name of a component

[Man-wai Chang ToDie (33.6k)]

What's the name of the component (looks like a magnetic coil) in a 56k
dial-up modem that's responsible for making the modem to go on or off hook?

And how could I tell whether it's a quality one?

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[Man-wai Chang ToDie (14.4k)]

A relay. It either works or it doesn't.

I turned on the modem speaker, and discovered that the relay was NOT
holding the line off-hook properly. I could hear lots of click-click
sound. I took the modem back to the store *TWICE* before getting a good
one.

I wonder whether its manufacturer did any QC on that product line....

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[Man-wai Chang ToDie (14.4k)]

A relay. It either works or it doesn't.

So what's the price difference between a good and a cheap relay?

Do they all use the same voltage? Could I buy a good relay to replace
the cheap ones on any modem and expected a more reliable modem?

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[Franc Zabkar]

So what's the price difference between a good and a cheap relay?

You should be able to find one for less than US$5.

Do they all use the same voltage?

Probably +5V.

Could I buy a good relay to replace
the cheap ones on any modem and expected a more reliable modem?

I've had problems with stuck contacts in certain relays. The symptom
is that the phone line is always off-hook. I've also had faults in the
chipset where the output pin that operates the relay has failed. In
such cases I hard-wired my own hook switch and operated the relay
manually.

You could always buy a modem with a silicon DAA, ie no relay, but
these have their own problems.

One thing to watch out for if replacing the relay is a reverse EMF
diode across the coil. If this is missing in the circuit, then it
should be incorporated into the relay itself. A missing diode will
cause the back EMF to kill the driver chip or the transistor. I
believe these relays must also be especially rated for
telecommunications applications, eg 4kV isolation.

- Franc Zabkar

 

[david]

So what's the price difference between a good and a cheap relay?

Do they all use the same voltage? Could I buy a good relay to replace
the cheap ones on any modem and expected a more reliable modem?

You're joking right? Are you really this ignorant?

http://en.wikipedia.org/wiki/Relay

 

[Man-wai Chang ToDie (14.4k)]

You're joking right? Are you really this ignorant?

http://en.wikipedia.org/wiki/Relay

Yes, I am not a EE guy.

So what is the difference between a relay and a switch?


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[Paul]

Yes, I am not a EE guy.

So what is the difference between a relay and a switch?

A relay, is a switch, with an electromagnet and plunger
connected to it. When you put current into the coil of the
relay, the metal plunger moves, and causes the switch contacts
to close. In a sense, a relay is used to amplify a lower
power signal - a low voltage like 6 volts, when fed to the
relay coil, allows some higher voltage switch contacts to be
opened or closed. The coil of wire is electrically isolated
from the switch contacts. That provides a measure of isolation,
desired between a low voltage control circuit (your modem),
and the high voltage on the phone line (-48V).

http://en.wikipedia.org/wiki/File:Relay_symbols.gif

A switch on the other hand, is just the contacts on the
right of the picture. For example, the light switch in
your room, requires someone to move the plastic toggle
to make it work.

Paul

 

[Man-wai Chang ToDie (14.4k)]

A relay, is a switch, with an electromagnet and plunger

connected to it. When you put current into the coil of the
relay, the metal plunger moves, and causes the switch contacts
to close. In a sense, a relay is used to amplify a lower
....
A switch on the other hand, is just the contacts on the
right of the picture. For example, the light switch in
your room, requires someone to move the plastic toggle
to make it work.

Then why "ethernet switch hubs" were not named "ethernet relay hubs"?

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[Paul]

Then why "ethernet switch hubs" were not named "ethernet relay hubs"?

Terms like relay and switch, are overloaded in the English
language, so can be used in a number of contexts. Ethernet
is largely purely digital, and the switching refers to
packet switching.

Paul

 

[Dave]

Yes, I am not a EE guy.

So what is the difference between a relay and a switch?

Very little. Typically, a switch requires some kind of human power to
operate. A relay uses electricity (usually) to throw a switch. -Dave