[AMRadio] HV Capacitor question

Jim Candela jcandela at prodigy.net
Sat Jul 22 08:10:08 EDT 2017

I received the caps from Mouser, .033uf 3KV, Polypropylene, axial package. All three measure .032uf exactly with a Fluke 87 DVM. Switch to the 0-200mv range, and I read 0.00mv!! If I heat the cap body it stays there at 0.00mv. If I heat either end, I get about 2mv when HOT, and the polarity depends on which side I get hot. This is most likely because each lead is crimped inside causing a thermocouple from dissimilar metals being joined. This is the same effect I've seen with metal film resistors. I need to try these caps in the actual circuit mentioned.
I have another cap story. Here in central Texas the weather has been HOT. My home AC system is about 17 years old, and has had trouble keeping up with the heat. I noticed that the blower motor in the air handler closet has been slowing down, and when that happens, the evaporator coils are prone to icing over. Some years ago I had a bad motor running capacitor (it was open, and bulging), and i was able to find a replacement on a Saturday. It was a 10uf 370vac can type. I measured the cap earlier this week, and it was close to 11 uf with my DVM on CAP range. Desperate, I bought another from Granger for $5. No change! Dam!! Already got a quote...$6500 for a new system.... :-(
Remembering this thread, and rooting in the garage, I unearthed a 10uf 250vdc Wima brand polypropylene capacitor...With capacitors like this the AC rating is usually 60% of the DC rating, so a 250vdc cap would be a 150vac AC cap. That is a long way from 370vac!! So what could happen? It could short out and cause a fire? I added an inline 5A fuse, and then I try it.....Guess what, the air handler roars to life at its more youthful vigor, and the house is noticeably cooler as the vents are blowing a larger volume of air. Last night, 104 outside, the AC was cycling off and on holding the setpoint of 75. With the can cap, the AC would stay on for hours as the temp inched up 75, 76, 77, 78...
I think my fan motor is failing, and the lower ESR of the polypropylene capacitor enables the old motor to behave itself. That capacitor is cold..no heating.

      From: Jim Candela <jcandela at prodigy.net>
 To: sr2000 k9axn <hallicrafterssr2000 at k9axn.com>; "AMRadio at mailman.qth.net" <AMRadio at mailman.qth.net> 
 Sent: Tuesday, July 18, 2017 1:25 PM
 Subject: Re: [AMRadio] HV Capacitor question
"Take a look at Notes 1, 2, and 3"
Some good reading for sure. On the Central Electronics 20A the 6BA7 mixer B+ bypass cap (C73) is usually a .005uf Silver Mica. Change that out with a ceramic Z5U, and the gain drops a bunch, especially on 40m.The mixer uses a high Q tuned circuit, and the RF circulating current passes through that cap. I once tried an equal value polypropylene, and the gain was too high!! I had to put about 2 ohms in series with it to make the circuit behave. Those polypropylene's are low ESR caps with good high frequency capability.
Back to my electrostatic clamp controller, I just ordered some poly caps from Mouser. The value is .033uf @ 3KV.  Two of those should replace six of the .01uf ceramic Z5U's in there before (2 groups of 3).
My thinking is that the "bad" Z5U's that can generate up to 1 volt into 10 meg-ohm when hot just have a infinite number of series R-C networks within, and to deplete the stored charge, perhaps I can short them out and put them in an oven at 100 Celsius for several hours. Might be a fun experiment. Would also like to put a microamp meter across the cap, heat it, and see if I can achieve a reading above zero ua...Might find that I have a new form of power for a QRP rig!!  :-)

Thanks for the comments guys.

      From: sr2000 k9axn <hallicrafterssr2000 at k9axn.com>
 To: Jim Candela <jcandela at prodigy.net>; AMRadio at mailman.qth.net 
 Sent: Monday, July 17, 2017 11:37 AM
 Subject: Re: [AMRadio] HV Capacitor question

Take a look at Notes 1, 2, and 3


Kindest regards Jim K9AXN

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