Relays, Relays - Choosing the correct one!

[Peter_wadley]

Olla!

I haven't gotten around to working with relays in my projects yet and would like to do so..

I understand how they work. However, there seems to be so many different variations available.. im at a loss for choosing the right one.

Basically, Id like to use a PIC to control 120V AC appliances.

None of which exceed a current of 10A.

So I guess that would mean I need a NO (Normally open) .. correct?

What other specifications would it need to be?

Im pretty sure I read that low voltage DC can switch higher voltage AC in the WIKI relay page.. which is pretty much the point of relays in the first place

Would I need to use a transistor to increase the output current of the PIC when switching the relay??

Also a diode should be put in parallel with the relay correct?

Best regards,

 

[Speakerguy]

Yes and yes to your last two questions.

 

[gerty]

Or you could go with solid state relays, if you like. They can be turned on with 3-32 volts @ 20ma typically.They are available in a couple of packages including circuit board mount.

 

[Peter_wadley]

Hey,

I just got back from the store..

I bought one relay to try.

Im pretty confused about how to use it though...

On the package it says:

5VDC SPDT 360mw 70ohm COIL

10A@ 28VDC 10@ 125VAC

There are 5 pins

2 are one side and 3 are in a triangle on the other side.. in the triangle is a picture of a coil.

Can anyone provide a relay tutorial link?

BTW, it is a small black plastic square if that makes any difference at all.

 

[Speakerguy]

You need to supply at least 70mA on the primary coil (the two pins) to get it to switch. This is most easily accomplished with a logic-level N-channel FET used as a low side switch fed from the output of your PIC. Use a diode snubber on this side of the relay.

The three contacts on the other side are the SPDT switch. Which pin is the pole and which two are the throws, I dunno. You'll want to use the pole and one throw, leaving the third pin NC'd. This will make it work so that you can turn the appliance on and off (you don't need the second throw for that).

 

[Peter_wadley]

Hey ok Ive identified the

Coil leads..

So I need to apply 70 ma of 5VDC to these two leads in order for switching to occur RIGHT?

The common..

Do I connect either wire of the AC supply to this lead?

The Normally open and Normally closed.. I just run the other AC wire through a LOAD and then to whichever of these leads RIGHT?

Thank you

 

[Hero999]

A good way of saving power is to power the relay at full duty cycle for a few hundered miliseconds to turn the relay on then PWM it at <50% duty at >20kHz to keep it on whilst using less than half the power.

 

[Peter_wadley]

A good way of saving power is to power the relay at full duty cycle for a few hundered miliseconds to turn the relay on then PWM it at <50% duty at >20kHz to keep it on whilst using less than half the power.

Thanks for the tip Hero

Yeah it does strike me as kind of waste full using power to hold a switch.. but I guess the coil wattage is really discrete compared to the common - NO and common - NC wattage..

I understand relays now, Thanks for your help all.

 

[Hero999]

Relays have quite a large amount of hysteresis, a 12V relay might turn on at 9V and off at 4V. When the relay is pulsed off for sort periods the current carries on flowing through the freewheel diode (due to its inductance) keeping it on and providing the current never drops below the minimum holding current it won't turn off.