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Car Relay Driving Circuit Techniques

These two projects (car relay driving circuit techniques) both address the same problem but in very different ways.

The problem that both designs address is where multiple relays are used in an enclosed space specifically inside a car relay box control panel.  When driving relays the current in the armature causes heating and with a large number of relays in a small space can cause unacceptable heat increase.

The key point about the relays is that they have both a pick up or turn on current and a holding current (the current required to keep the relay closed) which is lower than the pick up current.

In normal designs it may not be critical to use only the holding current but to reduce heating this is the most important action to be undertaken.

The usual way of driving relays is to individually drive the armature using a transistor connected to the armature and from there to the power supply without worrying about holding current.  

To create a low holding current driver you would need to reduce the transistor current flowing after having ensured that the pick up current has had effect i.e. to drive it long enough so that the relay closes.  One method of doing this is to use two control pins from a microcontroller one for the pickup current and one for the holding current to drive the transistor base current at different levels.  

This would however mean using a large amount of control pins (30 for 15 relays) which would infrequently change state and would waste the resources of a microcontroller.  

The designs below both automatically adjust themselves to the current requirements made on the armature and both alter that current when a new relay is activated.  In addition they both revert to low current holding mode and this is all down without any microcontroller in sight i.e. very elegant solutions.

Additionally they allow multiple relays to be added into the circuit with out altering the basic design of the system i.e. You don't have to add a new drive circuit for each relay added.

The first one takes a process and control approach where a logic circuits and fixed timers are used to current boost the relay driver current using an impulse drive.

Multiple Relay Drive Control Circuit Design 1

The second one takes an analogue circuit design approach where a current sensing resistor and opamp  is used to create a RC timed pulse that boosts the current drive for a short time.

Multiple Relay Drive Control Circuit Design 2

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