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MKP-MKT capacitors

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LAN_403

Unleashed

Mon Jun 18 2012, 10:52PM

Registered Member #5171 Joined: Tue Jun 05 2012, 11:32AM
Location:
Posts: 67

Hello everyone.
Today i'm asking you a very weird question(probably).
I'm building a Teslacoil, following this guide.

Now, to the shop they didn't have the MKP capacitors but only MKT.
They said it's the same thing, but i can't fully trust, because i know that any changes on resonant caps would affect the final result.
So, can i use these MKTs as well, or they have to be MKP?
Thanks in advance.

Josh Campbell

Tue Jun 19 2012, 12:18AM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

Just some basic info. I'll let someone who has used both of them in a TC speak to performance variances.

MKT is a metallized polyester film capacitor, MKP is a metallized polypropylene film capacitor.

Polypropylene:
This type have a very low dielectric loss. They are also an ideal alternative to polystyrene capacitors.

Polyester:
This type offer good stability, a large range of values at low cost and are the most widely used capacitor for general applications.

PhilGood

Tue Jun 19 2012, 01:18AM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Nothing weird with your question

Is it for tank capacitors ?

afaik, polypropylene are better because of their self-healing capabilities.

Unleashed

Tue Jun 19 2012, 01:31AM

Registered Member #5171 Joined: Tue Jun 05 2012, 11:32AM
Location:
Posts: 67

It's not for tanking(i guess) but for resonant caps.
A friend told me that a MMC would work better.
Anyway i prefer MKT if it's ok as well.. because they have in the near shop.

zzz_julian_zzz

Tue Jun 19 2012, 01:40AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Are you going to build an SSTC or DRSSTC? I dont think that the capacitor on the primary side is there for resonance tuning of the system, so you probably go for an SSTC.?. (also, the guide you posted is an SSTC but with Capacitor on prim) MKP stands for metallized polypropelene film, MKT is for metallized polyester film. Now, for your application with high current flowing across caps, you might wanna a higher current handling capability capacitors(MKP) , polyester can work also, in performance output they not too much differential, but the polyester will suffer more because they are not normally designed for higher current pulsed apps like DRSSTC. -since its an SSTC, the current is not that high(400+ Amps), the current depends on the Fres, and ON-time from the interrupter.

I personally think you can make it with MKT(with right capacitance value), but you have to at least measure the current on the prim which flows on ur caps, and then check on the Datasheet if it's covered on the rating and also check for thermal rise.

EDIT: Also, i re-checked the bridge schematics and there seem to be a lacking component on it which is very very critical (especially when it is ur first time to build) The snubber capacitor. This capacitor catches the transient voltages made form the inductance on your layout on the bridge,, You can have 5 uf /400V MKT connected on the + and - of the C7 on this schematic

But put it as close as you can on the Irfp460 mosfets. You might want to also add a little more resistance on the gate of the mosfets, to 10 ohms, this will add clearance from the OFF time form the previous alter pulse from other side mosfet pair before the other pair turns On and conduct. THanks.

@ Unleashed, What do you mean by not tanking and only for resonant caps? THanks

Unleashed

Tue Jun 19 2012, 02:13AM

Registered Member #5171 Joined: Tue Jun 05 2012, 11:32AM
Location:
Posts: 67

i guess that the 0.68uF caps on the bridge schematic are not for tanking, but for resonance isn't that correct?
If not, please forgive me, it's the first time i build a tc.
Anyway, what you mean with snubber capacitor?

zzz_julian_zzz

Tue Jun 19 2012, 03:04AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

As I have indicated on my earlier post, The snubber capacitor "snubs" high voltages in transient states that can cause destruction of your Half bridge switches (MOSFET) due to over voltage. This voltage varies from 2x to 4x of your system voltage(330vdc).

Also, energize the Bridge using a Variac for soft start, or If you dont have one, parallel atleast 5 20 Watts/ 500 Ohms resistor and series it on the (+) of C7 after the bridge diode.

Ben Solon

Tue Jun 19 2012, 04:49AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

The capacitor in a sstc is only for rc blocking purposes, and will not carry any large currents, so either should do fine. Snubbers or tvs diodes may help catch any user mistakes, but when a sstc runs normally, the avalanche capabilities of the irfp450 will stop any small blips.

zzz_julian_zzz

Tue Jun 19 2012, 08:31AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Yap, but it would be better to have one of those snub caps since the USER is on his first time.

Ash Small

Tue Jun 19 2012, 09:58AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

It looks to me like the 0.68uF caps C8 and C9 are 'series resonant' caps. 'Parallel resonant' caps are 'tank caps' (as far as I'm aware), because, with the inductor (primary) they form a 'Tesla tank circuit'. If I'm not mistaken, the current is much higher in a tank circuit (parallel resonant) than in a series resonant circuit, because the voltage is higher (the voltage rises in a tank circuit).

I'd try the polyester caps, as you can get them locally, and see if they get hot, but instead of using two MKT caps in parallel, I'd try eight, in four strings of two caps each, which will give the same capacitance (1.36uF), but will spread the current over eight caps, instead of just two.

(Someone please correct me if I've made a mistake)

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