PIC relays control

[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-

[flipper_md]

Bi-stable Relay, “Latching “, usually means a pulse turns it on, a second pulse turns it off.

I would really like a relay that behave like that!
If it exist and the PIC can run it, that's what I want !!
But the spec sheets aren't very descriptive in that manner...(attached Panason TN spec)

double coils latching relays ask for 40mAh, and need 2 pins...

About FET and other electronics switcher, i'm trying to avoid tehm because they draw some power just for operating.

About Diodes, let's say I want to protect the PIC from the relay, I would use a 5V Zener?, or I need to add a safety margin? 4.5V?

Lots of Amps in the coil effect of a relay?


Thanks for all added info, i'm slowly but surely getting somewhere! 8)

[Pic_User]

Hi Dom,

I would really like a relay that behave like that!
If it exist and the PIC can run it, that's what I want !!
But the spec sheets aren't very descriptive in that manner...(attached Panason TN spec)

double coils latching relays ask for 40mAh, and need 2 pins...

About FET and other electronics switcher, i'm trying to avoid tehm because they draw some power just for operating.

About Diodes, let's say I want to protect the PIC from the relay, I would use a 5V Zener?, or I need to add a safety margin? 4.5V?

Lots of Amps in the coil effect of a relay?


Thanks for all added info, i'm slowly but surely getting somewhere! 8)

from the data sheet:
Panasonic TN Relays
1 Coil latching
TN2-L-5 V
5V 20mA 250-Ohm coil

The TN2-L-5 V should work directly from standard (25mA) PIC I/O pins.

I gave incorrect / incomplete information about latching relays in a previous post.
A single coil latching relay changes state when the polarity on the coil is reversed.

+++++ Info +++++
Single coil latching relay
DC Power to coil transfers contacts.
Contacts maintain after coil power is removed.
To transfer contacts to original position, DC Power to the coil is reversed.
Contacts maintain after coil power is removed.
+++++
Coil suppression circuit:
Circuit to reduce the inductive switch off voltage peak of the relay coil (EMC protection)
The latching relay protection diode would have to allow either polarity.
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1889&stc=1&d=118567495 4" />
Then your PIC would Make I/O A high and I/O B low or I/O B high and I/O A low, to latch or unlatch the relay.

We should mention that using the transistor in place of the relay does not use very much battery power.
But, the PIC has to remain in control of the pin driving the transistor.
I would consider using a logic level CMOS power transistor.
-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

[Pic_User]

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.
Hope I have helped....

Bill12780

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1886&stc=1&d=118564652 0" />
5 V/100 Ohms = 50 mA
5 V/400 Ohms = 12.5 mA (half the Imax of the PIC)

[bill12780]

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1886&stc=1&d=118564652 0" />
5 V/100 Ohms = 50 mA
5 V/400 Ohms = 12.5 mA (half the Imax of the PIC)

oops....Sorry....it was late when I posted that. I think I was going for the four(4) key on the keypad and fat fingered it to the one(1) right below it.
(I mean 5/.0125 is easy math I do in my head all the time...had to be a typo...cant belive I was that tired! hahaha!)

Pic_user...You obviously have WAY more experience with this.

I will yield to your expertise here and bow out. I just thought that I could return the favor for all the help that I get here.

Thanks for correcting that.

Bill12780

[Pic_User]

Hey Bill,

I do exactly the same thing with resistors. I think in current, and sometimes go down in resistance value, to lower the current, oops. I also say “bigger resistor” when I mean a bigger current, oops. We all do that type of mis-speak, don’t worry about that.

You are more than welcome, to discuss alternate ways to do things. There is usually more that one way to solve electronics problems. Just as there is more than one way to program an algorithm. The fun of the forum is the multiple solutions discussed. Let the user decide which way they want to go. Or let their electronic junk-box decide for them. Please, always put your ideas into any thread that you want to help with. Your electronics experience is of great value.

I posted that CMOS schematic from your description. You had said you didn’t really have any easy way of making a schematic. If you look for a drawing program called “paint.NET” it is free and way ahead of MSPaint.

We are all in this together,
-Adam-

[bill12780]

Thanks Adam,

I appreciate your kind words of encouragement.

I just kinda felt like a dumb A** if you know what I mean. I feel like a leach around here most of the time because I am still very new to the PBP and the digital world in general. always asking question that I am sure to some are so elemetry they are bored to tears! I have not had to use it this sort of thing much in my career. Hence...Ignorance...

I will spout-off when I think I can help. But PLEASE if I am wrong just swat me on the nose with a rolled up newspaper!

Maybe one day I can actually be of some help! hahahaha

Thanks for the tip on the proggie...I will check it out. If I was at my work I would have just slapped something together in Eagle really fast. But I have never download it here at home.

Thanks again! (I need to program an F-key for that line)

Bill12780