Capacitor failure

[Wendymott]

Hi Peeps...I am building a Higher power SSB PA supposedly 180W but probably nearer 130W
I made a 3 band Low pass filter using T68-6 torroids... see circuit attached. I only tested it with a lashed up PSU on 80M, into a 100W 50R dummy load. With a single tone I was getting some 60W, and using a dual tone was getting a very nice non distorted output. However when I audio modulated (mike) I was getting peaks of 120W and a "POP" and blue smoke...
A Cap.. circuit ref C2 1no had failed. As I had no idea what the voltages might be I bought 1 KV rated caps from ebay (uk supplier) but probably sourced from China. Looking at the Minikits LP Filter they use 630V SMD caps, so I thought that 1KV would be ok.
In your honest opinions..should I uprate to 2kv, or put it down to the caps not being honestly rated.
I have used leaded caps as SMD was not necessary, so I am sure there are peeps out there that will know .

[G6Tanuki]

If the failed cap is in the multiple-Pi-section LPF I really doubt the issue is one of voltage rating.

I'd say it is more a current-rating issue.

120W in a 50-Ohm-impedance environment - plus modulation - that's several Amps of circulating RF.

I would be looking to achieve the needed capacitance by wiring multiple lower-value caps in parallel so the circulating RF current is distributed between them.

[Radio Wrangler]

Two companies have reputations for making ceramic capacitors that don't flash up glowing red and fail in high power RF applications... ATC (American Technical Ceramics) and Dielectric Labs inc. It all comes down to current rating at RF frequencies

Other capacitor makers did RF power specials over the years but these are lost amidst mundane types.

Expect them to be more than reassuringly expensive

I put 300W pulses at 1GHz through 0805 size SMT jobs. £12 in singles from RS.

As Tanuki says, split the capacitance into a load of parts in parallel to share current and keep the 1kV rating for each of them if you want to use non-special parts.

David

[dsergeant]

What type of capacitor? In a high-ish power low pass filter ceramic caps are probably unsuitable. As G6TAN says the issue will be high circulating currents. Years ago when I was playing with a resonant loop antenna on 136kHz where the tuning components had 20A or so of RF circulating in them I totally destroyed a few caps and needed to use polystyrene capacitors from the Philips 376 series (see http://davesergeant.com/loops.htm). Random components from eBay will probably not be up to the job.

73 Dave G3YMC

[Wendymott]

Thanks David and Dave and Tanuki... So I wonder how Minikits get away with it...I have not driven the Minikit filter, as it was dis assembled as a "bought for looking at".
I will rebuild the bought one and see where the flames come out of that Hi Hi..
I will re design the filter pcb as a "one filter" jig and test it with multiple caps to share the load.
I will also purchase a number of lower values of caps from ebay and test in situ.

[Wendymott]

Hi Peeps... I bought a series of different values and now.. in the 80M filter C1 = 2 X 100pf + 1x56pf.. C2 / C3= 4X220pf + 100pf, C4 = 2 X 100pf + 22pf... making a stand alone test pcb.. fingers crossed..

[Wendymott]

Tests went fine... No smoke or pops... thus new pcb made to accommodate the new values.. Thanks for the inputs.