04-04-2020, 06:35 PM
(This post was last modified: 04-04-2020, 06:35 PM by Mike Watterson.)
As a rule of thumb decide what the peak AC voltage is and pick a DC rating a little higher.
So a capacitor on the anode of a valve with a transformer load might need HT x 2 plus a safety margin.
I did work in QA in a capacitor factory for while. The capacitors were tested with static DC and sometimes -55 to +125 °C. For some applications the DC voltage was twice rated and for an hour.
Pulse rating is complicated and depends on dielectric material, metallised or metal foil, aluminium or silver, frequency /power/duration, if the material/construction is self healing, goes open circuit or short.
See also capacitors rated X (across mains), Y (Mains to "Earth" or chassis) and some usually only ceramic types are X Y.
Also ceramic have both widely different dielectric types (temp co, piezo effects, K (how much times vacuum capacitance is multiplied, voltage rating per mm, loss at higher frequencies). Also ceramic can be multilayer which is much lower inductance than a rolled foil type). The thickness of the metallisation and area of the dielectric affects power handling.
I made a tuned 1 MHz 20 W SMPSU once and was surprised to see the ceramic capacitor suddenly glow and explode into sand. A high power class D amp needs carefully chosen caps on the first stage of the filter.
Capacitors have more range of values, materials, voltage and power handling maybe than anything? Printed coils do exist. But printed capacitors are common and at high frequencies the connection is to a point of a cheese triangle shape.
Film caps can be metallised plastic or using separate aluminium foil. Common materials are: Polystyrene, PTFE (Teflon), Polycarbonate and the most common is Polyester = Mylar (Magnetic tape and photo film) = PET (as in bottles).
Paper is rubbish unless in hermetically sealed cans. "Oil", "Wax" is all usually just variations of paraffins. Candle is a Paraffin, as is diesel and kerosene. They all gradually absorb water!
Electrolytic types are using a chemically created thin insulating film on the tantalum or aluminium, which is why reversal is catastrophic. It's why they go can high ESR with age or have high leakage and why controlled reforming often works. Like a lead acid car battery, the polarity can be reversed on some Aluminium types if you are careful and know how. Not recommended.
So a capacitor on the anode of a valve with a transformer load might need HT x 2 plus a safety margin.
I did work in QA in a capacitor factory for while. The capacitors were tested with static DC and sometimes -55 to +125 °C. For some applications the DC voltage was twice rated and for an hour.
Pulse rating is complicated and depends on dielectric material, metallised or metal foil, aluminium or silver, frequency /power/duration, if the material/construction is self healing, goes open circuit or short.
See also capacitors rated X (across mains), Y (Mains to "Earth" or chassis) and some usually only ceramic types are X Y.
Also ceramic have both widely different dielectric types (temp co, piezo effects, K (how much times vacuum capacitance is multiplied, voltage rating per mm, loss at higher frequencies). Also ceramic can be multilayer which is much lower inductance than a rolled foil type). The thickness of the metallisation and area of the dielectric affects power handling.
I made a tuned 1 MHz 20 W SMPSU once and was surprised to see the ceramic capacitor suddenly glow and explode into sand. A high power class D amp needs carefully chosen caps on the first stage of the filter.
Capacitors have more range of values, materials, voltage and power handling maybe than anything? Printed coils do exist. But printed capacitors are common and at high frequencies the connection is to a point of a cheese triangle shape.
Film caps can be metallised plastic or using separate aluminium foil. Common materials are: Polystyrene, PTFE (Teflon), Polycarbonate and the most common is Polyester = Mylar (Magnetic tape and photo film) = PET (as in bottles).
Paper is rubbish unless in hermetically sealed cans. "Oil", "Wax" is all usually just variations of paraffins. Candle is a Paraffin, as is diesel and kerosene. They all gradually absorb water!
Electrolytic types are using a chemically created thin insulating film on the tantalum or aluminium, which is why reversal is catastrophic. It's why they go can high ESR with age or have high leakage and why controlled reforming often works. Like a lead acid car battery, the polarity can be reversed on some Aluminium types if you are careful and know how. Not recommended.
