# A relay that can switch from one source to another - what's that called?

I want to be able to use a microcontroller to select the power source, from among two, for a 100A DC application.

All the relays I see are ON / OFF, rather than a selector.

So, I'm looking for something with 5 connections:

• Signal input (say, 3.3v digital, which when activated causes it to "switch")
• Power input 1 (which I'll connect to a battery bank)
• Power input 2 (which I'll connect to an AC Adapter)
• Power output (which I'll connect to the load)
• Common ground (for the voltage of the signal input)

I'm probably not searching for the correct piece of equipment, but I can't seem to determine what this might be called.

• Search the terms "pole" and "throw" – Makoto Nov 13 '16 at 18:43
• You are after a SPDT relay. I doubt you will find one that will operate with 3v3 though. – Terry Gould Nov 13 '16 at 18:47
• single pole double throw. SPDT. Note that a relay for 100A DC will be large and expensive. Consider using MOSFET's instead. The problem is that when you break the circuit (open the relay) an arc will form, and this will partially melt the contacts. In an AC relay, the arc extinguishes during the zero crossing. In a DC relay, the arc will not extinguish until the contacts are separated by a large distance. So the contacts degrade very rapidly. – mkeith Nov 13 '16 at 18:49
• Why can't you just get two? You'll need two control lines but you'll be able to not only switch between sources, but also have both at once or none at all as you will have two independent switches. – Sam Nov 13 '16 at 22:54

If the ground/common/negative side of your circuit can remain connected, then you need to "switch" only the "hot"/active side (voltage not disclosed) side. That can be done with a SPST (Single Pole, Single Throw) switch or relay.

However there are several things to be aware of if you want to do something like this:

1. CURRENT Switching such a high current will require a large and expensive relay. These are becoming harder to get and more expensive as this kind of switching is done with semiconductors (transistors, thyristors, etc.) here in the 21st century.
2. VOLTAGE Switching a high voltage will require essentially a "mains-rated" relay which will add even more to the price.
3. DC vs AC Switching DC is more difficult than switching AC. Because AC naturally passes through 0V many times per second (50Hz or 60Hz) Breaking high-current DC requires a special kind of relay designed for such applications. This will add even more to the price.
4. CONTROL VOLTAGE You will not find any relays that can switch 100A @ ???V which can be operated from 3.3V. You will need probably a multi-stage relay driver circuit to allow the 3.3V logic-level to control another circuit with enough power to activate such a large relay.

You make it more difficult to answer your questions when you do not include the working voltage. It limits us to generic answers which may not be applicable to your situation.

There are plenty of relays which switch a common connection (your output) between a normally open and normally closed terminal. Relays generally handle lower current applications so you might be better off looking at contactors, which are just larger versions of relays with more durable contacts. If you're unable to find a contactor with NC and NO contacts on the one unit, you can also find mechanically interlocked contactors which only allow one contactor to be energised at any one time. This would be the safest option if you are concerned with component failure and don't want your supplies to be crossed. Your proposed signal voltage to the relay/contactor coil of 3.3V could be a problem. Stacking relays - having a relay with a 3.3V coil switching a 24V or 240V supply to the main relay/contactor - would get around this but brings more complexity and areas for component failure into the design. Also, if you are considering this for your household switchboard, be aware that some regulators require solar panels and other off-grid power supplies to be controlled by a particular switch in your switchboard. These are designed to prevent off-grid power sources from feeding the grid when the grid is down after a storm etc. Linesman could go to work on the grid, thinking it has been powered down, only to find voltage there, preventing them from working safely.

Galco.com sells ABB mechanically-interlocked contactors with 24VDC coil and contacts rated to 105A for the low, low price of \$743.42 (probably not including shipping...) Item# AE75M-30-11-81

Try searching for a transfer or changeover relay.

100A is fairly high for a contactor or relay. You could, in theory, use two 100A contactors, but I would assume fairly bad things could happen if both sources were connected together, so you probably want a device that has a mechanical interlock to make that very unlikely (also called 'break before make').

It's very unlikely you will be able to find a 100A device that will operate from 3.3V directly. Most likely it will draw a fairly large amount of power (a watt or more) from a higher voltage supply such as 12V. You can still use a 3.3V control signal but you would need a driver.

There are '100A'/'120A' Chinese and automotive-style relays on offer that are SPDT such as JQX-62F-1Z/2Z (Chinese part numbers are common to many makers, with varying ratings, approvals and quality). I would have doubts about their ability to carry 100 or 120A continuously- Chinese amperes tend to be on the small side. Coil resistance (12V) is stated at 130$\Omega$ so you'd need about a 100mA driver and 12V supply. Something with serious ratings would probably draw more.

Higher voltage DC usually uses magnetic blowout to extinguish an arc, but at 12-28V it's not necessary.