Relay confusion

You typically drive a relay from a digital signal with an NPN transistor. You'd typically only use an IC driver if you wanted to drive a bunch of relays. The ULN 2003 is a common device for this.

You can get 5V "high sensitivity" relays which can be driven directly from a digital logic output, but they are usually your smaller signal type relays, not the big beefy ones.

Dave :)

** Groper alert !

** OK, cop this.

Relays operate using #direct current, so the inductance of the coil is irrelevant to the basic operation.

Relays are made in a variety of voltages, simply to suit the needs of various users.

Relays with higher voltages have MORE turns of finer gauge wire on the coil than those with low voltages.

MORE turns of wire means higher resistance and inductance values.

Relays vary in the amount of power needed to make the contacts engage.

Small generally relays use less power than big ones.

Get it ?

( # Excepting those specially designed to use 50/60 Hz AC voltage )

..... Phil

Hi, Rog,

A given relay family (all alike except for DC coil voltage) will tend to have identical coil power requirements. So a 24-volt relay of type X will use half the current of a 12-volt type X, so its coil resistance will be 4x as much.

The 24-volt guy will have twice the number of turns (of smaller wire) so will have 4x the inductance. With 4x inductance and 4x resistance, the L/R time constant stays the same, so pullin/dropout timing will be mostly unchanged.

Of course, as the armature moves in, the magnetic path gets shorter, so the inductance may increase considerably after it's energized!

Normally one would tie one end of the coil to, say, +24 volts, and switch the other end to ground (or not) with a transistor or a fet or something. So coil current is determined by the supply voltage and the coil resistance, not by some value programmed into the driver. A diode is usually placed across the relay coil to keep the inductive flyback from trashing the transistor when it turns off.

Small relays can be driven directly from logic gates, often without even needing a diode.

AC relays are a bit more interesting.

John

Sorry to say this and no offence intended. Electronics 101 starts at your nearest technical institute next year. After that you may be ready understand these things. News groups are good for single questions but they are not all that great for a basic understanding of the subject.

--
John G

Wot\'s Your Real Problem?

"John G"

** That is a VERY cruel remark.

The OP announced his inexperience completely honestly.

** The OP has almost certainly done what jokingly passes for "'Electronics 101" these days.

Only it was all done on a PC running a dodgy simulation program.

Only the instructors barely knew more than the students.

Only no REAL items of electrical or electronic hardware were available for student perusal - let alone the sort of free experimentation ESSENTIAL for the development of a *feel* of their " raison d'etre " .

** Says who?

Ever read the name of this NG ????

...... Phil

On Thu, 29 Jun 2006 19:55:25 +1000, in message , "John G" scribed:

I respectfully disagree. The poster had a question germane to the newsgroup, presented it completely and matter-of-factly, and got a very good answer from John Larkin[1]; an answer that was probably of good use to a great many readers of the group, including myself. It was one of those little things that I've not spent a lot of time worrying about, and now I know more! I've been enlightened!

[1] No offense to Phil, but your answer was just a bit snippy. :-)

--

If life seems jolly rotten, there\'s something you\'ve forgotten,
and that\'s to laugh and smile and dance and sing!

"Alan Bullshit "

( snip sickening arse licking)

** LOL .....

A typically narky throw away remark from one of Larkin's fellow autistic c*ck suckers - who could not be "enlightened " even if a 4 foot long fluoro tube were shoved up him.

....... Phil

I'll jump to the defense of the OP too, it was a perfectly valid question asked nicely. The OP clearly has done some form of electronics

101 (not that it matters), but didn't know much about relay driving which is fair enough.

You can get a great understanding of a subject on newsgroups! Some people give detailed and through replies on a subject, often much better than you'll get in any text book or electronics class. Basic relay operation is not hard to explain. Other times you can often peiece together comments from different replys and understanding of a subject can fall into place quickly from that.

Dave :)

Hmmm... that makes the dropout time slower than pullin, given a low-impedance drive, or a diode clamp. Never thought of it that way before. It would be interesting to measure the deenergized/energized coil inductances of a few DC relays.

The delta-L is an important design factor in AC relays, and allows them to be more reliable than DC relays in some situations.

John

not exactly sure what your looking for but how ever. maybe its the pull and holding currents your looking at? relays require more current to get the contacts toward the magnetic core than it does to hold it there. as the contact arm gets closer to the core the magnetic field becomes stronger! thus once it's fully on it may only require half the current to hold it on than what it did to get it started. in fuel injected engines you have what is called a Peak and Hold injector which in it self, is a magnetic actuator.. more current is applied to get it to the full open position and then less current is needed to hold it there ! in logic this could be stated as hysteresis;

--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

relays are crude mechanical devices, get the voiltage approximately right and they'll work after a few microseconds.

as you have noticed they are inductive and so won't respond instantly to a voltage, and using a constant current drive can get a slightly faster response.

similarly sized relays need a similar ammount of magnetism to operate the contacts, to get that from a lower voltage needs less resistance: thiker wire and fewer turns carrying more current.

total watts will be in the same ballpark.

Bye. Jasen

"jasen"

** Strewth - I gotta see one of them.

....... Phil

Hello all,

Thanks to all (well, almost all ;) ) for the throrough explanations. I also am glad to see that I am not the only one who thinks that I did not overstep the bounds of this ng.

The reason why I need a relay, is because I want to multiplex the power supply of a DDR RAM ---they can sink up to 2A of current. Otherwise, I would went with a simple cmos switch.

-Roger

There are bounds?

er, milliseconds.

Expect nasty bounce and stuff. Electronic switching might be better.

John

On Fri, 30 Jun 2006 01:39:33 +1000, in message , "Phil Allison" scribed:

Ah. A true sociopath. How incredibly boring.

--

If life seems jolly rotten, there\'s something you\'ve forgotten,
and that\'s to laugh and smile and dance and sing!

Oops! I meant a few milliseconds,

Bye. Jasen

seems an odd use for a relay. they are kind of slow in comparsion to ram chips. and I meant milliseconds no microseconds. Bye. Jasen

#

I want to multiplex the RAM's power, not one of it's data signals. Since a DDR RAM can draw roughly up to 2A of juice, I do not know of another alternative than a relay. A response time of in the order of ms is the price I have to pay :(.

-Roger

Ok, I'm curious, *why* do you have to switch power to DDR RAM? Is this some sort of custom design?

BTW, be careful doing this, you may not be able to simply throw a relay in series with the RAM, signal integrity issues could really come into play here.

Dave :)