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sopawilly • Apr 9, 2009

Capacitors working in series

Hi everyone,

I'm using Capacitors working in serie to handle High Voltage, and I'm afraid they might burn down o the first use for unequal voltage between different capacitors

The circuit is this one:


The Batt represents a 1.7kV DC source, the capacitors are these ones

Evox-Rifa | Passives | Capacitors | Capacitors, Aluminium | Screw Terminal - ALS31A, PEH200 85°C

that are all 500 V and 220 uF from the same rack

The Inductors are 12 uH self made, but i dont think that's relevant for it

i've heard about protecting them with resistance, but couldn't find a clear schemma about how to do it, or how to dimension those resistance, I'd appreciate any additional info about that.

Thanks for reading
Guillermo Perez
hello,you can use relays or doides to protect your capacitors from over-current!!!
Rifaa • Apr 11, 2009
Well. The capacitor rating in this configuration is 2KV, which is quite adequate provided the VBatt is from a battery. I recommend U add one more for each bank if it is a rectified Voltage source.
But I wonder why u have inductors, I presume your source is an AC rectified DC. Am I right, if so I recommend to increase the capacitor Voltage ratings since ripple will be there and may be higher than the capacitor ripple ratings, U have to get the cap's tech data sheets to be sure.
And another fact is using inductor will produce AC current with in the LC Circuit ( LC tank circuit). So u better check AC handling capacity of the capacitors.
Are you aware that the total capacitance is around 150 uf.
And as for resistance, I wonder why you need them to protect the caps.
As I know, series resistance is used for very large capacitance to minimize the inrush charging current to protect the conductor and the Bridge rectifier or diodes, still these resistors are switched out of the circuit ones the capacitors has charged.
Or you can use a parallel resistor across each bank or one resistor at the output can be used to discharge the lethal high voltage ones the circuit has switched off.
But this has to be calculated accordingly to the desired no load + ripple current ( if source is rectified ) of the circuit. I say you'll need a high resistance with appropriate power rating to the environment this circuit is used.
Tell me more and I can help.
sopawilly • Apr 14, 2009
Thanks both for your replies,

Rifaa, I'lll try to give you the info about those detail,

I didn't explain why I was working with this circuit,it should store the energy from that DC source, and deploy it in one single pulse that should light a flashlamp at the right of it, according to flashlamp manufacturers, this should create a pulse of 0,3mS and 210 J.If everything works, this should excite a Nd:YAG laser rod and start a small laser spot. But the laser part is not that important for that, it should be the same as if it was for a very big flashlamp from a camera.

The DC source will be DC rectified, but a comercial one, so I hope it has some nice stability, not too much ripple.

And the idea of the resistors was based in something i found,

If you have two identical capacitors in series this is further simplified to:
[​IMG] This series circuit offers a higher total voltage rating. The voltage drop across each capacitor adds up to the total applied voltage.
Caution: If the capacitors are different, the voltage will divide itself such that smaller capacitors hog more of the voltage! This is because they all get the same charging current, and voltage is inversely proportional to capacitance.
Worse yet, if one capacitor is slightly leaky, it will gradually transfer its voltage to the others, possibly exceeding their voltage rating in turn. And if one of them punches through its dielectric barrier, it can then damage others in a cascading fashion. This is why series capacitors are generally avoided in power circuits

That comes from this source:

Capacitors - Multiple Capacitors

So, they should be working when the capacitors are getting charged.

I know these resistances will get quite hot, I'll use 1W resistances, because they should dissipate around 0,7W each.

I hope this can help you to help me, and thanks again for your time and attentions

reachrkata • Apr 14, 2009
You say that you need the stored energy to excite a flash lamp for a short duration with high energy. I would then be careful about designing (or for that matter even using) the inductors, since they block fast transition of current.

zulmanun • Apr 14, 2009
hi i think u need to use nonpoler capacitor when charged capcetor discharger due to flash on at small instent of 3ms time it will preduce highe voltage spike or ocillation becouse of capcitor and inductor which is about in kv and also in KA . poler capcitor only have capacity to get there respective polarity voltages if we apply oppsit voltage w.r. t polarity it will damage capcitor.
ok u apply and replay me thanks
Rifaa • Apr 15, 2009
Hmmm. the capacitor theory part is correct and polarized electrolytic with low ESR is best prefered for igniting flash lamps. Low ESR results in very low capacitor loss and very good ripple rejection and fast charging and holding capability
I cannot agree on adding resistors to the circuit as it will hinder it's operation if you are going to use the charged voltage to ignite a lamp of any sort, because resistors will discharge the caps as soon pwer is interrupted and the final voltage will be lower than u need by the time you use it.

One more thing I don't get it is how the heck u are going to get that high voltage without using charge pump curcuit. 😕

On second thought you can use a charge pump to charge the capacitor to any desired voltage with low DC input. The short circuit energy that is used to ignite any lamp can be increased by simply using more caps in series.

Or may be you might already have tht.
So if confused, just shoot what u are doing in here and I will be glad to help.
:dance: Hope you get a taste of the HV from the caps, it will be quite a stinger ( just kiddin )..... yet still then only you will be a . Right!!
sopawilly • Apr 21, 2009
Hey, sorry for not repplying before, and thanks for the help.

For how to get the 1.7 kV I have a DC power supply of 30kV and use Voltage Dividers, I know it's really overrated, maybe I'll switch to another regulable HV DC Power Supply, but I need other people to allow me to use it, so, who knows, maybe I'll have to do with the big toy.

Thanks, bye
Rifaa • Apr 24, 2009
Any time.
I guess this thread can be locked now mods.

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