Info on polarized capcitator

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rattus

Senior Member
Derek and Sam,

I must admit that many rectifier circuits connect the diodes directly to the cap. And I just realized that the larger current surges occur only at power on. In the steady state, the charging current is determined by the rate of voltage change, that is,

i(t) = C(dv/dt)
 
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Physis 3

Guest
One of the (many) things I like about you Rattus is that you'll make sure to understand something. :wink:

Edit: Error B
 
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urganb

Member
Location
Upstate N Y
replies to capcitor question answers

replies to capcitor question answers

Many thanks to everyone for replies .Really apreciated.Cap will be charged with NO load in circuit. Have 15, 25, and 60amp fullwave rectifiers on hand. Though the larger the better????? After cutting OFF AC power to rectifier ,2nd switch will be used to induce nanoburst of power from charged cap to pair of solenoid type coils 1 north oriented and 1 south oriented. May end up using less or more ufs,depending on strength of magnetic field generated.Do not know how to calculate ohms (size)of resister betweem DC power to cap, to prevent savage inrush of current when charging completly discharged cap, or how to calculate ohms(size ) of safety bleedown resister that will used via a normaly open switch to make sure cap is dead. Also feed resister should be on positive sides?? Or makes no differance??Any fomula will need to be self explanitory for me to use. Again,thanks to ALL,Ben :-?
 

rattus

Senior Member
Physis 3 said:
One of the (many) things I like about you Rattus is that you'll make sure to understand something. :wink:

Edit: Error B
Sam,

And the women say I am good looking too!
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Ben thanks for the clarification. I had to go back and re-read your original post. I caught something I missed, so please clarify.

Are you converting directly from line AC? If so this is dangerous, it can be done as any SMPS does exactly that. Not a good idea for beginners or amateurs. I suggest you use an isolation transformer.

When converting directly using SMPS technology, then you do inset a resistor called an in-rush limiter. However the resistor is installed before the rectifier, not after between the rectifier and filter cap.

For your application the size in ohm’s is not too important, what is important is the amount of TIME you want to charge the capacitor. It takes 5 Time Constants to charge a capacitor. 1 Time Constant equals the resistor in ohm’s time the capacitor value in farads, or Tc=RC. Keep in mind pwer rating of the resistor. It is all ohms law.

Here is a web site to do the math for you and explains things a bit.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html

If you want to see a design of a SMPS using the inrush limiter resistor here you go. I have built a few SMPS around this basic design.

http://www.smps.us/12volt-powersupply.html

I will give you a hint of what the resistor size should be, the answer lies in your rectifier size in amps so you do not burn it up. So if you are using say a 60-amp bridege supplied by 120 VAC RMS, no less than 2-ohms is a good place to start. Use an isolation transformer and forget about the resistor.

Also consider the power rating of the resistor along with the duty cycle. It all revolves around ohm's law. Think about this for a while and come back and ask some educated questions.
 
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Physis 3

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I once built an arc making toy using two neon display transformers. I forget the numbers but it was somewhere in the neiborhood of 32,000 volts. The reason I bring this up at all is to mention that I spent tons of attention on the safety issues related to what I was doing. I knew full well that it could be very dangerous, not that I was scared, but I was very much concerned that nothing stupid could happen.
 

steve66

Senior Member
I think it bears repeating:

Use an isolation transformer and wear safety glasses. I think it is likely you will produce fireworks at least once.

Since the peak voltage is 170V, you probably want a capacitor rated at least 300V.

Oh, and a disclaimer: there are probably a hundred other things that you haven't asked, and we haven't thought to mention. You alone are responsible for the safety of yourself and others.

I's supprised this thread wasn't closed on the "nonprofessional" rule. Not that I like to see people "diss'ed". But again, its likely you will produce fireworks.

Steve
 

rattus

Senior Member
steve66 said:
I think it bears repeating:

Use an isolation transformer and wear safety glasses. I think it is likely you will produce fireworks at least once.

Since the peak voltage is 170V, you probably want a capacitor rated at least 300V.

Oh, and a disclaimer: there are probably a hundred other things that you haven't asked, and we haven't thought to mention. You alone are responsible for the safety of yourself and others.

I's supprised this thread wasn't closed on the "nonprofessional" rule. Not that I like to see people "diss'ed". But again, its likely you will produce fireworks.

Steve
If a transformer is used the secondary should be center tapped. Then you can use two rectifiers to create a "full-wave rectifier circuit". For 120V, the turns ratio should be 2:1. For 240V, it should be 1:1.

You could also do this without a transformer by connecting the rectifiers to L1 and L2, but you would not have the isolation. (Assuming a 120/240V service)
 
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Physis 3

Guest
Urganb, why don't you just tell us what you're making? Mostly just cause I'm curious but we could probably be more helpful too.

Just considering the bleed resister I'd be looking at it in two ways.

First, it's a load on your capacitors.

Second, the resister is going to essentially be a little heater.

If you use a 33k at 170 volts it will shunt 5.15 mA. So you'll will need to be able to dissipate .876 watts. Usually I like to use twice the wattage for power type stuff so I'd use a 33k 2 watt resister. If I could live with it taking about 5 mA.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Physis 3 said:
Urganb, why don't you just tell us what you're making? Mostly just cause I'm curious but we could probably be more helpful too.

Just considering the bleed resister I'd be looking at it in two ways.

First, it's a load on your capacitors.

Second, the resister is going to essentially be a little heater.

If you use a 33k at 170 volts it will shunt 5.15 mA. So you'll will need to be able to dissipate .876 watts. Usually I like to use twice the wattage for power type stuff so I'd use a 33k 2 watt resister. If I could live with it taking about 5 mA.
I would like to know also. From re-reading the post a few time I am inclined to pull the plug on this thread as I suspect the OP has no biz doing this.

Sam to your point of using a bleeder of 33K would mean assuming a 10Kuf cap translates to a 30-minute discharge cycle, which IMO is useless. Also if the point is to charge the cap, turn off the input power indicates he want to discharge the cap into something other than a bleeder, which is also pointless IMO.

So urganb,
you had better convince me you know what you are doing, or I will pull the plug on this thread.
 

dbuckley

Senior Member
Physis 3 said:
I once built an arc making toy using two neon display transformers. I forget the numbers but it was somewhere in the neiborhood of 32,000 volts. The reason I bring this up at all is to mention that I spent tons of attention on the safety issues related to what I was doing. I knew full well that it could be very dangerous, not that I was scared, but I was very much concerned that nothing stupid could happen.
Take a look here, specifically the second photo from the bottom...
 
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Physis 3

Guest


Is this the image? If that's the one you're refering to I don't see what your point is. Could you elaborate?
 
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Physis 3

Guest
dereckbc said:
Sam to your point of using a bleeder of 33K would mean assuming a 10Kuf cap translates to a 30-minute discharge cycle, which IMO is useless. Also if the point is to charge the cap, turn off the input power indicates he want to discharge the cap into something other than a bleeder, which is also pointless IMO.
You're exactly right Dereck. Given the parameters I'm pressuming, 10k or less would have to be a rather big resister and also sort of defeat the purpose of needing so much capacitance.

Edit: And the voltage is gonna drop to somewhere a lot safer way before it's completely discharged.
 
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dbuckley

Senior Member
Physis 3 said:


Is this the image? If that's the one you're refering to I don't see what your point is. Could you elaborate?
It is indeed the image, and it relates to you saying "I once built an arc making toy using two neon display transformers", the mention of 32KV, and "I spent tons of attention on the safety issues related to what I was doing".

There is something inherently funny about the use of the word "safety" in conjunction with 32KV spark generators and/or Jacobs ladders, all of which are by definition unsafe. Not to mention the use of the word "toy" and 32KV spark, and "safe". Big Clive clearly thought it was funny, as do I, but the PAT testing jobsworth at the BBC clearly didn't get the joke...
 
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Physis 3

Guest
Do have any idea how many volts are driving your TV or computer CRT if you're using something from only a few years ago?

The reason I mentioned the Jacob's ladder is precisely to point out the concept of being safe and paying attention to doing that.

Edit: Error B

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dbuckley said:
There is something inherently funny about the use of the word "safety" in conjunction with 32KV spark generators and/or Jacobs ladders,
I see it as the opposite, it would be inherently "not" funny to leave out safety when there's 32kv near you.
 
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urganb

Member
Location
Upstate N Y
polorized capcitor charging

polorized capcitor charging

Hi. Thank again for valuable replies and concernes for safety.I guess i did not fully explain use in my 2nd post(#23). Am building a magneto charger to recharge tired north and south alnico magnets contained in a 1 1/2 inch dia.rotating armature as used in a magneto. For charging will be using 1 north and 1 south electromagnetic(solinoid type) coils . Armature will be solidly retained between 2 coils by machined 1 1/2 inch bolted down cross bars. Fly back diodes will be used. A bleed down resistor will be used via a normaly open switch. Switch closed after charging magnets,then reopened for next cycle.After ac power is shut off,fully charged capcitator, via a 2nd switch,Will discharge instantly into 2 copper wound coils with 1.1/2 mild steel cores.AS am using many turns and each coil is 5 inch long ,hopfuly will be able to use smaller cap,but still have option of larger cap if more gauss is needed.YES,there are chargers out there but they will only charge older horseshoe magnets,not alnico.Yes, there are still a few places i could send them to at reasonable cost.Do not wish to,am building my own .Sorry for such a long post,Yours Ben. PS To moderator.If you were a doctor and i a terminil patient,would you pull the plug quicker because of my lack of education?? Hope ya got a sense of humor,Ben.
 

steve66

Senior Member
urganb said:
Hi. Thank again for valuable replies and concernes for safety.I guess i did not fully explain use in my 2nd post(#23). Am building a magneto charger to recharge tired north and south alnico magnets contained in a 1 1/2 inch dia.rotating armature as used in a magneto. For charging will be using 1 north and 1 south electromagnetic(solinoid type) coils . Armature will be solidly retained between 2 coils by machined 1 1/2 inch bolted down cross bars. Fly back diodes will be used. A bleed down resistor will be used via a normaly open switch. Switch closed after charging magnets,then reopened for next cycle.After ac power is shut off,fully charged capcitator, via a 2nd switch,Will discharge instantly into 2 copper wound coils with 1.1/2 mild steel cores.AS am using many turns and each coil is 5 inch long ,hopfuly will be able to use smaller cap,but still have option of larger cap if more gauss is needed.YES,there are chargers out there but they will only charge older horseshoe magnets,not alnico.Yes, there are still a few places i could send them to at reasonable cost.Do not wish to,am building my own .Sorry for such a long post,Yours Ben. PS To moderator.If you were a doctor and i a terminil patient,would you pull the plug quicker because of my lack of education?? Hope ya got a sense of humor,Ben.
Doctors don't pull the plug due to lack of education, only due to lack of insurance:)

But seriously, most of the people on this forum are electrical contractors and engineers. There is a policy about not helping non-professional people. Trust me, its not because we don't like helping people, its more like a liability thing.

If you are doing more of an experminetial or hobby electronics, there are probably other forums more geared toward that aspect. You might even be able to find other people who are trying to accomplish the same thing.
 

dbuckley

Senior Member
If the OP really wants a high energy discharge, then it's worth remembering that energy stored in a capacitor is given by J=0.5*C*V^2, with a handly on-line calculator here.

The jist of the matter is using a (much) higher voltage is usually a good approach, as for the same enregy a much smaller capacitor bank is required, and the currents involved in the discharge are much lower. Also, all things being equal, the discharge is much faster. The downsides are it's well, a higher voltage, so safety and good engineering become very very important; the chances of surviving a belt off a high voltage cap bank are very low.

I did have a few paragraphs of helpful information here, but on reflection I've deleted it, as one will either know how to build high voltage assemblies safely, or one won't, and I don't think I should be the start point for one who may not.
 

urganb

Member
Location
Upstate N Y
polorized capacitor

polorized capacitor

Hi Dereck.You are pretty much correct.I reconfigure a few older F.M. mags for unique Harley applications such as the old VLs. Also Dual point setup and single point internal, manual advance. Can not use the newer rare earth type(Morris,Hunt)as drive end shaft is to short and cam to small dia. for dual points. To possibly aleviate some of the fears for my safety, AM uneducated, but with my various jobs and exposure to more than a few mechanical and electrical hazzards, i did not get to be 76 years old by being real stupid. I built, with NO assistance ,the 110/220 full wave DC power supply for my 1950s Monarch 10EE lathe that has given me 21 + years of trouble free service.(Did blow one of the two 10 amp fuses on the 110 ac input lines because of a LOCKED rotor) The caps on some of the older tig welders i have repaired use caps as large as some car batteries and COMMAND respect.Glad to see moderators DO have a sense of humor.Again ,Many many thanks for intrest and replies,yours,Ben. PS. Of the 6 electricians i have been aquainted with,the only thing they knew abour DC, was the fact they had a battery in their pickup truck!!
 
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