BISTABLE RELAY DRIVER （3）

Published:2011/8/11 21:51:00 Author:Phyllis From:SeekIC

By B. Chiron

Switching by logic level (Figure 3)




With Vcc = 5 V and Cde = 5 V the driver switches the relay to the ON position (10 ms pulse). When Cde returns to 0 V, the OFF state is switched on (20 ms pulse). Take into account that the driver will attempt to switch the relay to the OFF state when Vcc drops below about 3.7 V. The success of this operation is wholly dependent on Vcc.

Advantages

- Uncluttered, only one part needed.
- Suitable for 5-V powering.
- No capacitor required

Disadvantages

- Slightly lags the control signal (approx. 20 ms).
- Vcc not allowed to exceed 5.5 V.
- If Vcc drops too quickly, the relay may not make it to the OFF state.

Adaptations

- Also works with 2-coil relays.
- For a simple method of supply voltage detection it is sufficient to connect Cap and Cde (although in that case the supply voltage has to drop very slowly).

Dusk/dawn switch (Figure 4)




Principle

Direct current furnished by solar cells charges capacitor Cext. When the voltage across Cexl exceeds about 4.4 V, the relay will switch to ON. When the light intensity drops, the voltage across Cap drops also. At a level of 3.7 V, the relay switches to OFF again.

Advantage

- No battery required.

Disadvantages

- Relatively long charge and discharge times (approx. 1 minute assuming two solar cells exposed to daylight).
- Large hysteresis: much more light is required to switch the relay to ON than to OFF

Adaptations

- For poor light conditions, use solar cells that supply a higher voltage. Protect the circuit with a low-drop voltage regulator (LP2950CZ-5).
- Although the configuration may also be used with a 2-coil relay, this option is less appealing because the capacitor becomes larger and with it the charge time.
- This type of circuit (typ. current consumption 20 mA) represents a cheap alternative to an optocoupler input. Depending on the source and the length of the cable it may be necessary to protect the driver’s Cje input.
- Employ a power relay if larger currents are to be switched.

Controlling the mains supply in an USB device (Figure 5)




When the PC is switched on, capacitor Cext is charged via a current limiting resistor. Once the capacitor voltage has risen to 4.4 V, the relay follows the state of the USB microcontroller, switching to ON. When the PC is switched off, the relay returns it its initial state. In this v/ay, the supply in a peripheral device is switched synchronously with the PC.

Advantage

- OFF state guaranteed when PC is switched off.

Disadvantages

- Slightly lags the control signal (approx. 20 ms).
- Supply voltage not to exceed 5.5 V.
- Driver is relatively expensive.

Adaptations

- The relay in the example is a power relay, hence the value of the capacitor. Depending on the power to be switched and the desired degree of isolation, a small relay may be chosen. A single-coil relay allows a smaller capacitor to be used.
- If the peripheral is to receive its supply voltage together with the PC, the microcontroller is not required. Simply connect the 5 V of the USB connector to Cje and fit a 100-kfi resistor between Cde and ground.

Choice between DRT5 and DRL5

In the three above examples we discussed a few aspects of these new driver ICs. The choice of the type of driver is closely related to the application and the relay to be controlled. Below is a short discussion of the parameters for these drivers and some advice to choose the right type.

DRT or DRL: The DRL5 driver is suitable for use in equipment with a supply or control voltage greater than 4.75 V. By contrast, the DRT5 may be used at a lower supply level starting at 4.4 V. This difference in supply voltage is also relevant to applications using an external capacitor.

Maximum current of the control input: This parameter is especially important if the driver is used without a power supply. In that case, capacitor Cext needs to be charged from, say, a logic gate. Depending on the allowed or desired control current we choose the version with current limiting at l mA, 10 mA, 20 mA or 50 mA.

Control pulse length: This time has to exceed the ’minimum switching time’ required by the relay. Possible times are 5, 10, 20, 30 or 50 ms. The OFF switching pulse is twice as long, which allows the value of the external capacitor to be optimized if necessary.

Really useful!

Compared with existing solutions, the DRT5/DRL5 drivers enable relays to be controlled with very low currents indeed, take the switching characteristics of the relay fully into account and guarantee the state of the relay when the equipment is switched off. Unfortunately the down side of the new drivers is their relatively high cost. Of course, we hope that the price will drop with volume production. In any case, these new devices offer the electronics designer extra options in circuits where energy consumption is a prime consideration, such as all newly designed equipment powered by batteries or solar cells.

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