BISTABLE RELAY DRIVER （2）

Published:2011/8/11 21:29:00 Author:Phyllis From:SeekIC

By B. Chiron

What to take into account

- To begin with, there’s the minimum drive signal length, which should not be confused with the relay’s energizing time. We’re looking at the time needed to make the relay change over to its new position and remain in that state. If this time is specified in the datasheets, it is usually called ’min. signal width’ or ’pulse length’. Lacking this information we may use the ’set time’ and ’reset time’ specifications and add a safety margin.

- Minimum switching voltage: ’must operate voltage’, thus is the lowest voltage at which the relay is guaranteed to toggle. It is essential to check whether the drive voltage reaches this level during the minimum pulse duration.

- Maximum switching voltage: just as with a classic relay, this voltage may not be exceeded.

- With regard to isolation and contact ratings, the same applies as with traditional relays. If a relay is used as an isolator for the sake of electrical safety in a circuit, then a lower coil resistance results in better isolation properties.

- Distance to other relays: some manufacturers specify a smallest distance to other relays in order to minimize mutual disturbances.

- Maximum energizing time: some relays do not withstand continuous driving, having been optimized for the duration of the switching pulse only. With this type of Relay, pay special attention to an accurately defined switching pulse.

Some really thrifty control circuits

Relay drive with hold by way of switching contacts (Figure 1)




Principle

When a logic High level is applied, Tl will conduct and current will flow through coil ’SET’. As soon as the relay pulls in, Tl blocks, cutting the relay coil current. When the control signal goes logic Low, the same happens with T2 and the ’RESET’ coil.

Advantages

- Excellent for equipment that needs to be turned on, even in the case of a malfunction or a slowly rising supply voltage.

- Suitable for a 5-V supply.

- No capacitor required.

Disadvantages

- One relay contact is sacrificed for the control method.

- Isolation sets higher demands to board layout.

- Depending on the relay type used, the relay switching time may be shorter than the prescribed minimum switching time, hence a risk of intermittent
switching is created.

- The position of the contacts when powering off depends on the decay time of the supply voltage.
Adaptations

- Add energy buffer for switch-off phase.

- Add bridge rectifier for single-coil variant.

- Delay switching contacts to increase switching time.

Relay drive with a series capacitor (Figure 2)




Principle

At an input voltage of 12 V, CI is charged, causing a brief current through the coil so the relay toggles. Next, the current remains limited to level determined by Rl. If the input voltage drops abruptly to 0 V, the capacitor discharges via the relay coil with a reversed current, causing the relay to change to the reset state.

Advantage

- Simple structure for applications with clearly defined
switching voltages.

Disadvantages

- Short pulses at the input may charge or discharge CI without the relay switching (for example, while the supply is switched on or off).

- Functions badly at switching voltages of just 5 V.

Adaptations

- Schmitt trigger to be added when faced with slowly rising or dropping supply voltages.

- Modification possible for 2-coil relays.

- Possibility to add an energy wise control input.

The DRT5 driver

The Quasinil company offers a series of driver ICs specifically designed for driving bistable relays. The ICs are best described with the aid of a few examples. In the first of these, the basic operation is explained, while the second allows us to benefit from one of the most interesting and worthwhile characteristics: low-energy drive. A third example, finally, shows an example illustrating guaranteed switching levels.

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