What does a relay really do?!?

[DYORD]

Why in cars... it gives more power to your headlights or your horn.. but still using the same power supply.

How about in airsoft?! What will it do good to your AEG's?

[Murdoch]

Nothing significant other than making it slightly more complex. A relay is just a device which uses an electromagnet to operate a switch. Generally the benefit is you can use a low voltage to switch on/off a high voltage. Since you are dealing with only one voltage level in your aeg, and the wire lengths are minimal, there really isn't any benefit to using a relay. If your mechbox and motor demand higher currents and voltages to operate you would be better off with a mosfet switch. It will switch the higher currents without the arcing and heating associated with mechanical switches.

btw, if you care, the only reason your headlights burn brighter is because the current doesn't have to run from the battery to your console, through the switch, and back to the light. Thats a lot of resistance for 12v to push through and by using a relay, you lower that resistance and get a brighter light.

[ILLusion]

Originally Posted by Murdoch

Since you are dealing with only one voltage level in your aeg, and the wire lengths are minimal, there really isn't any benefit to using a relay. If your mechbox and motor demand higher currents and voltages to operate you would be better off with a mosfet switch. It will switch the higher currents without the arcing and heating associated with mechanical switches.

Isn't a MOSFET a relay?


Some useful reading:
http://www.airsoftmechanics.com/home...nt/view/23/26/

[shadowninja]

Originally Posted by ILLusion

Isn't a MOSFET a relay?


Some useful reading:
http://www.airsoftmechanics.com/home...nt/view/23/26/

no not realy

Pronounced MAWS-feht. Acronym for metal-oxide semiconductor field-effect transistor, a common type of transistor in which charge carriers, such as electrons, flow along channels. The width of the channel, which determines how well the device conducts, is controlled by an electrode called the gate, separated from channel by a thin layer of oxide insulation. The insulation keeps current from flowing between the gate and channel.
MOSFETs are useful for high-speed switching applications and also on integrated circuits in computers

[ILLusion]

Thanks for the clarification.

I think my confusion comes from the fact that Systema calls their MOSFET unit a "relay unit", but on closer inspection, the "relay unit" actually consists of two MOSFET switches connected together.

[mcguyver]

A relay can also be used to switch higher current loads than what a switch is capable of handling. This is the reason it's used for automotive applications and is also the theory behind its use in AEGs. This is by far the most common use for a relay, which can have many guises from the transistor to the Bosch-style relay for cars to huge contactors to drive large electric motors in industrial operations.

You could use a relay to switch a motor in an AEG with only a small current actually running through the trigger (switch) assembly. A FET does this in a small package and this is the real benefit of a FET, arc reduction aside. A relay will do the exact same job, but will be in a larger package and there will be some increased lag as the relay coil charges and drives the contacts closed. And a relay will be much more durable and less susceptable to cemf and thermal damage than any FET.

That's all.

[MadMax]

Relays typically use a small electromagnet to pull a contact reed across the major current path. This has some practical differences from transistor switches, particularly MOSFET switching devices.

Because a coil has to be charged to pull a physical contact, relays have a sizable "on time". This is the delay between applying a voltage to the coil and acheiving contact in the moving reed. Relay on times are around 1ms IRC.

Conversely FETs can acheive on times in the 50ns range which makes them useful for signal transmission as well as power applications.

Because relays have physical gaps which close, they can create sparks at opening and closing which can actually weld them shut.

However there are some significant benefits to using relays over transistors.

Relays are typically very tolerant to static discharge. It's pretty hard to fry a coil with a static spark. Conversely FETs are quite susceptible to static discharge.

FETs also incur a 0.7v drop across their current path no matter what the amp rating of the FET. This means that FETs will always dissapate 0.7v of voltage. For a 9.6v circuit, this means that you'll burn 7% of your power in your FET.

Relays are not subject to 0.7v diode losses, but it's hard to find small relays rated to 30A (I'm not sure there are any).

Because of the semiconductor nature of FETs, they can pass current in one direction only. This is generally ok for AEGs which only drive their motors in one direction, but it can complicate motor control in other devices. It is important to connect reverse bias diodes across motor current paths to provide a path for back EMF to dissapate when motors are shut off.

For those taking electrical engineering or technical courses, DC motors behave partially as strong inductors. They do not like discontinuous current flow and can spike very high voltages to drive current when the current supply is disconnected.

The consequence of this is that very high voltages can be developed when a motor is disconnected from the power supply (which is the cause of much of the motor sparking) which can damage FET banks. Connect a reverse biased diode in series with a low resistance resistor (maybe 50 ohms?) across the motor terminals to provide a back EMF path instead of blasting it across the FET bank.