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By JOHN CLARKE Con t r ol h i gh - cur r en t loa ds w i t h t h is DC Relay Switch Want to switch power to a high-current load using a circuit capable of supplying just a few milliamps? No problem – build and use the SILICON CHIP DC Relay Switch. I T’S OFTEN NECESSARY to switch power to a device that requires a current of several amps in order to drive it. The problem is, the device that’s required to do the switching may only be capable of supplying just a few milliamps. Such a circuit might be capable of switching on a LED but that’s about all. The way around this problem is to use a relay with heavy-duty contacts to switch the power. However, your electronic switching circuit may not even have sufficient power to drive a relay coil – at least not directly. This DC Relay Switch board is the answer to that type of situation. It utilises a heavy-duty relay with 30A Main Features • • • • • • • • Automotive-style high-current relay Operates from 12V DC power supply Suitable for low-voltage switching only (up to 50V DC) Activated by low current Isolated input to provide flexible switching options Can be activated using a low-voltage AC signal or an oscillating signal Relay-on LED indication Normally open (NO) and normally closed (NC) relay output terminals 76  Silicon Chip contacts, runs from a 12V supply and requires just 400mA of signal to trigger the relay. That’s made possible by using an optocoupler and some simple electronic circuitry to drive the relay. What’s more, the input trigger signal does not have to be ground referenced. This means that you can drive the relay board from just about any DC signal, whether it normally sits at around 12V, 5V or 0V. It can even be driven by low-voltage AC or by a signal that is rapidly switching on and off. Current drive In practice, the DC Relay Switch requires a current to drive it rather than a voltage. A signal current of just 400mA or more switches the relay on and when there is no current, the relay switches off. In practice, this means that you can drive the relay switch board using an external circuit that normally drives a LED. When the LED is on, the relay is on and vice versa. Alternatively, the relay board can be connected so that the relay is off when the external LED is lit. If the LED is multiplexed (ie, switch­ ed on and off) at a fast rate, then the relay board can be configured to switch on the siliconchip.com.au Parts List Fig.1: the circuit is triggered by applying a signal to optocoupler OPTO1. When the phototransistor in OPTO1 turns on, it turns on transistor Q1 and this then turns on transistor Q2 which drives the relay and LED1. relay while ever the LED is being driven by the switching circuitry. A LED on the DC Relay Switch board provides on/ off indication for the relay (ie, it lights when the relay switches on and goes off when the relay is off). As shown in the photos, the DC Relay Switch comprises a small PC board that includes the relay, the optoisolator, two transistors and various other minor components. It is powered from a 12V DC supply via an on-board screw terminal block. A second 2-way screw terminal block is used for the trigger signal inputs. External connections to the relay contacts can be made using either PCmount spade connectors or a 3-way screw terminal block. The spade connectors are best for high-current applications. Finally, the PC board can be fitted inside a small plastic (UB5) utility case, if this is required. OPTO1 from breaking down and dissipating too much power if a reverse voltage is applied. In this case, D3 conducts and limits the voltage across the LED to a safe value (ie, to about 0.6V). When current flows in the optocoupler LED, the optotransistor conducts and supplies base current to transistor Q1 via the 22kW resistor from the 12V supply rail. This switches Q1 on which in turn switches Q2 on via its associated 1kW base resistor. And when Q2 switches on, relay RLY1 also switches on, as does LED1. The 10kW resistor between Q1’s base and ground ensures that Q1 switches off when the phototransistor in OPTO1 turns off. Similarly, the 1kW resistor between Q2’s base and emitter ensures that this transistor switches off when Q1 switches off. The 1mF capacitor on Q1’s base is necessary if the input is driven using How it works OK, let’s see how the circuit works – see Fig.1. As shown, the input trigger signal is applied to the LED inside optocoupler OPTO1 via a 1kW resistor. This resistor limits the LED current to less than 12mA for a 12V signal and to less than 5mA for a 5V signal. Diode D3 prevents the LED inside siliconchip.com.au 1 PC board, code 05211061, 46 x 61mm 1 UB5 box, 83 x 54 x 31mm 1 SPDT PC mount horn relay (Jaycar SY-4072, Altronics S 4206A or equivalent) (RELAY1) 2 2-way screw terminal connectors (5.08mm pin spacing) 1 3-way screw terminal connectors (5.08mm pin spacing) 3 PC mount 6.4mm spade connectors 1 2-way pin header (2.54mm pin spacing) 4 M3 x 12mm countersunk Nylon screws & nuts 4 3mm Nylon washers 4 M3 nuts 1 jumper shunt Semiconductors 1 4N28 optocoupler (IC1) 1 BC549 NPN transistor (Q1) 1 BC327 PNP transistor (Q2) 2 1N4004 1A diodes (D1,D2) 1 1N4148 diode (D3) 1 3mm red LED Capacitors 1 220mF 16V PC electrolytic 1 1mF 16V PC electrolytic Resistors (0.25W, 1%) 1 22kW 1 2.2kW 1 10kW 3 1kW an AC signal or some other switching signal. This capacitor is connected into circuit using link LK1 and filters the resulting signal on pin 4 of OPTO1 to produce a steady DC voltage. This ensures that Q1 remains on while ever the input signal is applied. Note that LK1 is only necessary for AC input signals. It can be left out of circuit (ie, the 1mF capacitor is disconnected) for DC trigger signals. Operating The Circuit From 24V DC Want to operate the DC Relay Switch from 24V DC? Here’s how to do it: • • • Use a 24V relay instead of a 12V relay – eg, the Altronics S 4208A 24V 30A relay (Jaycar do not have a 24V version). Increase the voltage rating of all capacitors to 35V. Change the 2.2kW resistor in series with LED1 to 4.7kW 0.25W. November 2006  77 Fig.2: install the parts on the PC board as shown in this layout diagram. Be careful not to get transistors Q1 & Q2 mixed up – they may look identical but Q1 is a BC549 (NPN) while Q2 is a BC327 (PNP). Diode D2 provides spike protection for transistor Q2 when the relay is switched off. It shunts the back-EMF voltage spike generated when the relay switches off – a necessary precaution to prevent “punch-through” of the transistor. Power for the circuit can be derived from any suitable 12V DC supply (eg, a plugpack or battery). Diode D1 provides reverse polarity protection, while a 220mF capacitor decouples the supply. Construction The DC Relay Switch is built on a Warning! DO NOT use this DC Relay Switch to switch 240V AC mains voltages. The relay is not designed to do this and it is dangerous to connect mains to the bare PC board. If you do need to switch mains voltages, then use this board to trigger an external mains-rated relay. A suitable mains switching relay was published in the May 2006 issue of SILICON CHIP. PC board coded 05211061 and measuring 46 x 61mm. This fits inside a UB5 box and is secured using four M3 x 12mm countersink Nylon screws and nuts. A 3mm Nylon washer is used between the PC board and the case at each mounting point, to lift the board clear of the base. Fig.2 shows the parts layout on the PC board. Begin by checking the PC board for any defects such as broken tracks and shorts between adjacent tracks. That done, check the corner hole sizes – these should all be 3mm in diameter. In addition the holes for the relay pins and the screw terminal blocks must be large enough to accept these parts. Once all the hole sizes are correct, begin the assembly by installing the resistors. Table 1 shows the resistor colour codes but it’s a good idea to also check them using a digital multimeter, just to make sure. Next, install the diodes and the optocoupler (OPTO1), making sure they go in with the correct polarity. Follow these with the capacitors, transistors Q1 & Q2, the LED and the relay. Take care with the polarity of the capacitors and LED. Transistors Q1 & Q2 come in iden- tical (TO-92) packages so be careful not to get them mixed up. Q1 is an NPN BC549 type, while Q2 is a PNP BC327 and the circuit won’t work if you transpose them or install them the wrong way around. As mentioned previously, you can use either a 3-way screw terminal connector or PC-mount spade connectors to make the external connections to the COM, NO & NC relay contacts. Use the spade connectors if the relay terminals are to carry currents in excess of 2A via. Finally, install the 2-way pin header for LK1. The link itself can be left out if you intend to trigger the board using a DC input signal. Alternatively, install the link if you want delayed switch-on and switch-off for the relay, or if you intend using an AC input signal (see below). Testing OK, now for the smoke test. You will need a 12V DC supply rated at about 150mA to power the board. Connect this to the +12V and 0V terminals, making sure you get the polarity right. Initially, when you apply power, nothing should happen. You can now Table 1: Resistor Colour Codes o o o o o No.   1   1   1   3 78  Silicon Chip Value 22kW 10kW 2.2kW 1kW 4-Band Code (1%) red red orange brown brown black orange brown red red red brown brown black red brown 5-Band Code (1%) red red black red brown brown black black red brown red red black brown brown brown black black brown brown siliconchip.com.au Fig.3: the various triggering options. In (a) the relay board is triggered by a signal that goes from low to high (+5V or +12V); in (b) by a signal that goes to 0V; and in (c) by an external circuit that turns on an indicator LED. check if the circuit works by connecting the negative (-) signal input to 0V and the positive (+) input to the +12V rail. When you do so, the relay should immediately switch on and the LED should light. How to use it Fig.3 shows three different circuit configurations that can be used to trigger the relay board. Fig.3(a) shows how to turn the relay on using a signal output that goes high (ie, to 5V or 12V). Conversely, Fig.3(b) shows how to rearrange the wiring so that the relay turns on for a signal output that goes low (ie, to 0V). Fig.3(c) shows how to drive the relay board from a circuit that normally powers a LED. Note that if the LED is multiplexed when it is lit (ie, switched on and off at a fast rate), the relay may chatter on and off. Inserting link LK1 to connect the 1mF capacitor into circuit should stop this chattering. In all three above cases, if you want delayed switch-on and switch-off for the relay, increase the value of the 1mF capacitor. A value of 220mF will give a nominal 1-second delay. Note that it is important that the trigger circuit be capable of providing the required current to the relay board input. The relay board will draw about 3mA when there is 5V between its “+” and “–” inputs and 10mA when there is 12V between these terminals. If this exceeds what the trigger circuit can deliver, then the 1kW resistor in series with pin 1 of the optocoupler can be increased. Doubling this resistor (eg, to 2.2kW) will halve the current requirement but if you ultimately go too high in value, the optotransistor may not turn on sufficiently to drive the relay circuit. The minimum recommended trigger current is 400mA. This corresponds to using a 22kW resistor in series with OPTO1 for a 12V power supply and a SC 7.5kW resistor for a 5V supply. Looking for real performance? 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Order by phoning (02) 9939 3295 & quoting your credit card number; or fax the details to (02) 9939 2648; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. siliconchip.com.au November 2006  79