• I am applying 24Vdc to an MTL7787+ to drive a 24V relay coil but it doesn’t work. Why not?

• MTL show the Safety Description of the zener barriers on the data sheet, but it only shows voltage, resistance and current. A number of field equipment certificates also show power. Do I have to confirm that the barrier has a value for power that is acceptable and how can I check this?

I am applying 24Vdc to an MTL7787+ to drive a 24V relay coil but it doesn’t work. Why not?

You are not taking into account the resistance of the barrier.
For optimum efficiency, the resistance of the relay coil should be the same as the resistance of the barrier (i.e. “matched”). However, this means that you will get only half the supply voltage (12V) at the relay coil.
You should therefore use, for example, a 12V, 400 ohm relay coil.

Rt (Total resistance) = Rb (Resistance of barrier) + Rc (Resistance of coil)

Isupply (Current flowing in circuit) = Vsupply / Rt

Vc (Voltage at coil) = Rc x Isupply.

Alternatively you could use an MTL774X product which provides simple “volt-free” contacts in the safe area to operate the relay coil.

MTL show the Safety Description of the zener barriers on the data sheet, but it only shows voltage, resistance and current. A number of field equipment certificates also show power. Do I have to confirm that the barrier has a value for power that is acceptable and how can I check this?

Yes, you have to ensure that the barrier power value is not greater than the power value given for the field equipment.
The maximum power transfer between the safe area and the field takes place when the impedance of the source is equal to the impedance of the load. This is called the Matched Power.
This is the worst case condition and so this is used to calculate the power output from a barrier. Under these conditions the power can be expressed as:-

Matched Power = Voc x Isc / 4 (W)

For a typical 24V dc barrier, MTL7728+, with safety description 28v, 93mA.

28 x 0.093 = 2.604

2.604 / 4 = 0.651W

However the power value for each channel of a barrier is provided in the Maximum Cable Parameters table for each barrier series.