GB2125219A - Moisture removal within capacitors - Google Patents

Abstract

The presence of moisture in the dielectric of capacitors will affect the capacitance and is particularly disadvantageous where circuitry is stored for long periods between use. An electrical capacitor (11) comprises electrodes separated by a dielectric material. The capacitor has a case (14) of brass sealed at one end by a glass wall (16) and filled with dessicant e.g. silica gel to absorb moisture from the dielectric material and electrodes. The case is hermetically sealed to prevent ingress of moisture. <IMAGE>

Description

SPECIFICATION Capacitors Electrical capacitors comprise, in their simplest form, a pair of electrodes and dielectric material separating the electrodes. Most dielectric materials have a certain water content which cannot be completely removed by a drying process and the presence of water affects the dielectric constant of the material and hence the capacity of the capacitor. The presence of water on the surface of dielectric material, considered microscopically, causes the surface area to increase and hence the capacitance of the capacitor which is proportional to the area of the plates, also to increase. Similarly, the presence of water within the dielectric affects the dielectric constant of the material.If a circuit including the capacitor has been tuned to a particular frequency, the effect of water within the capacitor during storage will cause the capacitance to change so that the circuit will operate on a different frequency. This is not acceptable when apparatus must be ready for immediate use on withdrawal from long-term storage without time being available for retuning every circuit.

it is possible to stop further moisture entering the dielectric material by sealing the material and electrodes from the atmosphere, but the residual water content of the material present before sealing will still affect performance.

According to the present invention, there is provided an electrical capacitor comprising dielectric material, a pair of electrodes separated by the dielectric material, sealing means for sealing the electrode and the dielectric material from the atmosphere and a dessicant within the sealing means for abosorbing moisture from the dielectric material.

An example of the invention will now be described with reference to the accompanying drawing which is a schematic representation of a sealed capacitor.

A capacitor 11 is cylindrical in shape, and has axial copper wires 12 and 13 extending from respective ends from connection to a circuit.

Although this shape of capacitor has been illustrated, it should be realized that the invention can be applied to any shape or size of capacitor.

A hermetically sealing case 14 of brass is provided for the capacitor, the wire 1 2 extending through a hole in the base of the case 14 and being soldered thereto at 1 5. The size of the case 14 is chosen to allow maximum space around the capacitor 12 within the case 14 without making ths capacitor assembly so large that it cannot be used in its intended application. The space between the capacitor 11 and the case 14 is filled with silica gell and the mouth of the case 14 is closed by a glass-to-metal seal 1 6 secured to the mouth of the case by a solder joint 1 7. The copper wire 1 3 passes through a central hole in the seal 16, the seal 1 6 providing insulation between the wires 12 and 13 which if not present would cause the capacitor 11 to be shorted out.

Although the silica gell will act to absorb moisture from the capacitor 11, it is preferred that as much moisture as possible should be driven out of the case in the first piace. The assembly, assembled as so far described, is then placed in a heated and evacuated oven so that water within the case is driven out of the case 14 and extracted by the partial vacuum. After heating, the assembly is sealed up by applying a soldered joint between the wire 13 and the glass-to-metai seal 16 at 18.

Although some moisture may still remain within the capacitor, the silica gell will absorb this moisture and avoid the moisture changing the capacitance of the capacitor 11. As it may take a little time for the moisture in the capacitor 11 to be asorbed by the silica gell, the assembly should be left to age for a few days before criticai adjustments are made depending on its capacitance. The heating and evacuation step mentioned above is not essential, but considerably decreases the ageing time required.

It has been found that capacitors according to the invention posses a better stability of capacitance by a factor of about 10 compared to those which do not contain a dessicant within the hermetically sealed case. This is true of dielectric materials such as polyesters which normally absorb water to a high degree where previously orders of stability better than 1% were difficult to obtain as well as to material such as polypropylene which have a very low water absorption coefficient.

Capacitors liable to be subject to stress are often encapsulated in epoxy resin material but this does not form a hermetic seal to keep moisture out and it is still necessary to provide a separate hermetic seal case around the resin. The epoxy resin may be formed with a cavity to contain the dessicant.

Silica gell has been mentioned as the dessicant, but any suitable material can be used.

1. An electrical capacitor comprising dielectric material, a pair or electrodes separated by the dielectric material, sealing means for sealing the electrodes and the dielectric material from the atmosphere and a dessicant within the sealing means for absorbing moisture from the dielectric material.

2. A capacitor as claimed in claim 1 wherein the dessicant is silica gel.

3. A capacitor as claimed in claim 1 or 2, wherein the sealing means comprises a brass casing, and wires are provided extending from said electrodes through the casing, at least one of said wires being insulated from the casing.

4. A capacitor as claimed in claim 3, wherein said casing has a glass end wall attached thereto.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Capacitors Electrical capacitors comprise, in their simplest form, a pair of electrodes and dielectric material separating the electrodes. Most dielectric materials have a certain water content which cannot be completely removed by a drying process and the presence of water affects the dielectric constant of the material and hence the capacity of the capacitor. The presence of water on the surface of dielectric material, considered microscopically, causes the surface area to increase and hence the capacitance of the capacitor which is proportional to the area of the plates, also to increase. Similarly, the presence of water within the dielectric affects the dielectric constant of the material.If a circuit including the capacitor has been tuned to a particular frequency, the effect of water within the capacitor during storage will cause the capacitance to change so that the circuit will operate on a different frequency. This is not acceptable when apparatus must be ready for immediate use on withdrawal from long-term storage without time being available for retuning every circuit. it is possible to stop further moisture entering the dielectric material by sealing the material and electrodes from the atmosphere, but the residual water content of the material present before sealing will still affect performance. According to the present invention, there is provided an electrical capacitor comprising dielectric material, a pair of electrodes separated by the dielectric material, sealing means for sealing the electrode and the dielectric material from the atmosphere and a dessicant within the sealing means for abosorbing moisture from the dielectric material. An example of the invention will now be described with reference to the accompanying drawing which is a schematic representation of a sealed capacitor. A capacitor 11 is cylindrical in shape, and has axial copper wires 12 and 13 extending from respective ends from connection to a circuit. Although this shape of capacitor has been illustrated, it should be realized that the invention can be applied to any shape or size of capacitor. A hermetically sealing case 14 of brass is provided for the capacitor, the wire 1 2 extending through a hole in the base of the case 14 and being soldered thereto at 1 5. The size of the case 14 is chosen to allow maximum space around the capacitor 12 within the case 14 without making ths capacitor assembly so large that it cannot be used in its intended application. The space between the capacitor 11 and the case 14 is filled with silica gell and the mouth of the case 14 is closed by a glass-to-metal seal 1 6 secured to the mouth of the case by a solder joint 1 7. The copper wire 1 3 passes through a central hole in the seal 16, the seal 1 6 providing insulation between the wires 12 and 13 which if not present would cause the capacitor 11 to be shorted out. Although the silica gell will act to absorb moisture from the capacitor 11, it is preferred that as much moisture as possible should be driven out of the case in the first piace. The assembly, assembled as so far described, is then placed in a heated and evacuated oven so that water within the case is driven out of the case 14 and extracted by the partial vacuum. After heating, the assembly is sealed up by applying a soldered joint between the wire 13 and the glass-to-metai seal 16 at 18. Although some moisture may still remain within the capacitor, the silica gell will absorb this moisture and avoid the moisture changing the capacitance of the capacitor 11. As it may take a little time for the moisture in the capacitor 11 to be asorbed by the silica gell, the assembly should be left to age for a few days before criticai adjustments are made depending on its capacitance. The heating and evacuation step mentioned above is not essential, but considerably decreases the ageing time required. It has been found that capacitors according to the invention posses a better stability of capacitance by a factor of about 10 compared to those which do not contain a dessicant within the hermetically sealed case. This is true of dielectric materials such as polyesters which normally absorb water to a high degree where previously orders of stability better than 1% were difficult to obtain as well as to material such as polypropylene which have a very low water absorption coefficient. Capacitors liable to be subject to stress are often encapsulated in epoxy resin material but this does not form a hermetic seal to keep moisture out and it is still necessary to provide a separate hermetic seal case around the resin. The epoxy resin may be formed with a cavity to contain the dessicant. Silica gell has been mentioned as the dessicant, but any suitable material can be used. CLAIMS

1. An electrical capacitor comprising dielectric material, a pair or electrodes separated by the dielectric material, sealing means for sealing the electrodes and the dielectric material from the atmosphere and a dessicant within the sealing means for absorbing moisture from the dielectric material.

2. A capacitor as claimed in claim 1 wherein the dessicant is silica gel.

3. A capacitor as claimed in claim 1 or 2, wherein the sealing means comprises a brass casing, and wires are provided extending from said electrodes through the casing, at least one of said wires being insulated from the casing.

4. A capacitor as claimed in claim 3, wherein said casing has a glass end wall attached thereto.

5. A capacitor as claimed in any one of claims 1 to 4, wherein the components have been dried before the casing is sealed.

6. An electrical capacitor substantially as hereinbefore described with reference to the accompanying drawings.