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'[EE]:Contactor vs. SSR'
2001\11\02@095646 by Madhu Annapragada

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Hello:
If one wants to remotely turn a 48VDC, 10A inductive load on and off using a
logic level control signal (as opposed to putting a switch in series with
the load) is it permissible to use a solid state relay as opposed to a
contactor? For that matter, from a regulatory point of view , what does a
contactor do that a solid state relay doesn't? Forgive me if this question
sounds naive, but I have been unable to track down the answer in any of the
books on my desk.
Thank you
Madhu

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2001\11\02@144414 by =?iso-8859-1?B?Tm9tZWwgoA== ?=

A solid-state relay can accept much lower input currents and voltages to turn it on and much longer life.

Here are the benefits I found:
-opt coupled for protection (eliminates signal noise onto data line and protects digital components)
-high current surge capabilities
-small size
-low current consumption
-longevity of life (much longer)
-reliable switching
-quiet (silent)
-less bounce at output (usually caused by reed)

bad:
-usually low current (though high current is available but expensive)
-killed by over voltage spikes (just requires protection (Caps to ground))
-when they die, they die closed (on, but some have protection)
-cost a little more
-require heat sinks for high current (increases life expectancy)

The opt coupling allows superb isolation. All you need is a LED, a light detector, and a high current triac if you want to build one yourself.

Looking at some datasheets, I found that some operate at 3 to 30v with 7 to 16ma input current. The specific one I am looking at now allows a switching speed from 25 to 70Hz.

"SSRs have some disadvantages. Their output is quite easily damaged by over-voltages, they are restricted to single-pole, normally open, configurations and they have much larger on-resistance than conventional relays. If SSRs are not driven properly or become damaged, they can act like rectifiers, resulting in a half-waving AC supply to the load." (Electronic Times, http://www.electronicstimes.com/story/OEG20010829S0014)

well, that s all I care to find.
If you have relatively low switching speeds for a non critical application (you can replace the relays easy), then I would say use contactors. These allow much higher currents and all you would really need would be a buffer at the output of your digital device and maybe amplify it to drive the relay. You can find them at almost any auto parts store.

{Original Message removed}

2001\11\02@151601 by Madhu Annapragada

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Thanks Nomel:
So if I understand you right, a contactor is essentially a mechanical relay?
Or does a contactor offer something unique in terms of protection that sets
it in a different category from a garden variety electromechanical relay?
Thanks
Madhu




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2001\11\02@153231 by Douglas Butler

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I generally find the term "contactor" used in multi-horsepower or
multi-kilowatt applications.   One book I have defines a contactor as a
switch used in power circuits over 20 amps.  A contactor is a big switch
that may be thrown manually, electrically, hydraulically or by other
means.

Sherpa Doug

> {Original Message removed}

2001\11\02@153704 by steve

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> Here are the benefits I found:
>  -high current surge capabilities

You have to be careful with this one.
The current ratings with a SSR are more absolute than with
contacts. For example, if you are switching a light bulb and your
contacts are under-rated for the cold current, you'll get away with it
at the expense of contact life.
Not so with an SSR so you have to be sure that you cater for worst
case situations.

> bad:
> -killed by over voltage spikes (just requires protection (Caps to
> ground))

See above - A cap can take you over the instantaneous current
limit.

You know what is inside them so just use the same precautions
you would for a discrete triac/mosfet solution.

Steve.

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Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
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2001\11\02@155258 by t F. Touchton

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Contactors I have worked with also included a "heater" circuit.  This is a
small coil that heats up as current is drawn.  It is connected to a
mechanical thermal sensor, and shuts the contactor off like a circuit
breaker if an over current condition exists.  The interchangeable coil is
selected based upon the current draw of the load.

If I may be so bold to ask... what is a "Sherpa"??

Scott F. Touchton




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I generally find the term "contactor" used in multi-horsepower or
multi-kilowatt applications.   One book I have defines a contactor as a
switch used in power circuits over 20 amps.  A contactor is a big switch
that may be thrown manually, electrically, hydraulically or by other
means.

Sherpa Doug

> {Original Message removed}

2001\11\02@160358 by Douglas Butler

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> If I may be so bold to ask... what is a "Sherpa"??
>
> Scott F. Touchton

I had a summer job as a porter at a camp in Maine.  The porters were
called "Sherpas" from the people that live in the Himalayas.  My uniform
shirt had my title and first name => Sherpa Doug.

In college I got a reputation for helping people move furniture and
other large awkward objects.  That combined with the shirt caused the
name to stick.

Douglas Butler (aka Sherpa Doug)
Senior Engineer
Imetrix Inc.
1235 Route 28A
P.O. Box 152
Cataumet, MA 02534-0152
tel. (508) 564-6460
Fax (508) 564-6860
@spam@dbutlerKILLspamspamimetrix.com

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2001\11\02@173114 by Madhu Annapragada

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Many thanks for the clarifications.
One last time...in summary, if I am switching a 48VDC, 10A inductive load
then  I can do with a properly rated electromechanical relay right?
I am thinking of the 40A, T92 series board mount DPDT relays from TYCO
electronics. I will be controlling the coil of the relay using a simple
toggle switch that will be mounted on the front panel of an instrument.
Thanks
Madhu


{Original Message removed}

2001\11\02@174644 by Randy A.

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In a message dated 11/2/01 5:32:08 PM Eastern Standard Time,
RemoveMEmadhuTakeThisOuTspamCONECTIV.NET writes:


{Quote hidden}

Madhu:

Yes, you can use an electromechanical relay for your application.  Just be
sure to put a diode across the coil (if it is a DC coil) and a suppression
diode across the contacts as well if you are switching a DC load.  If you are
using an AC coil you should use a RC network for suppression across the coil
and nothing is required across the contacts with the AC normally.

I hope this is of some help.

Randy Abernathy
4626 Old Stilesboro Road NW
Acworth, GA 30101
Phone/Fax: 770-974-5295
E-mail: spamBeGonecnc002spamBeGonespamaol.com
We repair, install and service industrial woodworking machinery, specializing
in SCM / SCMI machines.

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2001\11\03@021212 by jeethur

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Yeah, a contactor is what I use to switch large 3 phase motors.
It basically is a relay with a 230 volts (or 110 elsewhere)
Coil. So, I drive the Contactor with a small relay.
SSRs are small, neat and good(most of the times). But when it comes to
inductive loads, SSRs and be a big PITA. Last month I had built a PIC
based timer for a friend. I use a MOC3041 based SSR circuit. It worked
perfectly
For non inductive loads. But when my friend connected a 5 HP motor to
it,
The SSR would not completely turn off. Tried everything but the problem
Persisted. So, I just used a plain old relay to do the job and its been
Working fine since then.

Jeethu Rao
http://www.jeethurao.com


> {Original Message removed}

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