PIC relays control

[flipper_md]

Hi All

I'm starting a little projet with a PIC chip, since I discovered that PIC Basic can make it easy for me

I have some programming skills, but I'm very poor at selecting components, so may someone more aware of the electronical side help me?

I'm want to use a 16F84A or 16F628A (i have both actually), and If possible i want to drive some low power relays directly from the chip!?

Panasonic and Omron makes 5V, 20mAh, 250ohm relays that are close to the output power of the PIC pins, but anyone ever made it work?

I'd like a single coil latching relay, maybe the TN2-K-5V of Panason..

Next thing is how to drive it safely, knowing that I should add zener diodes somewhere to dampen the coil release.
Being a single coil latching relay, I will need two pins and so, current will flow both direction.
I would prefer a relay that changes state everytime it get a pulse, so driven by only 1 pin, but I haven't seen that anywhere.


k, enought on the relays, it's already scary long

ttl

Dom

[paul borgmeier]

Flipper,

Where are you? In the states you can get these (below) .. they work great using one output pin but they do not latch. I used them in testing equipment years ago and they were perfect. No zener needed, just a flyback diode across the coil. I drove mine direct from a PIC output at 5 volts since the coil resistance is 250 ohm.

http://www.radioshack.com/sm-compact...i-2062478.html

[bill12780]

Ok here is the deal,


You MUST use a diode across the coil of ANY relay (unless its got one built in) Its for the back EMF (colapsing magnetic field) of the coil when you un-energize it.

If your looking for a latching type of switch depending on what your doing you have lots of options. relays with NO and NC contacts (probly your best bet), scr's, Triacs etc..

If your worried about not being able to drive the coil from the PIC. You can just use a FET. The input impedence of the gate of a FET is really high and takes almost no current to turn on. Just make sure to use a series resistor in the gate circuit in case the FET ever fails (If the fet ever shorted gate to whatever it could hurt the PIC)

Oh, one other thing to consider is what your switching. If your switching AC with lots of current you will need a snubber circuit to reduce the arching on the relay contacts. I just went through this on my predesesor's motor control circuit at my work. His "new" design would fuse the contacts together about the thrid or forth time the motor started.

Well that is my two cents (probly all its worth) hope it helped. If you want to post a schematic of what your doing I would be more then happy to look it over.

Best!
Bill12780

[ohararp]

Bill, do you have a schematic or part number for this control circuit with snubber by chance?

[flipper_md]

hi folks

i'm happy to know that it's at least possible to do

Radio Shack has become CircuitCity around here, and they don't carry much of electronics anymore...
I will probably make an order at Digikey.

I'm looking to drive two 12v (DC) HelmetCam, who drink 130mAh approx. each.
(but it runs very hot... I wonder how it can only be taking 130...)
At first I wanted to have some kind of soft power switching, because that whole thing is plugged into a camcorder, but this sounds very complex to me so I will use Zener diode in the circuit to prevent overvoltage peak.
KIS system

I need Latching because I won't be switching too often, and the circuit can run for more 6-7hours non-stop.

thanks all!



Dom

[Pic_User]

Hi Dom,

As I see it:
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1883&stc=1&d=118558263 1" />
You would need a 5V relay to drive it directly.
The PIC 25mA limit would drive 200 or more coil Ohms.
Not:
Mono-stable Relay, normal relay that stays energized while a voltage is on the coil.
But:
Magnetically Latching, or Mechanical Latching Relays
Bi-stable Relay, “Latching “, usually means a pulse turns it on, a second pulse turns it off.
Single Coil Latching (1 coil latching), usually means the second pulse goes to same connection.
A single PIC I/O could give a pulse to turn it on, then later a second pulse would turn it off.

Dual Coil Latching (2 coil latching), usually means a pulse on coil A turns it on, a pulse on coil B turns it off.
This would require 2 I/O pins for control.

Here is a hook-up if you want to use a 12V relay.
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1884&stc=1&d=118558290 6" />

You could also use a SCR that would latch with an impulse from a PIC I/O pin. It would be more difficult to un-latch.
-Adam-

[Pic_User]

Hi ohararp,

Bill, do you have a schematic or part number for this control circuit with snubber by chance?

Here is a simplified example of a snubber for AC.
And a “fly-back” diode for DC. Note polarity.
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1885&stc=1&d=118558358 0" />

-Adam-

[bill12780]

OK...

The snuber I spoke of actually is acrossed the "contacts" of the relay. You only need this if your driving an inductive load or a really high current circuit. Like say a large motor or amplifier.

the "flyback" diode is used across the "coil" of the relay (reversed biased) to stop the colapsing field in the relay coil...Not in the load. (Although if your load was a DC inductance I supose it would work there too...)

You can use a Zener to clamp the voltage if your concerned about spikes. But if your power supply is well regulated you should not have any problems swithing 130 mills. Just make sure you have good decoupling.

You could use a SCR like was mentioned. You will need to remove the source voltage to get the scr to "unlatch" or turn off. This might be a good option but would require more engineering. (transistor to remove the voltage from the Anode long enough for the SCR to turn off and slow enough that it does not trigger itself)

For the kind of current your looking at. I would use a n-channel FET. I dont really have any easy way of making a schematic for you but I will try and explain.

Take the HIGH from the pic and run it through a series resistor that will limit the current to about half the Imax of the PIC (about a 100 ohm should give you about 12mA off the top of my head) hook the resistor to the gate of the FET put the load (your camera) in the source circuit and ground the drain.

Oh...And this is assuming that your cameras run on DC. If your switching AC then you will need a triac.

By using a FET you will reduce your overall Current drain. It takes alot less current to keep a FET on then it will to hold a realy.

If you look you can find a latching Relay. I personally have never used them so am not to familar with them. But I would bet they are more expensive then using a standard realy or FET.

If you would like I will look at my schematics tommorow at work and get the actual values I used for my snubber. I think it was .01uf and 10K if I remember right but don't hold me to that.

Hope I have helped....

Bill12780