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Request EE advice, please

Anthony P

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I am attempting to construct a home-built capacitor for a dye laser flash lamp. I am using 5mil Al sheets 10"x 15" active area and 12"x18" 5mil mylar. I assembled and clamped 40 layers together. Capacitance read 0.74uF (great). Voltage will be 20kvdc.
The problem is that the resistance is not infinite. As a good capacitor should, resistance starts at "short" and quickly climbs... but only to about 3megs. When I test commercially made HV caps, the resistance reaches infinity (200meg meter limit). When connected to HV supply, it does not build up a charge or store one.
I expected it to be "leaky" but was hoping submerging in oil would prevent this. I have not yet tried the oil as I wanted to do as much testing as possible before making that mess.
Any advice would be greatly appreciated.
 



Cyparagon

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Are you holding the test leads with your fingers during this test? Did you clean the mylar prior to assembly? Try peeling the layers apart slowly and see if/how the resistance changes.
 

Anthony P

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Thank you for your reply, Cyparagon. I am no EE.
The test leads were clipped on. Metal and mylar were cleaned as I went. I did disassemble the whole thing layer by layer. Resistance climbs to infinity( over 200Meg) until I get to about 10 layers. I have just re-assembled with double mylar layers. At 40 layers I read a capacitance of 380nF, and resistance climbs very, very slowly(10min or more) to 8.4 meg... still not infinity. Have not placed in oil yet.
I have read that these types of caps tend to be "leaky". I wonder if leaking vs charging from ohms test is reaching an equilibrium point. Further I wonder If the oil bath will stop or reduce leakage.
I will be ready for HV test and oil this weekend.
As a plan "B", I ordered a Maxwell 20kv 1uF from e-bay. I wish they had posted their add a month ago, before I started.
 

Cyparagon

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I don't see how oil will help here, although I am possibly missing something. If your dielectric is 3Meg, placing an infinite resistance in parallel with it changes nothing. You would need to put the oil in "series" with the existing dielectric, which would physically mean separating the metal layers further with the oil. I don't think that's how these are constructed.

As you probably know, 20kV on 8 megs is 50 watts. That's several orders of magnitude over what is acceptable for a capacitor. Sounds like you need a better dielectric.
 

Anthony P

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I don't see how oil will help here, although I am possibly missing something. If your dielectric is 3Meg, placing an infinite resistance in parallel with it changes nothing. You would need to put the oil in "series" with the existing dielectric, which would physically mean separating the metal layers further with the oil. I don't think that's how these are constructed.

As you probably know, 20kV on 8 megs is 50 watts. That's several orders of magnitude over what is acceptable for a capacitor. Sounds like you need a better dielectric.
I agree. There are only 2 parts: The conductor and the insulator. It will not accept a hv charge at all. Acts like a short on the power supply. Even if it was some leakage situation I should still be able to get some voltage into it. I will start researching other dielectrics. I really thought mylar was a good choice.
Thank you for your help. I will let you know how it goes.

EDIT:
To clarify, a single layer of mylar reads infinite resistance from metal plate to plate on either side. When stacked more than about 10 layers, resistance decreases. My thoughts were that charge was bleeding off from the edges of the plates and contacts. I will rebuild with laminated polyester pouches for each layer.
 
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