AC/DC vs. DC-Only Configuration of SSR's

When using a small solid state relay (SSR) to turn on a DC load, it can be useful to understand how the internal configuration can affect its circuit performance. Connecting it for DC-only operation can provide significant benefits over the more common AC/DC configuration.

Most small SSR’s utilize two N-channel MOSFETs with their sources connected back to back and acting as a switch to pass a load current when the internal infrared LED triggers their gates, as seen here:

image

Note that this configuration allows current to pass through both MOSFETs, connected in series, in either direction from pin 6 to 4 or pin 4 to 6 when the gates are triggered by the infrared LED. Therefore, when connecting the load through pins 4 and 6 and leaving pin 5 open, one can pass either an AC or DC current though the device.

image

However, if one only needs to pass a DC current of a known polarity through the SSR, there is an alternative method of connecting the device which can offer significant benefits. This alternate connection is shown here:

image

In this configuration, the DC current follows two parallel paths, going into both pins 4 and 6 and out through pin 5. Since the current is going through the two MOSFETs in parallel rather than in series, the internal resistance is reduced to about 1/4 of the series connection and the current carrying capacity is nearly doubled. Additional benefits are significantly reduced heating under load and reduced voltage drop across the SSR because of the reduced resistance.

On Digi-Key’s website, all of the SSR’s maximum load currents are specified assuming a series connection because some can only be connected this way (some omit to the common source pin access – pin 5 in the schematic above).

So, keep this in mind when searching for an SSR which will satisfy your load current requirements. You may be able to save some money and improve performance by selecting a part with a lower load current rating and connecting it for DC-only operation, as shown above.

2 Likes