SPDT Realy - a poor man's Mosfet or the ultimate switch protection?

[StillAlive]

Definition: [I]"SPDT Relay : (Single Pole Double Throw Relay) an electromagnetic switch, consist of a coil (terminals 85 & 86), 1 common terminal (30), 1 normally closed terminal (87a), and one normally open terminal (87). When the coil of the relay is at rest (not energized), the common terminal (30) and the normally closed terminal (87a) have continuity. When the coil is energized, the common terminal (30) and the normally open terminal (87) have continuity. The diagram below shows the relay at rest, with the coil not energized.

Why do I want to use a relay and do I really need to?
Anytime you want to switch a device which draws more current than is provided by an output of a switch or component you'll need to use a relay. The coil of an SPDT relay that we most commonly use draws very little current (less than 200 milliamps) and the amount of current that you can pass through a relay's common, normally closed, and normally open contacts will handle up to 30 or 40 amps. This allows you to switch devices such as headlights, parking lights, horns, etc., with low amperage outputs such as those found on keyless entry and alarm systems, and other components...." Hey, wait a minute, that's us!

Why not to use a SPDT relay to safely switch our power-hungry motors thus protecting our precious contacts? I believe that's the whole purpose of Mosfet relays. Btw, I know there are more advanced Mosfet applications out there (and i'm pretty sure these can be done with relays too).

So why to use delicate, relatively expensive electronic setup, if you can do it simpler and cheaper? We don't see automotive industry using Mosfets, it's all good old relays. The only drawback IMO of using a relay is it's size, which is usually bigger than a Mosfet assembly.

So what do you think?

BTW, 12 30A auto relay successfully installed and operational in my friend's AUG, more to come.

I'm attaching a simple diagram for connecting a relay. It is in spanish(?) illustrating how to connect a car headlamp, but it is exactly the same with AEG.

[Drake]

Those pics aren't working (the first three). Even posting the URL in manually in another window doesn't work

[mcguyver]

Originally Posted by StillAlive

BTW, 12 30A auto relay successfully installed and operational in my friend's AUG, more to come.

Ummm, no. A relay requires time to energize the coil, time that coupled with the spool-up of the gearbox to fire means that you have increased lag. People who use a FET use it to help redce lag. Plus, a relay costs about as much as a trigger switch, especially if you use a quality relay like a Bosch, and it requires energy to close the switch, and does not eliminate arc formation. It moves the arc from the switch to the relay.

A relay will also take up room, room that most guns simply do not have in abundance. A Bosch-style relay is about 1" x 1" x 2", a complete waste of space, especially in an AUG.

A FET is not the best answer either, as it has reliability issues, and is susceptable to the inductive spike generated by the motor. But it's a better option than a relay.

[namloot]

I have a Extreme-Fire MOSFET switch in my Marui M14, it is protected against the inductive spike from the motor.

It noticably increased my rate of fire and trigger response.

[Drake]

Originally Posted by namloot

I have a Extreme-Fire MOSFET switch in my Marui M14, it is protected against the inductive spike from the motor.

It noticably increased my rate of fire and trigger response.

Care to comment on the Extreme Fire FETs? After a lot of shopping around, their SW-AB but I'm still trying to determine how reliable they are and how "worth it" they are.

[namloot]

Thier homepage:
http://extreme-fire.com/

I have thier SW-S in my Marui M14:
http://extreme-fire.com/SW-S.html

It was very easy to wire and install.

The M14 has plenty of room in the stock for a MOSFET switch. I bought a wiring harness for a "large battery, large stock AEG", from my local airsoft store, and added the MOSFET switch to it, along with the connecting wires for the trigger switch and +battery lead to the motor. I removed the stock battery harness, and put my modified one in it's place. The TM M14 has all the electrical connections on the outside of the gearbox, so I didn't have to actually rewire anything, just move the wires around. If my MOSFET switch ever fails, I can simply remove the harness I made, with the MOSFET, and put the stock harness back in.

[StillAlive]

Originally Posted by mcguyver

Ummm, no. A relay requires time to energize the coil, time that coupled with the spool-up of the gearbox to fire means that you have increased lag. People who use a FET use it to help redce lag.

Hmm, you have a point there.. Although, my friend haven't noticed any lag, but i believe you're right. Thanks.

Originally Posted by mcguyver

Plus, a relay costs about as much as a trigger switch, especially if you use a quality relay like a Bosch, and it requires energy to close the switch, and does not eliminate arc formation. It moves the arc from the switch to the relay.A relay will also take up room, room that most guns simply do not have in abundance. A Bosch-style relay is about 1" x 1" x 2", a complete waste of space, especially in an AUG.

Never used Bosch ones, we tried Takamisawa ones and one other brand (don't remember). Most of them were relatively small, but still a bit larger than carefully soldered mosfet assembly. And yes, a relay (as i found out recently) costs about as much as a DYI mosfet.

Originally Posted by mcguyver

A FET is not the best answer either, as it has reliability issues, and is susceptable to the inductive spike generated by the motor. But it's a better option than a relay.

That's why we thought of a relay 1st place - reliability. Only last week i sow 3(!) "commercial" mosfets burning out (including the Systema one).

OK, so let's try a FET. We've made some research and finally decided upon this design http://www.red-alliance.net/phpBB2/viewtopic.php?t=3836
Many thanks to the red alliance. We haven't used the exact trigger model they had, but the relevant parameters are very close. Another change we've made was adding a small radiator to the assembly to help with the heat.

So that's what came out of it.

[Naglfar]

A Relay still uses a mechanical switch, you aren't cutting down on the trigger resistance, nor are you eliminating the current arcing between the contacts.
Like someone already said, you're drawing extra current and using up time to magnetize the coil that contacts the poles.