I have a circuit which connects two 6v 10ah batteries in parallel to two motors which together draw ~2A. This circuit has a DPDT switch which I would like to replace with a DPDT relay and have the Raspberry Pi drive the switching.

When doing my research, I came across different types of relays with various terminologies and different ways of using them.

What are the different types of relays ? (SSR, DPDT, SPDT, Mechanical, Electromagnetic etc.)

Do relays and switches always correspond to each other ? e.g. if there is a DPDT switch, then there is a corresponding DPDT relay ?

What is a difference between a Signal relay and Power relay ?

What is coil voltage/current vs contact voltage/current ? e.g. if a contact voltage is listed as "120VAC/0.5A, 24VDC/1A" can I still use it to drive motors which draw ~2A with a 6v 10ah battery ? Are there standard ratings that relays always come in ?

When do we need to use a transistor instead or along with a relay ?

Note : I have been through quite a few questions on this community and others. I am open to modify the question iteratively to refine if required.


closed as too broad by Chris Stratton, Wesley Lee, Enric Blanco, Voltage Spike, Dmitry Grigoryev Jun 8 '17 at 6:59

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Part of your question is already answered under Relay: higher voltage allowed at lower current? \$\endgroup\$ – The Photon May 25 '17 at 17:59
  • \$\begingroup\$ At present this question is too broad - an answer to the whole thing would be on the scale of a the textbook chapter or at least wikipedia page on relays, demonstrating that you should be looking at resources like that rather than asking here. If you want to edit this into a singular, specific question that does not duplicate one already here it might survive, but at present it is not a fit for the mission of stack exchange sites. \$\endgroup\$ – Chris Stratton May 25 '17 at 18:03
  • \$\begingroup\$ Reed Relay 0.5, Signal<=2A gold-plate, Power>2A, Opto ( logic) Transistor, cheap,fast non-isolated, FET high current and specs ia.omron.com/product/cautions/36/safety_precautions.html I expect you to read AND comprehend all of this \$\endgroup\$ – Sunnyskyguy EE75 May 25 '17 at 18:08
  • \$\begingroup\$ Another option would be to use a BJT or Darlington (PN2222 and TIP120 are popular in hobby applications) to drive a relay. This could be cheaper if you already have some "jelly bean" components. \$\endgroup\$ – Caleb Reister May 27 '17 at 6:52
  • \$\begingroup\$ Thanks for the help so far ! I will check out Reed / BJT / Darlington. \$\endgroup\$ – dparkar May 30 '17 at 4:28

What are the different types of relays ?

The three most common types of relay are

Form A corresponding to a SPST normally open switch.

Form B corresponding to SPST normally closed.

Form C corresponding to SPDT.

Do relays and switches always correspond to each other ?

There are other types of relays than the three I mentioned above, but if you want to use one of them, you should make sure you know where you will acquire it and whether you trust your assurance of supply from that source.

What is a difference between a Signal relay and Power relay ?

A signal relay implies the expected switched signals are relatively low power and possibly high frequency.

A power relay implies the switched circuit will have relatively high current.

These terms are marketing terms rather than technical ones.

When do we need to use a transistor instead or along with a relay ?

When the control signal you have available is not able to supply enough current to activate your relay, a transistor is a likely way to resolve this.

  • \$\begingroup\$ I would say that "signal" and "power" are technical terms rather than (or as well as) marketing terms. Those terms relate to the materials and construction of the relay contacts, and other current-carrying parts. \$\endgroup\$ – Peter Bennett May 25 '17 at 19:36
  • \$\begingroup\$ Is ever a good idea to use a relay with high-frequency signals? \$\endgroup\$ – Caleb Reister May 26 '17 at 8:02
  • \$\begingroup\$ @CalebReister, I wouldn't use a standard relay for RF, but there are relays specifically designed to switch RF signals. \$\endgroup\$ – The Photon May 26 '17 at 15:15
  • \$\begingroup\$ Also, it may be worth adding more to the transistor section. A RPi runs at 3.3V and could easily be damaged from inductive kickback currents generated when an electromechanical relay is turned off. \$\endgroup\$ – Caleb Reister May 27 '17 at 6:47
  • \$\begingroup\$ Thanks for the help so far ! Does DPDT also fall under Form C ? \$\endgroup\$ – dparkar May 30 '17 at 4:27

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