A Few DC Solid-state Relays

 By Georges Treels (France) (Elektor Electronics)

Good old electromechanical relays are relatively expensive where any significant current has to be switched and switching times must be short. One solution is to go over to solidstate relays (SSRs). In DC mode, MOSFETs offer a very interesting solution, and the various manufacturers today offer devices at less than £4 with amazing performance, in terms of both current and low R DS ( on y They're relatively simple to use, in both monostable and bistable modes, so why stint ourselves?

The following circuits will let you switch 10-60 A (or even more if you use configurations with MOSFETs in parallel), with very short switching times. Several configurations are shown, monostable and bistable, capable of switching a load with one side returned to either ground (high side switching) or the positive rail (low side switching). In addition, the monostable configurations offer galvanic isolation and can be driven by signals from 5-24 V, DC or AC. The bistable SSRs are controlled using a simple push-button and a little bit of logic.

Let's start with the monostable SSRs. Bridge B1 makes it possible to accept any input polarity in the case of a DC control signal, and rectifies the signal in the case of an AC control signal. The network R1, R2, D1 limits the LED current in opto-isolator IC1 . The base of the phototransistor in IC1 is connected to ground via R3; its emitter is connected directly to ground.

In the case of a load returned to ground, the gate of T1, a P-channel MOSFET, is driven directly from the collector of IC1. If the load is returned to the positive rail, the gate of T1, this time an N-channel MOSFET, is driven via T2, which inverts the output from IC1 .

C2, C3, D2, and D3 protect the MOSFET in the event of loads that are not purely resistive. Both bistable configurations use the same power stages as the monostables, with an N-MOSFET for loads connected to the positive rail and a P-MOSFETfor loads connected to ground.

IC1.A is wired as a simple flip-flop: with the switching threshold set by PI , IC1 .A output will change state each time button SI is pressed. R1 and CI avoid rapid oscillations while SI is pressed. IC1 gates B, C, and D directly drive the gate of the P-MOSFET in the case of a load returned to ground. IC1 .B inverts IC1 .A output when an N-MOSFET has to be driven (load returned to the positive rail). In both configurations, the relay remains off at power-on (safety feature).

Concerning the MOSFETs, the table lists a number of possible types. This list is far from being exhaustive and new devices come out regularly. Give preference to a low value of R DS(on) (dissipation) and a good dv/dt specification in the case of 'dirty' loads. Pay attention also to the V DS . Even though most of these transistors can take 60 V, this is not the case for either the optoisolators or the bipolar transistors used.

If you are designing a PCB for this type of relay, pay attention to the possibility of heavy currents being carried by the PCB track. For example, three SUP75P03-07 wired in parallel can pass over 200 A! Bear in mind that a PCB track with a copper layer 35 jam thick (i.e. standard) has a resistance of 48* 10 5 x L/ Wohms, where L(ength) and W(idth) are in mm.