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I need to control a load of 12v DC and 30A, Can I control the load with an AC solid-state-relay 24-380v and 40A? If it isn't possible, what should I use? Thanks.

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    \$\begingroup\$ If this is a 1-off design and doesn't require any certification, you are probably fine. However if you need to get certifications (i.e. UL) you will eventually need to get one with the proper DC ratings. \$\endgroup\$
    – Jim
    Aug 11 '16 at 16:18
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    \$\begingroup\$ Uh-uh. It's an AC SSR which means it's triac based and he's switching DC. \$\endgroup\$
    – Transistor
    Aug 11 '16 at 16:25
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    \$\begingroup\$ Heh ... it might be able to switch the DC ... on... but it probably relies on zero crossings to switch off, and DC doesn't have enough zero crossings... \$\endgroup\$ Aug 11 '16 at 16:49
  • \$\begingroup\$ The answer is no, you wouldn't want to do that. Use a normal relay. \$\endgroup\$
    – Voltage Spike
    Aug 11 '16 at 17:18
  • \$\begingroup\$ An SSR rated only for AC depends on the AC load current passing through zero in order to switch off. As Brian Drummond said, this doesn't happen very often for DC. You need (depending on e.g. how many times the load will be switched) a relay or DC rated SSR. \$\endgroup\$ Dec 3 '17 at 21:36
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No you can't.

enter image description here

Figure 1. The SSR-40 DA.

I need to control a load of 12v DC and 30A ...

The device clearly shows that:

  1. The output (pins 1 and 2) are AC. It can not switch DC.
  2. The output will switch 24 V to 380 V AC. Even if it could handle DC your voltage is too low.

If it isn't possible, what should I use?

A relay or a DC SSR with voltage and current ratings to suit.

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    \$\begingroup\$ "It can not switch DC." Half right. It can switch DC on just fine. What it cannot do is switch it off. \$\endgroup\$ Aug 11 '16 at 17:43
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    \$\begingroup\$ Ha! Not true - it's a zero-cross SSR! (OK, if you power the input before powering the output ... but that's not DC anymore.) Answer was pitched at the same level as the question. ;^) \$\endgroup\$
    – Transistor
    Aug 11 '16 at 18:00
  • \$\begingroup\$ It switches on and off on the zero cross - very true! +1 for fairness and content of your answer. \$\endgroup\$
    – Andy aka
    Aug 11 '16 at 20:42
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what should I use?

You can control a 30A DC load of any reasonable voltage with a suitable MOSFET. Your options are wider if the gate control voltage is 10 volts or above. For switching 12 volts, I'd look at 20 volt rated devices with an on-resistance in the milli ohm range. For a 10 milli ohm on-resistance at 30 A, the power dissipation is going to be 9 watts so, some form of heatsinking will be required.

You can get a few MOSFETs that are around 1 milli ohm of course.

To make this work with reliability you need to have an under-voltage lock-out system should the 12 volts drop to below 10 volts. This prevents the MOSFET being turned on when it can only achieve an on-resistance of maybe 100 milli ohm to avoid burn-out.

If you are switching an inductive load you will need to use a flyback catch diode rated potentially at full load current.

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Use mechanical relay. Input AC and output AC or DC

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    \$\begingroup\$ I don't think the AC input is necessary. There are plenty of DC-input relays that can switch 30 A at 12 VDC. \$\endgroup\$ Jun 13 '19 at 11:51

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