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PICList Thread
'Reed relays'
1996\12\18@104902 by Mark A. Corio

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Hi again,

I have some sensitive analog circuitry that I need to do some relay switching
in for varying some filter bandwidths and other things.  I am considering
using reed relays.  Does anyone have any comments about this?  I would like
to drive the relay coils direct from a PIC I/O pin (16C74A or 16C84.....not
sure yet).  Will the protection diodes on the PIC I/O pins be adequate for
coil suppression or do I need external diodes as with other relay types?  I
have no experience with reed types but believe the inductive kick should be
smaller from reeds.

In the past, for this type of application I have used low signal level
latching relays so as not to have a constant current flowing through the
coils......I have had problems with noise on the coil drive signals being
imparted on the signal in standard type relays.  This approach also let me
use a common ground plane without the coil current causing problems in the
return path.  Remember, I am very noise sensitive here.  In my current
application the relay (reed or otherwise) will have the coil driven by a
signal from another board (both ends of coil) without electrical connection
the board with the signal.  Are reed relays more sensitive to coupling of
noise to the signal from the coil since the contacts are surrounded by the
actuating magnetic fields?  Any experiences with this are of most interest.

Thanks again.

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel:  (716) 328-5850 --- Fax:  (716) 328-1144
http://www.frontiernet.net/~rmi/

***** Designing Electronics For Research & Industry *****

1996\12\18@105754 by Mike

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>Hi again,
>
>I have some sensitive analog circuitry that I need to do some relay switching
>in for varying some filter bandwidths and other things.  I am considering
>using reed relays.  Does anyone have any comments about this?  I would like
>to drive the relay coils direct from a PIC I/O pin (16C74A or 16C84.....not
>sure yet).  Will the protection diodes on the PIC I/O pins be adequate for
>coil suppression or do I need external diodes as with other relay types?  I
>have no experience with reed types but believe the inductive kick should be
>smaller from reeds.

Ypu'll still need to drive the coil - there are better ways...
Why not use analog multiplexers and switches ?

Mike
Perth, Western Australia

Socrates once gave the advice to "by all means get married... If you
get a good wife you will become happy, if you get a bad one you will
become a philosopher."

1996\12\18@124106 by optoeng

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Mark A. Corio wrote:
...
> I have some sensitive analog circuitry that I need to do some relay switching
> in for varying some filter bandwidths and other things.  I am considering
> using reed relays.  Does anyone have any comments about this?  I would like
> to drive the relay coils direct from a PIC I/O pin (16C74A or 16C84.....not
> sure yet).  Will the protection diodes on the PIC I/O pins be adequate for
> coil suppression or do I need external diodes as with other relay types?  I
> have no experience with reed types but believe the inductive kick should be
> smaller from reeds.

Energy stored in relay coil is

1/2 * L * i ^ 2

where L is the coil inductance and i is the current at the moment you
switch off the voltage.  This energy must be absorbed by something, and
that something is usually either catch diodes and/or output
transistors.  Due to the large number of turns in many sensitive relays,
their inductance can be surprisingly large.

Reed relays are less of a problem for these reasons:

1. The fine wire used to wind reed relay coils has a significant
resistance, so a significant portion of the energy dissipates in the
coil itself.
2. Whereas, conventional open frame moving armature relays have a
magnetic circuit that closes upon actuate, reed relays usually have a
solenoid construction with a lower inductance.
3. Signal switching (as opposed to power switching) reed relays have a
low actuation force and consequently, a low operating current.

Consequently, output structure damage is less likely with signal
switching reed relays.  Having said the above, since PIC output
protection diodes are not rated for energy absorption, I would still
recommend that you incorporate at least 1 protection diode per coil.

{Quote hidden}

There may be significant capacitance between the coil and the contacts,
and your layout may aggravate the problem.  However, since the magnetic
is largely unaffected by non-magnetic materials in the path, it is
possible to electrostatically shield the contacts.  I haven't looked
lately, but it used to be possible to buy shielded reed relays.  If not,
it shouldn't be too hard to add some shielding to a reed relay.  Copper
foil wrapped around the glass envelope of the reed will work: ground it
to your analog ground.  For best results, avoid letting the shielding
act like a bunch of shorted turns: leave a narrow gap so that the shield
makes a 'C' rather than an 'O' around the reed.


--

Paul Mathews, consulting engineer
AEngineering Co.
spam_OUToptoengTakeThisOuTspamwhidbey.com
non-contact sensing and optoelectronics specialists

1996\12\18@132127 by Martin McCormick

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In message <.....961218104807_1087348080KILLspamspam@spam@emout20.mail.aol.com>, "Mark A. Corio" writ
es:
>I have some sensitive analog circuitry that I need to do some relay switching


>return path.  Remember, I am very noise sensitive here.  In my current
>application the relay (reed or otherwise) will have the coil driven by a
>signal from another board (both ends of coil) without electrical connection
>the board with the signal.  Are reed relays more sensitive to coupling of
>noise to the signal from the coil since the contacts are surrounded by the
>actuating magnetic fields?  Any experiences with this are of most interest.

       I have mixed feelings about reed relays.
They are relatively quiet both electrically and mechanically, but they
are fragile and shock-sensitive.  While I don't know all of the constraints
imposed by the design, the first thing that comes to mind is to use a
CD4066 or even a CD4016 quad bilateral switch to do the audio switching.
Each gate on the chip is a pair of FET's which, when turned on, conduct
AC as long as the peeks don't exceed +- V(ss).  The DC control signals do not
appear in the audio path and the switching is clean and click-free.

       One could use an opto isolator to totally isolate the switching
board from the audio board.

       This is just a thought.  What say the rest?

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group

1996\12\18@135842 by Martin McCormick

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       I have goofed.  Just as I sent my last message, I discovered that
I had used the word peek when I should have used peak as in

>AC as long as the peaks don't exceed +- V(ss).

Of course the spell checker passed that with flying colors.

Martin McCormick

1996\12\18@141323 by sdattalo

face
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Martin McCormick wrote:
>
>         I have goofed.  Just as I sent my last message, I discovered that
> I had used the word peek when I should have used peak as in
>
> >AC as long as the peaks don't exceed +- V(ss).
>
> Of course the spell checker passed that with flying colors.
>
> Martin McCormick

I think we should ex-spell Martin from the list.

Mark,

Why do you use the relays? Do you need the low on resistance they
provide? Do you need the the galvanic contact-to-coil isolation?

If it's an isolation issue, have you considered solid state
relays? These are available with on resistances <20 ohms (which
is extremely large compared to a relay), isolation barriers
as large as 3kv, and contact ratings as high as 400V (if not
higher now).

Scott

1996\12\18@144621 by timetech

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Mark A. Corio wrote:
>
> Hi again,
>
> I have some sensitive analog circuitry that I need to do some relay switching
> in for varying some filter bandwidths and other things.  I am considering
> using reed relays...

Mark: Check out the International Rectifier opto-isolated electronic
relays. They do a better job at switching in analog circuits than all
but the most expensive reed relays. They can be driven directly from a
PIC processor pin with a series limiting resistor.

-- Tom Rogers

1996\12\18@162142 by Bob Blick

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>sure yet).  Will the protection diodes on the PIC I/O pins be adequate for
>coil suppression or do I need external diodes as with other relay types?

If the PIC can deliver enough current to energize the relay, it can also
handle the turnoff current which, by the rules governing inductance, is the
same as the run current.

There is another issue you should look at, which is whether your power
supply voltage will be raised during turnoff. If you have a regulated power
supply, and the other loads are equal to or greater than the relay current,
then the regulator should track it no problem.

Don't use pin RA4 without a protection diode, it is open-drain(it's also
inverted).

As to your question of cross-coupling of signals between the coil and
contacts, I have no answer. Attach an earphone to the relay's switch
terminals and see if you can hear the power supply when the relay is engaged.

On an unrelated note regarding unwanted signals, I was building a device
with, among other things, a PIC, keypad, LED backlit LCD, and a piezo
speaker. I noticed that on alternate keypresses the speaker would make a
continuous background noise until I pressed another key. I had it so each
time you pressed a key you got a beep out of the speaker, and the beep was
255 togglings of the speaker pin. Every other keypress would leave the pin
in a "high" state, and the piezo speaker was essentially attached across the
power supply. The sound I was hearing was the small variation in the power
supply as the backlight dimming routine toggled the backlight power every
100 microseconds, changing the load and upsetting the power supply ever so
slightly. I changed the beep routine to leave the speaker pin at 0 volts
upon exit. It's amazing how sensitive piezo speakers are at 5KHz, the ripple
wasn't more than 0.03 volts pk-pk.

Cheers, Bob

1996\12\18@175339 by fastfwd

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Bob Blick <PICLISTspamKILLspamMITVMA.MIT.EDU> wrote:

> Don't use pin RA4 without a protection diode, it is open-drain(it's
> also inverted).

Bob:

Inverted?  How so?

-Andy

=== Andrew Warren - .....fastfwdKILLspamspam.....ix.netcom.com                 ===
=== Fast Forward Engineering - Vista, California          ===
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1996\12\18@211715 by optoeng

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Bob Blick wrote:
>
> >sure yet).  Will the protection diodes on the PIC I/O pins be adequate for
> >coil suppression or do I need external diodes as with other relay types?
>
> If the PIC can deliver enough current to energize the relay, it can also
> handle the turnoff current which, by the rules governing inductance, is the
> same as the run current.

Not necessarily so.  The structures that sustain the flyback current are
not necessarily the same as those that conduct the forward current, and,
if sufficient current flows into the substrate, the whole chip can turn
in to a thyristor!

{Quote hidden}

--

Paul Mathews, consulting engineer
AEngineering Co.
EraseMEoptoengspam_OUTspamTakeThisOuTwhidbey.com
non-contact sensing and optoelectronics specialists

1996\12\18@224139 by Steve Hardy

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> From: "Mark A. Corio" <Mcoriospamspam_OUTaol.com>
> I have some sensitive analog circuitry that I need to do some relay switching
> in for varying some filter bandwidths and other things.  I am considering
> using reed relays.  Does anyone have any comments about this?  I would like
> to drive the relay coils direct from a PIC I/O pin (16C74A or 16C84.....not
> sure yet).  Will the protection diodes on the PIC I/O pins be adequate for
> coil suppression or do I need external diodes as with other relay types?  I
> have no experience with reed types but believe the inductive kick should be
> smaller from reeds.

One trick for suppressing inductive flyback is to place a 'shorted turn' around
the coil.  This could be in the form of a cylinder made of copper foil,
soldered to form a tube.  If it fits inside the coil (and around the reed)
then so much the better.  This technique turns an inductor into a resistor
although, depending on coil geometry, there may still be sufficient
leakage inductance to cause some flyback.  Best put a zener in parallel.

{Quote hidden}

Again, the copper sleeve may be grounded (to signal ground) which will give
additional protection against capacitively coupled noise.  There may be a very
small magnetic coupling to the signal if the reed is slightly helical: very
small indeed, considering the 'turns ratio'.

Regards,
SJH
Canberra, Australia

1996\12\18@224321 by Bob Blick

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>Not necessarily so.  The structures that sustain the flyback current are
>not necessarily the same as those that conduct the forward current, and,
>if sufficient current flows into the substrate, the whole chip can turn
>in to a thyristor!


I beg to differ. Please check the data sheets, and you will see something
like this:

"Input clamp current, IOK(VI<0 or VI>VDD).....+-20mA"

I realize that output current is rated at 25mA, but the fact of the matter
is that PICs have good protection on most of the pins, they are not your
average CMOS chip.

Cheers, Bob

1996\12\18@233229 by Steve Hardy

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> From: Scott Dattalo <@spam@sdattaloKILLspamspamUNIX.SRI.COM>
>[cut]
> Why do you use the relays? Do you need the low on resistance they
> provide? Do you need the the galvanic contact-to-coil isolation?
>
> If it's an isolation issue, have you considered solid state
> relays? These are available with on resistances <20 ohms (which
> is extremely large compared to a relay), isolation barriers
> as large as 3kv, and contact ratings as high as 400V (if not
> higher now).

Uh, I think the guy wanted low level analogue control, not
phase control of his electric furnace.  (Steve waxing sarcastic
here...)

I think for the ultimate in low noise application he is correct
in selecting relays (so long as he doesn't want to switch too
rapidly).  The mercury-wetted contact has much lower noise than
a FET (its 'RDSon' is heaps lower) and there is less capacitive
coupling from the control - FET gate capacitance is at least
a few pf.

Regards,
SJH
Canberra, Australia

1996\12\19@104342 by optoeng

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Bob Blick wrote:
>
> >Not necessarily so.  The structures that sustain the flyback current are
> >not necessarily the same as those that conduct the forward current, and,
> >if sufficient current flows into the substrate, the whole chip can turn
> >in to a thyristor!
>
> I beg to differ. Please check the data sheets, and you will see something
> like this:
>
> "Input clamp current, IOK(VI<0 or VI>VDD).....+-20mA"
>
> I realize that output current is rated at 25mA, but the fact of the matter
> is that PICs have good protection on most of the pins, they are not your
> average CMOS chip.
>
> Cheers, Bob


Agreed that the protection diodes are rated as you state, but I stand by
what I said.  It is not generally true that such diodes can withstand
the SAME current as the output transistors, and the thyristor latch-up
problem is well-documented.
--

Paul Mathews, consulting engineer
AEngineering Co.
KILLspamoptoengKILLspamspamwhidbey.com
non-contact sensing and optoelectronics specialists

1996\12\19@130332 by sdattalo

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Steve Hardy wrote:
{Quote hidden}

Low noise, Low level, small signal, pick your favorite subjective
analog term. Solid state relays such as CP Clare's (formally
Theta-J) LAA-110 perform extremely well in a high-performance
analog circuits. These devices are essentially opto-isolators
with MOSFET outputs instead of the more typical Bipolar transistor
outputs.

Will they perform well in your application? I have no idea. But
I've used them in several designs. In one case, the solid state relays
were used to 1) Multiplex 24 differential analog signals, 2) Switch
Gain settings, and 3) Switch voltage references (part of the
compensation system). Three ranges were available +/- 1V, +/- 5V,
and +/- 15V. About 14.5 bits of dynamic range was obtained while
operating over a -20 to 70 degree C temperature range. (14.5 bits?
yes 14.5 bits; I was using a V/F converter for the A/D converter.
The usable counts were approximately 2^14.5 ~ 24,000.) Granted,
the bandwidth of the signals was extremely low, but it worked
for my application.

Some of disadvantages of these SS relays are:
1) The Capacitance of the contact is about 35pf when it is off.
Relays can be as low as 1pf.
2) The on resistance is between 1 and 20 ohms (depending on the
device you choose). Relays can be found with just milli-ohm on
resistances.
3) Can not phase-control Electric furnaces.

Some advantages
1) Cost. Last I checked they're about 2USD in qty.
2) Smaller foot print. 6pin through all dip and I've seen but
not used surface mount versions.
3) No inductive kick back when the relay is turned off.
4) No relay contact "bounce".
5) Speed; switching times vary from tens to hundreds of microseconds.
6) If you're lucky, you can phase-control an Electric furnace.

I'm sure there are more pros and cons.

Scott


'Reed relays'
1997\03\04@154025 by Marianne Peters
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Please do not send any more mail to this address.  I don't wish to be part
of your discussion--and I have no idea what you are talking about!!

Marianne

1997\03\05@070943 by Octavio Nogueira

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Hi Marianne,

It seems like you were signed to this discution list. To sign off just
send a message to spamBeGonelistservspamBeGonespammitvma.mit.edu with UNSUBSCRIBE PICS
in the body of the message.

Regards,

Octavio
========================================================
Octavio Nogueira
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========================================================

----------
> De: Marianne Peters <RemoveMEpetersspamTakeThisOuTSKYENET.NET>
> Para: PICLISTEraseMEspam.....MITVMA.MIT.EDU
> Assunto: Re: Reed relays
> Data: Terga-feira, Margo 04, 1997 03:16
>
> Please do not send any more mail to this address.  I don't wish to be
part
> of your discussion--and I have no idea what you are talking about!!
>
> Marianne

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