Latching relay driver

Hi Srikanth,

The ULN2003V12PWR cannot drive a latching relay, but can drive normal relays.

The reason is that latching relays require the voltage to be one direction to latch and the opposite direction to unlatch. The ULN2003V12PWR does not have the ability to reverse the direction.

There are many devices in the DRV88xx family that can be used to drive latching relays. Any brushed motor driver that meets the voltage and current specifications should do.

Hi,

ULN2003V12PWR may not have the ability to reverse the direction of voltage. But I can connect the end of relay coil to OUT1 and OUT2 and make only one of the OUTs high and the other low. A similar circuit is given in Figure 7 of datasheet ().

Won't the above combination work for a latching relay coil?

Hi,

Please refer to the below link:

http://m.eet.com/media/1126690/11142-figure_1.pdf

I want to build a similar circuit with ULN2003V12PWR without 47 ohm and 4.7 ohm resistors in the above circuit (and directly connect OUT1 and OUT2 to relay coil).

Please advise.

Regards

Srikanth

Hi Srikanth,

A circuit like what you show in the second link can be used, as long as the current and voltages specifications of the relay coil are met.

In order to close a latching relay, a specified amount of current is pulsed through the coil in one direction. To open the latching relay, the current is pulsed through the coil in the opposite direction.

So you need a voltage source on both sides of the coils. This voltage is not supplied by the ULN2003, and comes through the resistors shown in your second link. The resistors are there to limit the current from the supply when there is a direct path from the supply through the resistor to OUT.

Let's assume the latching relay to be used requires 50mA @5V to operate. This means the coil resistance is 100 Ohms. Many latching relays have a set voltage spec (the one I looked at was 75%). With this information, you can place a 20 Ohm resistor to 5V at each coil. This works out to 4V across coil when activated.

The 20 Ohm resistor on the output being pulled low results in an extra 250mA, but this is a temporary condition. Once the latch is opened or closed, the output can be turned off.

Depending on the duty cycle of the relay, this solution may work. If the duty cycle is too high, your system efficiency may benefit by using a full H bridge.