Products for USB Sensing and Control
Products for USB Sensing and Control

AC Solid State Relay - 120V 20A

ID: 3953_0

This SSR switches up to 120VAC/20amp and connects to a PhidgetInterfaceKit 0/16/16.

Replaced by 3953_1
Replaced by the 3959, an SSR with similar characteristics.

The 3953 DC Solid State Relay can be used to switch loads of up to 120A at 20VAC using the digital output from a 1012 - PhidgetInterfaceKit 0/16/16. The Relay comes with an optional protective cover to protect people from touching the exposed high power wires, and a varistor to protect your SSR from harmful transients and surges. All wires connect to this relay by screw terminals. If you are switching a high power load or find the relay is getting hot, please consider a heat sink. Heat sinks 3955 and 3956 are designed for use with this relay.

We've listed this SSR at 120V because the included MOV is only rated for 120V operation. If you decide that you do not need the MOV for surge protection, you can use this relay without the MOV at the manufacturer's specification of 240V.

Comes Packaged with

  • A Protective Relay Cover
  • A Heat Transfer Pad
  • An MOV (Metal Oxide Varistor)

Product Specifications

Electrical Properties
Relay Output Type SCR
Isolation Method Photoelectric
Dielectric Strength 2.5 kV AC
Control Voltage Min 3 V DC
Control Voltage Max 32 V DC
Load Voltage Max (AC) 120 V AC
Load Current Min 100 mA
Load Current Max (AC) 8 A
Load Current Max (With 3955 Heatsink) 15 A
Load Current Max (With 3956 Heatsink) 20 A
Load Surge Current 255 A
Turn-on Time Max 1 ms
Turn-off Time Max 11.6 ms
Physical Properties
Length 58 mm
Width 45 mm
Height 25 mm
Operating Temperature Min -30 °C
Operating Temperature Max 80 °C
Other Properties
Manufacturer Part Number HFS34/D-240A20PS-Y
Customs Information
Canadian HS Export Code 8536.49.00
American HTS Import Code 8536.49.00.75
Country of Origin CN (China)

Have a look at the Data Sheet for a complete set of specifications.

This relay connects to the 1012 - PhidgetInterfaceKit 0/16/16.
Just connect the positive control terminal of the SSR to the +5V terminal on the 1012, and connect the negative terminal to an open collector digital output on the 1012.


Heat Sinks

Have a look at the relay's characteristic curve in the Data Sheet to decide if you need a heat sink. Without a heat sink, the maximum current that can be handled by this relay is 8A up to an ambient temperature of 40°C.

In order to be able to switch currents of up to 20A, you have to use the 3956 - Heat Sink.


  1. When choosing a SSR, check the actual load current and the working ambient temperature. Refer to the characteristic curve in the Data Sheet and select the proper heat sink if required.
  2. Use the heat transfer pad between the SSR and the heat sink. Failure to do so can cause overheating and could damage the relay.
  3. Make sure that the SSR terminal screws are tight. If the screws are not tight, the SSR will get damaged by heat generated when the power is ON. Check the Data Sheet for recommended tightened torque.
  4. When an SSR fails, it most often fails permanently closed - leaving your load powered, and possibly creating a fire or safety hazard.
  5. Make sure that you use the relay within the product specifications.