RE: [CAD_CAM_EDM_DRO] servo E-stop circuit

A relay is a switch that is controlled, instead of by your hand, by an
applied voltage. Usually, the controlling voltage is different than the
voltage/current that is switched.

A contactor is a specialized relay designed for switching power to a motor.
Usually, a contactor will be suitable only for switching AC voltages(or at
least rated only for AC voltage). There may be DC contactors available, but
I have not seen them. The ratings on contactors are usually in volts, and
hp, so you can directly figure out what contactor to buy for your motor.

A circuit breaker is just that; it is designed to break or open the circuit
when a specific condition occurs; usually overcurrent. These devices are
designed to protect equipment, and are not usually rated to be constantly
switched on and off. There ARE circuit breakers that can be tripped(shut
off) by applying a voltage, similar to arelay, but I don't know if you can
also get ones that can be turned back on by application of a control
voltage. Most circuit breakers trip automatically, and can be reset only by
hand. Many can also be shut off by hand.

If the "contacts" are to be controlled by a voltage, rather than by your
hand, then a relay is what you need. Unfortunately, you need a relay that
has the current and voltage rating to do what you want. I don't know the ckt
you are talking about, so I can't say if you will need the full 15A rating,
but you will need to make sure that the relay you select has the proper
voltage rating AT THE 15A current rating.

The voltage ratings on relays have to do with what happens when the contacts
are in the process of closing or opening. When there is a small gap, arcing
can(will) occur, and this can lead to pitting of the contacts, which causes
more arcing, etc.

High DC voltages are among the hardest things on relay contacts. If you look
at relay contact ratings, you will see that a relay that is rated at, say,
10A@120VAC is usually also good for 10A@240VAC, but only 10A@28VDC. This is
because when you are switching DC, it is much harder on the contacts. Also,
you can't use Ohm's law(a very common mistake) to figure out how much
current is allowed at a different DC voltage. This is because the relay
ratings don't have to do with power dissipation(V*A). So, for instance, the
relay in the example above is probably NOT useable at 5A@48VDC. Relays rated
for higher than 48VDC do not seem to be very common.

All that being said, what is important is the voltage that the relay
contacts will see; in other words, if the contacts are open, what voltage
would you read across them with a voltmeter. And, if the contacts are
closed, what voltage will appear across them as they are opening. These are
the DC voltages that you must size the relay for. In a simple circuit, it is
just the value of the DC Power Supply voltage. In some of the bleed resistor
circuits I have seen, the switching takes place(at least the opening OR the
closing) at very low voltage levels.

Also, if you can arrange to switch to take place when no current is flowing
through the relay, I believe you will be ok using higher voltages on a relay
rated only for the lower DC voltages.

Search the archiv; there was quite a bit of posting on this subject a few
months ago-not sure if it was here, or at Geckodrive forum, or at DIY-CNC.

Andy Wander
Verrex Corporation


-----Original Message-----
From: Lance Hopper [mailto:snaggletto@...]
Sent: Thursday, July 15, 2004 9:21 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] servo E-stop circuit

Hello,
I need an E-stop circuit for my Gecko servo system. I've been
looking at Mariss' "servo E-stop circuit" in the files/circuits/misc
circuits section. I need help figuring out what parts to buy.

There are 2 NC (normally closed) 15A 'contacts' in the upper section
of the drawing that switch the braking resistor in during E-stop.
Are these more like relays, or contactors, circuit breakers or what?
I've done some looking at places like automationdirect.com, and they
seem to have them as 'inductive' motor starters (some are for
resistive loads).

What 'device' do I need here (please be specific)? Any
recomendations on where to get them?

If anybody thinks they have a better 'E-stop circuit' that a non-
electrical engineer can build, I'm all ears. Thanks.