Overview

Pololu Isolated Solid State Relay/Switch, SPST, bottom view with dimensions.

This board functions as a solid state, single-pole, single-throw (SPST) relay or switch that can be activated by a low-current control signal between 2.7 V and 40 V. The switching is performed by a pair of MOSFETs controlled by an optically isolated driver that galvanically isolates the load from the control inputs. The MOSFETs are arranged back-to-back to make the two outputs symmetric and equivalent, allowing bidirectional current control with the switch in any orientation relative to the load (for example, this board can be used for high-side or low-side switching). The board can be used as a replacement for a mechanical relay in many situations, and its solid-state design makes it immune to problems like contact wear and arcing that limit the service life of mechanical relays. It also offers silent operation and a smaller form factor (0.8″ × 0.32″ × 0.13″) than a typical mechanical relay with a comparable current rating. These devices can also be used for isolating logic signals, but note that the turn-on and turn-off times on the order of 1 ms make them unsuitable for signaling rates beyond a few hundred per second.

Two versions of the solid state relay/switch are available with different voltage and current capabilities:

• Pololu Isolated Solid State Relay/Switch, SPST, 30V, 11A
• Pololu Isolated Solid State Relay/Switch, SPST, 60V, 7A

Details for item #5420

Pololu Isolated Solid State Relay/Switch, SPST, 60V, 7A.

This version of the solid state relay/switch board can switch up to 7 A of current at voltages up to 60 V (absolute maximum). The switch has maximum turn-on and turn-off times of 1 ms, and its output MOSFETs have a typical combined resistance of 30 mΩ when the relay is enabled.

Using the solid state relay/switch

The load connection points are accessible on one side of the board while the control pins are routed to the other. The control circuit is powered by supplying 2.7 V to 40 V across the VIN and GND pins, and it is activated by a high (2.7 V to 40 V) control signal on the EN pin, which turns on the output MOSFETs. The EN signal turns on the output switch via an LED driver and an optocoupler (or opto-isolator) that provides the board’s electrical isolation. The output switch is made from a pair of back-to-back MOSFETs, so it is symmetric and bidirectional, and this board can be used as a high-side switch or low-side switch.

The board’s control circuit will typically draw up to 10 mA from VIN when EN is high (slightly less at voltages below 20 V). If your EN signal source can provide at least 10 mA, you can tie EN and VIN together—for example, by soldering a short piece of wire between the through-holes—to power and activate the control circuit with a single connection (since the board does not require VIN to be present when EN is low).

Three-wire and two-wire (with EN and VIN tied together) control options for the Pololu Isolated Solid State Relay/Switch, SPST.

Making connections to the board

The EN, VIN, and GND pins work with 0.1″-pitch header pins and are compatible with standard solderless breadboards

You can connect the board’s output terminals to your circuit in a variety of ways. For typical high-current applications, you can solder wires directly to the 0.045″×0.095″ slots. The slots can also accommodate 3.5mm-pitch and 5mm-pitch terminal blocks as shown below:

The current path slots can also accommodate a 1×2 0.2″-pitch header or a 1×3 0.1″-pitch male header with the middle pin removed (as shown in the pictures below).

Real-world power dissipation considerations

In our tests, we found that the 30 V version of the relay/switch could conduct about 11 A continuously, and the 60 V version could conduct about 7 A continuously, without reaching the thermal limit for the MOSFETs. Our tests were conducted at approximately 25°C ambient temperature with no forced air flow.

The actual current you can pass through the board will depend on how well you can keep it cool. The printed circuit board is designed to help with this by drawing heat out of the MOSFETs. Solid connections to the output pins (such as with thick soldered wires) can also help reduce heat build-up in the MOSFETs and board.

Warning: Exceeding temperature or current limits can cause permanent damage to the MOSFETs. If you are switching an average continuous current greater than about 2/3 of the specified maximum, we strongly recommend that you monitor the MOSFETs’ temperature and look into additional cooling if necessary.

This product can get hot enough to burn you long before the chip overheats. Take care when handling this product and other components connected to it.

Creepage and high voltage considerations

This product is not designed to or certified for any particular high-voltage safety standard. The PCB traces are laid out to provide at least 4.8 mm of creepage between the control side and the load side. A rough rule of thumb is that uncontaminated FR4 PCBs should have approximately 1 mm of creepage per 100 V_RMS of isolation. Note that this guideline implies a maximum safe voltage lower than the maximum isolation voltage rating of 3.75 kV_RMS for the TLP3906 optocoupler used on this board.

Warning: This product is generally intended for use below 30 V, and many of the application example pictures (e.g. use on a breadboard) are appropriate for such lower-voltage applications. These boards can be used at higher voltages, but working with higher voltages can be extremely dangerous and should only be attempted by qualified individuals with appropriate equipment and experience.

Schematic and dimension diagrams

Schematic diagram for the Pololu Isolated Solid State Relay/Switch, SPST.

The dimension diagram is available as a downloadable PDF (268k pdf).