Relay question

John "The Bloviator" Larkin's magic excess charge ?:-) ...Jim Thompson

-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 |

Spice is like a sports car... Only as good as the person behind the wheel.

Considering the source of the question, it certainly can't be an electrical reference.

He must mean the circuit should, if it feels like it AND the winds are right AND the moon is in the right phase, work sometime in the present, as in "The current situation".

Me? I'd prefer an antique to do the job:

mike

It is odd and somewhat creepy that you (JT) frequently ignore the topic under discussion and instead present off-the-wall ad hominem remarks about John Larkin. Such behavior is unprofessional and can do you no possible good -- you have nothing to gain by ad hominem posts.

t I

=A0 =A0 ...Jim Thompson

JT's career has reach the point where he has nothing to lose, if you get my drift.

I'll try to explain my problem clearly. I have a circuit with a bunch of reed relays in it to switch on and off different parts of my system. One particular relay in my system essentially electrifies a coil to create an electro-magnet. When I switch this relay on and off, the magnet works correctly. However, when the relay is switched off, there is something happening that i don't quite understand yet. When I switch off this particular relay, every other relay in my circuit is momentarily activated (< 1 second). I'm thinking this is because the load on my relay of interest is drawing a lot of current and when the relay is switched off, there is some sort of shock provided to some part of my circuit, causing all my relays to briefly switch on.

Anyway, I hope this problem explanation makes sense and someone can give me a solution I haven't tried yet. If I haven't provided enough detail about my problem, let me know and I will fill in the gaps. Usually, I'm pretty good at figuring out stuff like this, but I honestly don't understand this one. Help would be very much appreciated! Thanks. :-)

Sounds like your problem may be caused by back-EMF when you cut the current to the electromagnet, which is an inductor. Are you using anything (diode, snubber, etc.) in your circuit to catch the inductive kick?

As Bitrex says. This topic has been discussed here numerous times:

It sounds like you need a snubber.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Something about a cigarette..switch and don't fight? Translation: how about a complete circuit so that we can see current paths?

Are you fussy about what KIND of a cat to have? Some are VERRRY good at that...

[IMG] Relay_circuit.jpg[/IMG]

Thank you all for your help. I have added a link to the section of my circuit that I am referring to. As you can see, pretty simple design. The "Shutter" is essentially an electromagnet, so it's a high current load. There are five other similar relays in my design that all control different things (signals, solenoids, etc.), but none of them cause this backlash problem. However, they all temporarily activate when the "Shutter" relay turns off.

Like I said before, I probably sound like I don't know what I'm doing, but this is my first time really designing circuitry for a mechanical system like this. All my previous electronic experience is signal processing, so good capacitor placement solves 90% of those problems, lol. :-)

Thanks again for your help! :-)

Thank you all for your help. I have added a link to the section of my circuit that I am referring to. As you can see, pretty simple design. The "Shutter" is essentially an electromagnet, so it's a high current load. There are five other similar relays in my design that all control different things (signals, solenoids, etc.), but none of them cause this backlash problem. However, they all temporarily activate when the "Shutter" relay turns off.

Like I said before, I probably sound like I don't know what I'm doing, but this is my first time really designing circuitry for a mechanical system like this. All my previous electronic experience is signal processing, so good capacitor placement solves 90% of those problems, lol. :-)

Thanks again for your help! :-)

You need a snubber. At it's simplest you'd just use a diode across the shutter, similar to what you're doing at the coils of th ereed relay

If the shutter is sharing a common ground with the drive circuit, why aren't you using a transistor to actuate it, instead of a reed relay?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Travis-

Energy is stored in the metal core of the electromagnet. When the circuit is opened, the magnetic field collapses and generates a high negative voltage across the coil. (E=L*Di/Dt where Di/Dt is very high)

How critical would a time delay be, when the Shutter is deactivated? If you connected a diode across the Shutter, it would keep voltage from building up but would delay Shutter deactivation.

I see that diodes are already used across the reed relay coils.

Fred

OK; try the following in order:

1) disconnect the "shutter" solenoid and test operation; bet it works with no problem. 2) Use a snubber on the "shutter" coil similar to that shown on the relay coil; this might be sufficient. 3) If not, put a 10 ohm resistor in series with the diode, and then a resistor in series with the supply for the "shutter" relay - such that the resulting (lower) voltage is adequate for pull-in; increase supply voltage as much as you can so that this "extra" resistor is at least the same value as the "shutter" coil resistance.

y h

Silly question, I thought a 'snubber' was a series RC with R ~ coil resistance and C ~ L/R^2. But your comments seem to indicate that it's just a diode across the coil.

Thanks George H.

--
while the RC snubber is called a snubber, the diode across the
inductance cab be called a snubber diode, a flyback diode, a
freewheeling diode, a suppressor diode, or a catch diode. 

Quite an impressive list of AKAs! ;)

If you have two pieces of metal that will otherwise smack into each other in a machine, and you put a rubber block or spring+damper there, that is also called a "snubber". As are (I think) the funny round things on the ends of British rail cars and engines.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tim Wescott Inscribed thus:

They are called "Buffers" over on this side of the pond !

--
Best Regards:
                     Baron.