)
    Actually I want to avoid using 230 v to 110 volt transformer !!!
     
  7. dendad

    Well-Known Member

    Feb 20, 2016
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    Unless you have a constant low power load, a voltage divider is not the way to go.
    You may just have to get a step down transformer or new plug packs.
    What are you powering and what are the plug packs in question?
    Are they "standard" voltages and currents?

    Nowadays, it is really easy to get universal plug packs at very good prices. Even have a look in Op Shops or ask around for old ones as they are tossed out by the truck load.
     
  8. arishy

    Thread Starter Member

    Apr 26, 2014
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    The funny thing is ALL new dc adapters are 100-240 Volt , even the cheap ones. How did they achieve it is a puzzle to me. I am sure one of you can enlighten me how they did.??
     
  9. dendad

    Well-Known Member

    Feb 20, 2016
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    It is just a design consideration. The wide spread on the input voltage range is a bit of a trade off. If you make it for 110VAC or 240VAC, the supply could be a little smaller and a bit more efficient. A larger inductor and higher powered components are needed as the voltages and currents that the supply may be subjected to will be greater than a dedicated one.
    A lot of old PCs had a voltage selector switch to change the input. The supply was designed to run at the "same" voltage, bridge rectified for 240VAC and voltage doubled for 110VAC. Don't ask me how I know what happens when a 110VAC unit taken out of a ship was connected to 240VAC without flicking the switch........ OOPS!
    So called "universal" input supplies are a great idea and owe a lot to the world market place. The only bit that needs to be changed is the plug.

    Do you know how switch mode power supplies work?
     
  10. ArakelTheDragon

    Active Member

    Nov 18, 2016
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    Also the losses from a capacitive divider will be very big. Its much better to use a step down transformer.
     
  11. ebp

    Well-Known Member

    Feb 8, 2018
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    A divider made with ideal capacitors is lossless because it relies on reactance not resistance. Real capacitors have some losses due to unavoidable resistances.
     
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  12. MrAl

    AAC Fanatic!

    Jun 17, 2014
    5,268
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    Hello there,

    To get 9v out at 2 amps from the wall wart you would need a 2.6uf cap in series with the wall wart input. That would give you about 110vac at the wall wart input. However, if you remove the load from the wall wart the input to the wall wart could go up much higher even near 200vac which obviously could be a big problem.

    To get some kind of regulation you'd need two caps around 10 times that value (maybe around 30uf each) and form a voltage divider, then add the load. The wall wart would load the output somewhat. The bigger the caps the less the loading, as an infinite value for each cap would not show any loading, but 30uf is probably a good trade off between no load regulation and full load regulation.
    However, there are problems that come up here. First, there are two caps in series across the line so you'd need a surge limiting device maybe a small value resistor or surge thermistor. Also, you would not get any isolation from this although most wall warts are themselves isolated. Also proper fusing is needed.
    Another problem is things like this should be put into a box that can limit damage to the surroundings if something goes wrong and one of the caps blows up.
    So there are various problems that need to be handled by a professional that knows all of the risks beforehand and has taken the appropriate steps to mitigate these problems. It could get dangerous if not.

    Probably best to either use a good transformer or just get a wall wart that can be used at the 230vac line.
     
  13. arishy

    Thread Starter Member

    Apr 26, 2014
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    Thank you so much for taken the time to explain the risks involved with dealing with 230 v and messing with an adapter that is designed to do specific job. Dendad mentioned the fact that you can design a power supply that deals with two voltages and stated that actually the power supply was designed for ONE specific level ( which is the proper approach in design ) With some little tricks you accept one or the other but BOTH end up using the SAME circuit.

    What I was thinking is use the design of circuit to use different level And that is bad engineering.
     
  14. MrAl

    AAC Fanatic!

    Jun 17, 2014
    5,268
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    Hi,

    You mean change the cap values or change the output of the wall wart?
    Sometimes you have to be very specific so i know what version you are talking about.

    One thing though, i thought your original question was a very interesting question. It may actually be possible. Of course it would have to be looked into very carefully, and compare prices of AC rated caps that were around 30uf or so. Maybe not so cheap.
    I know transformer prices have gone up quite a bit over the past 10 years maybe. I looked for an isolation transformer and those things are expensive. The 230vac to 120vac (you apparently need) are cheaper though and you could get one for under 50 dollars (USD) that can easily do 2 amps on the output of the wall wart.
     
  15. dendad

    Well-Known Member

    Feb 20, 2016
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    I'm not sure what you mean there.
    A "proper approach in design" is to have your circuit suit the use, and if you want 100 to 260VAC input, design it for that.
    If your plug packs were 240VAC and you wanted them to run on 110VAC, a voltage doubler mod may be possible, but for it to go the other way, I see smoke.
    The transformers in your supplies are designed for about 150VDC (110x1.4) and it cannot run at twice the volts.
    A step down transformer is really the only way to go, else new plug packs. Anything else is just asking for trouble.
     
  16. MisterBill2

    Well-Known Member

    Jan 23, 2018
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    One other HUGE problem with capacitive voltage dropping in general is that it can not tolerate any harmonics because at any higher frequency the reactance is less and so the voltage drop is less and so often something gets burned up or otherwise destroyed. That is why some non-dimmable lights fail almost immediately when operated by an electronic switching device like a motion sensor or a day/night sensor. The switching action of the scr or triac creates harmonics and the device burns up. AND, it can also happen with some UPS systems and some generator systems. So the capacitive voltage dropper or voltage divider is a poor choice for almost all application that need to keep working.
     
  17. MisterBill2

    Well-Known Member

    Jan 23, 2018
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    The new power supplies that accept from 100 volts up to 250 or so volts all use switching mode power converters, while the older ones used fixed ratio transformers. The method of the change over varies, and some of them just accept a very wide range of input voltages.That is why many, but not all, small power supplies can be universal while none of the older ones could be universal.
     
  18. arishy

    Thread Starter Member

    Apr 26, 2014
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    If we ignore for a moment the economics of designing an object, ( that will classify the discussion as academic ) But if we ignore safety that will kill any project.
    Even with the switching power supply approach the engineers made sure to isolate the two parts of the circuit.
    But what I really do not understand is the lack of an approach to deal with AC high voltages ( in the domestic env.) in a safe way. Look at the products that deal with low dc voltage control. We can say we own it now,
    The fact that the AC is very dynamic ( frequency and harmonics for one, Being connected to the grid which like being connected to the internet , anything can happen. In other words we do not own it.
    Circuit design simply shy away from developing products that let us control that part in a cheap and safe way.

     
  19. MisterBill2

    Well-Known Member

    Jan 23, 2018
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    In this forum there are a large portion of participants who have not yet gained the insight to understand that a given voltage source will be deadly. Daily there are those who are clueless. And the fact is that if this group fails to clearly explain the hazard some people will die fairly hard. AND we choose to avoid being the cause of that happening. Thus we avoid presenting designs that when assembled by the unknowing will present a real hazard.
     
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  20. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
    2,870
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    There is also 12/24 swithable and not full range. Full range covers 95 to 285 AC.
    Why 95 V - Some parts of Japan have a 100 VAC system.
    What 285 - You get 277 from 3 phase systems for lighting.
    So, you need like +-5% or so from nominal.

    For 120/240 switching it depends on half wave or full wave rectifying the input. You'll get the same basic source voltage to work with or about 168 VDC. The bridge makes ground not ground on the new supply, so sparks fly when your not careful when servicing,
     
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