Relay Driver Network

Discussion in 'General Electronics Chat' started by Narwash, Apr 8, 2013.

  1. Narwash

    Thread Starter Member

    Jun 27, 2012
    Hi all,

    I'm trying to drive a Teledyne 732TN-5 Relay:

    It has an internal driver, so I only need 3.5V @ 1.5mA. The problem is that I also want undervoltage detection, so I'm using the 8211 undervoltage detector from Maxim Integrated:

    My understanding of the MAX8211 chip as is follows: when the threshold pin drops below the 1.5V internal reference, the output of the MAX8211 chip sinks up to 7mA of current until approx 0V is achieved at the output pin. Otherwise, the output is left floating. The resistor network is set up so that when the battery drops below the set minimum voltage, 5V in my case, the voltage at the threshold pin is 1.5V.

    I've attached a schematic of my attempt at a relay driving circuit that is dependent on the output of the MAX8211 chip. The idea is that I create a relatively constant current source using a FET and limit the current drawn from the battery to 7mA, the maximum current which can be sunk by the MAX chip.

    I then use a second FET, whose drain is attached to the output of the MAX chip, to drive the base of the internal driver of the relay (pin 10).

    Datasheets of the respective FETs:

    The problem is, I can't seem to get enough voltage at the output of the whole thing. I'll get around 5-6 mA of current, but only about 1.2V at the source of the second FET. It seems like an easy enough problem and I may of just selected the wrong FETs. I've been hitting my head against the wall for a couple days, however, and I was wondering if there was an obvious or stupid thing I'm missing.