A DC to DC solid state relay (SSR) switches a direct current load using a low-power DC control signal, replacing mechanical contacts with an optically isolated input LED and an output power semiconductor (typically a MOSFET or IGBT). Unlike AC SSRs that rely on zero-crossing detection and TRIACs, DC SSRs must manage continuous conduction, making thermal management and inductive kickback protection the primary design challenges.
DC to DC Solid State Relay Spec Sheet & Rating Table
When selecting a DC to DC solid state relay, the "Max Load Current" printed on the datasheet cover is often a best-case scenario assuming an infinite heatsink at 25°C ambient. To properly size your component, you must look deeper into the specification sheet.
| Manufacturer / Part Number | Input Control Voltage (VDC) | Output Load Voltage (VDC) | Max Load Current (A) | Output Type | On-Resistance RDS(on) (Ω) | Typical Price (USD) |
|---|---|---|---|---|---|---|
| Sensata / Crydom D1D07 | 3.5 - 32 | 0 - 100 | 7.0 | MOSFET | 0.090 | $22.00 - $28.00 |
| Omron G3VM-61G1 | 1.15 (LED Forward) | 0 - 60 | 4.0 | PhotoMOS | 0.045 | $4.50 - $6.00 |
| Panasonic AQY212 | 1.14 (LED Forward) | 0 - 60 | 0.55 | PhotoMOS | 0.850 | $2.00 - $3.50 |
| IXYS / CPC1908 | 1.2 (LED Forward) | 0 - 60 | 1.5 | PhotoMOS | 0.350 | $3.00 - $4.50 |
| Carlo Gavazzi RD1A480D10 | 4.5 - 32 | 0 - 48 | 10.0 | MOSFET | 0.050 | $35.00 - $45.00 |
Which Rating Column Governs This Load?
For DC SSRs, the governing column is On-Resistance (RDS(on)) combined with the manufacturer's thermal derating curve, not the "Max Load Current." Because DC current does not have natural zero-crossings to extinguish arcs or cool the die, the semiconductor is in a state of continuous conduction. The power dissipated as heat is calculated by P = I² × RDS(on).
For example, running 7A through the Crydom D1D07 (RDS(on) = 0.090Ω) generates 4.41W of heat. Without a properly sized heatsink, the internal silicon will exceed its maximum junction temperature (usually 125°C to 150°C) and fail catastrophically. Always calculate your I²R losses and cross-reference the thermal impedance (junction-to-case, θJC) before finalizing your part selection.
Input vs. Output Wiring & Protection
While legacy electromechanical relays use a "coil" and "contacts," a DC to DC solid state relay uses an Input (Control) side and an Output (Load) side. Wiring them incorrectly or omitting protection will destroy the component instantly.
Input Side (Control / "Coil" Equivalent)
The input side consists of an internal infrared LED inside an optocoupler. It is strictly polarity-sensitive.
- Polarity: Connect the positive control voltage to the '+' (or Pin 1) terminal and the negative/ground to the '-' (or Pin 2) terminal. Reversing this will not activate the relay and may reverse-bias the LED beyond its 5V breakdown limit.
- Current Limiting: You must limit the input current to the LED's specified range (typically 5mA to 15mA). If your control signal is 12VDC and the LED forward voltage (Vf) is 1.2V at a desired 10mA, calculate the series resistor: R = (12V - 1.2V) / 0.010A = 1080Ω. Use a standard 1.1kΩ resistor.
Output Side (Load / "Contact" Equivalent)
The output side utilizes a power MOSFET or IGBT. Unlike mechanical contacts, DC SSR outputs are often polarity-sensitive depending on the internal architecture.
- Polarity: For standard single-MOSFET SSRs, the load must be connected to the Drain and Source terminals exactly as marked. Reversing the load will forward-bias the MOSFET's intrinsic body diode, meaning the load will remain permanently powered regardless of the input control state.
- Flyback Protection: Inductive loads store energy in magnetic fields. When the SSR turns off, the collapsing field generates a massive reverse voltage spike (inductive kickback) that will punch through the MOSFET's drain-source breakdown voltage (VDSS).
Load-Type Decision Path: Resistive, Inductive, and Motor
Choosing the right DC SSR architecture depends entirely on the load's inrush characteristics and energy storage. Use the decision tree below to match your application to the correct semiconductor topology and protection scheme.
| Load Type | Examples | Inrush Characteristic | Recommended SSR Output | Mandatory Protection / Sizing Rule |
|---|---|---|---|---|
| Resistive | DC heaters, incandescent lamps, resistor banks | Cold resistance is lower; moderate inrush (up to 10x for tungsten). | Standard Power MOSFET | Size for 1.5x steady-state current to handle cold inrush. Heatsink based on continuous I²R. |
| Inductive | Solenoid valves, DC contactor coils, lifting magnets | Current ramps up slowly; massive voltage spike on turn-off. | MOSFET with high VDSS rating | External parallel flyback diode is mandatory. TVS diode recommended for fast-cycling valves. |
| DC Motor | Brushed DC motors, linear actuators, conveyor drives | Stall current can be 5x to 8x nominal running current. | IGBT or Oversized MOSFET Array | Size SSR for the stall current, not running current. Add RC snubber + flyback diode for brush noise. |
| Capacitive | Large filter banks, DC-DC converter inputs, LED drivers | Acts as a dead short at turn-on; massive current surge. | MOSFET with high surge rating (ISM) | Implement a pre-charge circuit or NTC thermistor. Verify SSR datasheet specifies a non-repetitive surge current rating. |
For deeper application notes on matching thermal impedance to these specific load profiles, refer to the Sensata/Crydom technical documents library and the Omron Solid State Relays technical guide.
Bench Testing & Failure Triage: Dead, Live, and Replace
When a DC control circuit fails to activate a load, you must determine if the SSR is the culprit. Because SSRs lack moving parts, their failure modes are strictly electrical and thermal.
How to Test an SSR Dead (Bench Test)
Remove the SSR from the circuit and set your multimeter to Diode Test mode.
- Input Side: Place the red probe on the '+' terminal and black on the '-'. You should read a forward voltage drop between 1.1V and 1.5V (the internal LED). Reverse the probes; the meter should read "OL" (Open Loop). If it reads 0.00V (short) or OL in both directions (open), the input optocoupler is dead.
- Output Side: Switch the meter to Resistance (Ω) mode. Measure across the load terminals. A healthy, un-triggered MOSFET SSR will read very high resistance (often >1MΩ) or show the forward voltage drop of the internal body diode if your meter's test voltage is high enough. If it reads near 0Ω, the output semiconductor has suffered a thermal short.
How to Test an SSR Live (In-Circuit)
Live testing verifies the SSR's ability to carry current without excessive voltage drop.
- Apply the control voltage to the input terminals.
- Set your multimeter to DC Volts and place the probes directly on the SSR's output load terminals (not the power supply, not the load—directly on the SSR screws/pins).
- With the load drawing current, measure the voltage drop. A healthy MOSFET-based DC SSR should show a voltage drop of less than 0.2V (often in the millivolt range).
- Diagnostic Threshold: If you read a voltage drop greater than 1.0V under load, the internal MOSFET is degrading, the die-attach solder is cracking from thermal fatigue, or the SSR is severely undersized for the current.
When to Repair vs. Replace
Always replace. Unlike electromechanical contactors where you can sometimes file down pitted contacts or replace a burnt coil, DC to DC solid state relays are hermetically sealed or potted in thermally conductive epoxy/silicone to manage heat and block moisture. Attempting to desolder the internal MOSFET from the substrate will destroy the optocoupler alignment and the thermal interface. If an SSR fails, identify the root cause (usually missing flyback diode, inadequate heatsink, or ambient temperature exceeding 40°C), correct the system design, and install a new unit.
