Control Circuit For Cooling Fans

Abstract

A control circuit for an electrically driven cooling fan has a relay for switching the fan in and out. Associated with the relay is resistance means for reducing the differential between the on and off points of the fan.

Description

This invention relates to a control circuit for an electrically driven cooling fan.

A control circuit according to the invention comprises in combination a resistor the value of which is dependent in use on the temperature of an apparatus cooled by the fan, a relay winding having a contact which when the winding is de-energised serves to operate the fan, a control circuit responsive to the value of said resistor for energising the relay winding when the temperature of the apparatus falls to a predetermined level, and resistance means which is coupled to the control circuit by way of the relay contact when the relay winding is energised, the resistance means reducing the required increase in the temperature of the apparatus before the relay winding is again de-energised to cause the fan to operate.

The accompanying drawing is a circuit diagram illustrating one example of the invention.

Referring to the drawing, a road vehicle incorporates a battery 9 having its negative terminal connected to an earth supply line 10, and its positive terminal connected to a supply line 11 through a series circuit including the vehicle ignition switch 8, diode 30 and a resistor 13. The road vehicle is driven by an internal combustion engine indicated at 17, the engine being cooled by flow cooling water within a conduit 16.

Located within part of the conduit 16 is a temperature-sensitive-resistor 15 one end of which is connected to the line 10, and the other end of which is connected through a temperature gauge 14 to the line 11. A junction of the gauge 14 and resistor 15 is connected through a resistor 24 to the line 10, and a variable point on the resistor 24 is connected to the base of an n-p-n transistor 25, the emitter of which is connected to the base of an n-p-n transistor 26 having its emitter connected to the line 10. The transistors 25, 26 have their collectors connected to the line 11 through a relay winding 22 bridged by a freewheel diode 31, and the winding 22 serves when it is energised to move a contact 23 out of engagement with a contact 21 into engagement with a contact 28. The contact 28 is connected through a variable resistor 29 to the base of the transistor 25, and the contact 21 is connected to the positive battery terminal through a fan motor 19, which when it is operating drives a fan 18 providing cooling air through a part 16a of the conduit 16.

In operation, when the engine temperature is low, the resistor 15 has a high value, and current flowing through the gauge 14 turns on the transistors 25, 26 energising the winding 22 so that the contact 23 moves into engagement with the contact 28 and the fan motor 19 is not energised.

When the engine temperature is high, the resistor 15 has a low value, and the transistors 25, 26 do not conduct sufficiently to energise the winding 22, so that the contact 23 is in engagement with the contact 21 and the fan motor 19 is energised.

Assuming that the fan motor 19 is operating then as the temperature of the resistor 15 falls, an increase in base current flows to the transistor 25, until at a predetermined temperature, the transistors 25, 26 conduct sufficiently to energise the winding 22, thereby moving the contact 23 back into engagement with the contact 20a and stopping the motor 19. However, the contact 23 now couples the base of the transistor 25 to the line 10 by way of the resistor 29, so reducing the base current of the transistor 25, and reduces the current flowing in the winding 22. This reduction is insufficient to de-energise the winding 22, so that the contact 23 remains in engagement with the contact 28.

As the fan motor 19 is not now operating, the temperature of the resistor 15 increases, and the current flowing to the winding 22 decreases, until at a predetermined temperature the winding 22 is de-energised, and the contact 23 moves into engagement with the contact 21 again to operate the motor 19.

It will be appreciated that the relay 22, 23 has the usual property in that the current requires to energise the winding 22 is greater than the current at which the winding 22 becomes de-energised. In effect, the resistor 29 serves when the winding 22 is energised to decrease the current flowing through the winding 22 by an amount less than the differential between the turn-on and turn-off currents of the winding 22.

The diodes 30, 31 are incorporated to protect the transistors 25, 26 against transients. The settable resistors 24, 29 determine the operating points of the fan 19.

Claims

I claim:

1. A control circuit for an electrically driven cooling fan, comprising in combination a resistor the value of which is dependent in use on the temperature of an apparatus cooled by the fan, a relay winding having a contact which when the winding is de-energised serves to operate the fan, an amplifier coupled to said resistor and responsive to the value of said resistor, said amplifier having an output decreasing with rise in temperature of the apparatus whereby when the temperature of the apparatus falls to a predetermined level the amplifier output energises said relay winding, and resistance means which is coupled to the amplifier by way of the relay contact when the relay winding is energised, the resistance means serving when the relay is energised to reduce the current flowing in the relay winding by an amount insufficient to cause the relay winding to become de-energised, so reducing the required increase in the temperature of the apparatus before the relay winding is again de-energised to cause the fan to operate.