US3407282A

US3407282A - Electric heating pad

Info

Publication number: US3407282A
Application number: US508280A
Authority: US
Inventors: August E F Fickweiler
Original assignee: Van Der Heem BV
Current assignee: Van Der Heem BV
Priority date: 1965-11-17
Filing date: 1965-11-17
Publication date: 1968-10-22
Anticipated expiration: 1985-10-22

Description

Oct. 22, 1968 A. E. F. FICKWEILER 3,407,282

ELECTRIC HEATING PAD Filed Nov. 17, 1965 INVENTOR. AUGUST E. F. FI'CKWEILER- United States Patent 3,407,282 ELECTRIC HEATING PAD August E. F. Fickweiler, Waddinxveen, Netherlands, as-

signor, by mesne assignments, to Van der Heem N.V., a corporation of the Netherlands Filed Nov. 17, 1965, Ser. No. 508,280

1 Claim. (Cl. 219519) The invention relates to an electric heating pad with a heater cable, comprises a central resistance core and a coaxial safety sheath, in which the resistance core is connected to the supply voltage through the normally open contacts of a relay and the heater coil and the normally closed contact of a bimetal switch, whilst the relay coil is connected to a relay supply voltage through the safety sheath and a resistor.

Such a heating pad must meet several safety requirements, i.e. that the supply voltage must be disconnected as soon as overheating of the heater cable threatens. Such an overheating can e.g. be caused by an interruption in the resistance core, in which case sparks can occur at the place of the interruption, which sparks can cause an inadmissibly strong local heating. In a known device, the relay energization is discontinued in that case, because the insulating substance between the resistance core and the safety sheath obtains a lower resistance due to the marked heating, whereby a partial short-circuit between the resistance core and the safety sheath occurs. The safety sheath is so connected in the energization circuit of the relay, which is connected to the supply circuit for the resistance core, that such a partial short-circuit leads to de-energization of the relay. It is a disadvantage of this device, that a very special insulating material between the resistance core and the safety sheath is required, whilst the operation is rather critical and in practice turns out not to be reliable under all conditions.

It is an object of the invention to provide an electric heating pad, in which an interruption of the resistance core leads to immediate disconnection of the heating pad, without necessitating the use of a special insulating material between the resistance core and the safety sheath.

It is a feature of the invention that the primary winding of a current transformer is connected in the series circuit of the resistance core, whilst the secondary winding of the current transformer is connected in series with the normally closed contact of the bimetal switch and together therewith constitutes the source of relay supply voltage.

These and various other objects, features, and advantages of the invention will be better understood from the following description taken in connection with the accompanying drawing, which shows a circuit diagram of an electric heating pad according to the invention.

In the drawing, the heater cable comprises a central resistance core 1 and a coaxial safety sheath 2, which by means of a suitable insulating material is insulated with respect to the central core 1. These components are the only components which are arranged in the heating pad itself, whilst the remaining components are all parts of the automatic controller for the heating pad.

The resistance core 1 is connected to the supply terminals 8 and 9 through the primary winding 3 of a current transformer, the normally closed contact 4 and the heater coil 5 of a bimetal relay and the normally open contacts 6 and 7 of a safety relay. As soon as the relay is operated by external influence, the normally open contacts 6 and 7 are closed, so that the resistance core 1 is energized. The current through the primary winding 3 of the current transformer causes a voltage across the secondary winding 10, which voltage causes a current through the resistor 11, the rectifier 12, the safety sheath 3,407,282 Patented Oct. 22, 1968 2, a resistor 13, the relay coil 14 and the heater winding 5 and the normally closed contact 4 of the bimetal switch, wherebythe relay coil 14 is energized and the normally open contacts 6 and-7 of the relay remain c1osed.- The current through the resistance core 1 not only causesa heating effect of the heating pad, but also of the heater winding 5 of the bimetal switch, so that after. some time the normally closed contact 4 thereof is opened. In that case, the current through the resistance core 1 is discontinued, so that the heat generation in the heating pad is discontinued and the current through the primary winding 3 of the current transformer is discontinued, so that the voltage across the secondary winding 10 thereof is also discontinued, but the relay coil 14 remains energized, since the full mains voltage is now present across the opened contact 4 of the bimetal switch, so that this voltage takes the place of the initial voltage across the secondary winding 10 of the current transformer.

After some time has elapsed, the bimetal switch has cooled down so far, that its normally closed contact 4 is again closed and the resistance core 1 is again energized. The interval during which the normally closed contact 4 remains opened is shorter as the ambient temperature of the bimetal switch is lower and its rate of cooling is consequently higher. Thereby, the resistance core 1 is energized during a larger part of the time, as the ambient temperature of the bimetal switch, that is the room temperature, is lower, so that the heat generated in the heating pad automatically adapts itself to the room temperature.

Disconnecting the heating pad can be effected by bring ing the relay in the inoperative condition mechanically, by interrupting the current supply by means of a separate switch or by interrupting the current through the relay coil 14, e.g. by short-circuiting this relay coil by means of the shown push-button 15 with normally open contacts.

The

resistors

11 and 13 can be so chosen, as is known per se, that the energization of the relay coil 14 is disrupted as soon as due to a mechanical damage of the heater cable, a short-circuit between the resistance core 1 and the safety sheath 2 occurs. The

resistors

11 and 13 are so chosen, that touching the safety sheath 2 due to a fault of the surrounding insulation cannot lead to dangerous consequences, whilst in that case too, the energization of the relay coil 14 is discontinued and the heating pad is disconnected. Although it would basically be possible to switch the heating pad on by temporarily bridging the normally open contacts 6 and 7 of the relay by means of a push-button switch with normally open contacts, this is not very well feasible in practice, since the contact distance of the opened contacts 6 and 7 must be fairly large for safety reasons, so that a very strong energization of the relay coil 14 would be necessary for having the relay attract its armature. This heavy energization current cannot usually be obtained if the

resistors

11 and 13 are chosen in conformity with the requirements set forth above, so that it will generally be necessary to have the switching-on effected by mechanical actuation of the relay. The capacitor 16 serves for smoothing the direct voltage which is supplied to the resistor 13 in series with the relay coil 14.

If an interruption of the resistance core 1 should develop whilst the normally closed contact 4 of the bimetal switch is opened, the voltage across that contact is interrupted, so that in that case too, the energization of the relay coil 14 is discontinued and the normally open contacts 6 and 7 are opened, whereby the resistance core 1 cannot be switched on again.

What isclairned is: r V a series circuit of the safety sheath, the relay coil and the 1. A safety system for an electric heating pad comsecondary winding of the transformer.

prising a heater cable having a central resistance core and a coaxial safety sheath, 2. safety relay having at least a nor- References Cited mally open contact and a coil, a temperature responsive 5 UNITED STATES PATENTS switch having a normally closed contact, an electrical supply source, a transformer, the resistance core cOn- 2914645 11/1959 Wallace 219-519 nected to the supply source through the normally open relay contact, the primary of the transformer and the BERNARD GILHEANY Pnmary Exammer normally. closed temperature responsive contact, an en- 10 BELL, Assistant Examiner.

ergy source for the coil of the safety relay consisting of

Claims

1. A SAFETY SYSTEM FOR AN ELECTRIC HEATING PAD COMPRISING A HEATER CABLE HAVING A CENTRAL RESISTANCE CORE AND A COAXIAL SAFETY SHEATH, A SAFETY RELAY HAVING AT LEAST A NORMALLY OPEN CONTACT AND A COIL, A TEMPERATURE RESPONSIVE SWITCH HAVING A NORMALLY CLOSED CONTACT, AN ELECTRICAL SUPPLY SOURCE, A TRANSFORMER, THE RESISTANCE CORE CONNECTED TO THE SUPPLY SOURCE THROUGH THE NORMALLY OPEN RELAY CONTACT, THE PRIMARY OF THE TRANSFORMER AND THE NORMALLY CLOSED TEMPERATURE RESPONSIVE CONTACT, AN ENERGY SORUCE FOR THE COIL OF THE SAFETY RELAY CONSISTING OF A SERIES CIRCUIT OF THE SAFETY SHEATH, THE RELAY COIL AND THE SECONDARY WINDING OF THE TRANSFORMER.