Relays again!

Started by c04203420 October 9, 2005
I was just wondering if someone would kindly explain a little bit
about relays and the bx-24.

I have been to many websites selling relays and the variety/price is
overwhelming. I particularly like the searching facilities of farnell
and their inventory is huge. What i dont understand is how to source
the right one for my project given the huge amount of information
pertaining to each. For example. On the web site i can search by:

Voltage, isolation
Voltage, load RMS min
Voltage, load RMS max
Current, load
Current, load min
Current, surge
Voltage, blocking
Current, surge single cycle
Voltage, control DC min
Voltage, control AC min
Voltage, control DC max
Voltage, control AC max
Output type
Voltage, input min
Voltage, input max

What am i looking for here. I'll give you an example. I need to
switch a 110watt light on average once a day. Do i really need a
solid state relay for this purpose and if so which one would be
ideally suited to my needs.

What do i need to calculate? Does the ssr need an additional power
source to switch the higher (AC) load? Gvien the search criteria
above what am i looking for. Any hyperlink pointers would also be
helpful. Especially from http://uk.farnell.com/

Thanks in advance.

Marc.


--- In basicx@basi..., "c04203420" <c04203420@y...> wrote:
> I was just wondering if someone would kindly explain a little bit
> about relays and the bx-24.

A relay has two main sections: the control section and the load
section. The load section is the part that switches your devices
(i.e. the load) on and off. The control section is the part that
causes the load section to activate and deactivate.

The load section has to be able to withstand the load voltage
(120VAC in your case) when it is in the "off" state and it has to be
able to withstand the load current (approximately 1 amp in your
case) when it is in the "on" state. The load section also has to be
able to withstand the transient currents and voltages that occur
when going from off to on and vice versa. Often, the
relay/switching device will have a "maximum surge current" rating or
an "inrush current" rating. Some devices even have a dI/dt rating
(referring to how fast a change in the load current can be
tolerated). If you're just switching incandescent lights you
needn't worry much about these. If you were switching highly
inductive or highly capacitive loads then you'd have to be concerned
about the transient conditions.

As for the control section, it is usually specified in terms of how
much voltage or current is required to initially activate the load
section and, in some cases, how much voltage or current is required
to maintain that activation. This is common for electromagnetic
relays but a solid state relay will generally just have a
minimum/maximum current requirement for load activation.

Another characteristic of these devices is the "isolation voltage".
That is a measure of how large the voltage difference can be between
the load section and the control section. Exceeding that limit may
cause the device to fail, possibly resulting in the high voltage on
the load side getting into the control circuitry - generally not a
good thing.

There are probably dozens or even hundreds of devices out there that
would work for your application. The selection will come down to
price, availability and suitability. For switching a 110W/120VAC
light bulb you need a relay that is rated for at least 120VAC load
voltage and at least 1A of load current. On the control side, you
need to find a device that can be controlled by a 5 volt logic swing
requiring a maximum of 20mA of control current (less would be
better). Note that you can always add a transistor to your circuit
that acts as an additional switching stage: the BX-24 controls the
transistor and it controls the relay which controls the load.
Opting for this design variation allows you to choose from a much
broader array of relays that require more control current than can
be provided by the BX-24. Moreover, since the transistor can handle
larger load voltages than the BX-24 you can also choose from relays
that require a higher control voltage, e.g. +12V, if such voltage is
available in your system.

From my quick review of the specifications of the SSR that Wim
suggested I would opine that it would serve your purposes well.
Given that, you then need to look into price and availability.

BTW, there is usually a good selection of SSRs and relays available
on eBay, often for good prices. If you're planning to be
experimenting with microcontrollers I'd suggest bidding on some from
time to time to add to your "parts box". It's quite useful to have
a good selection of all kinds of devices on hand so you can quickly
prototype a new idea. After you've proven the idea, you can then
procure parts more closely matched to the application if it is
justified.

Don


Thanks a lot for a very gentle introduction to relays. I was
particularly interested in what you said about adding a transistor to
switch a larger load from bx.

One other thing I would like to know. If i have ten loads does that
mean I will need 10 relays or is there an all in one killer relay
that will switch the lot for me?

I'll have in excess of ten different loads requiring to be switched
and considering that the average price here in rip off Britain for an
ssr is around 7 then the price soon starts to build up.

How could i switch multiple loads. Some being inductive some not. The
most inductive load i will have will be a 250Watt HPS lamp connected
to a ballast.

Thanks.

Marc.



> ... Switching the load on and off when the sine wave is near zero makes a much "quieter" environment.

While zero-switching sounds like a good thing, it can bring high
inductive kick voltages. The voltage rating of the SSR you've chosen to
switch an inductive load can be more important than the current rating.
A non-inductive load that you might want to parallel with an inductive
load will see that voltage spike at commutation; that might not be good.

I did a large seven-segment display using dual-tube 36" 30W flourescents
for each segment. A "ballast" (actually an autotransformer) for 120VAC
flashing flourescent fixtures will produce a ~1000-volt spike on the
load side of a commutating crossover-switched SSR. HID ballasts might
well do the same. Tom


--- In basicx@basi..., "c04203420" <c04203420@y...> wrote:
> If i have ten loads does that mean I will need 10 relays or is
> there an all in one killer relay that will switch the lot for me?

Assuming that all ten loads are to be switched simultaneously, you
can treat the ten loads as if they are one aggregate load and size
your relay accordingly. Depending on the particulars, that may be
more or less cost efficient than sub-dividing the loads. You'll
have to pencil it out.

The other factor to consider is that if you have multiple relays
that need to be driven by one BX output the load on the BX pin will
be the sum of the current requirements of the relays. Depending on
the particulars, the aggregate control current requirement may be
too large for the BX to handle directly. This can be addressed by
adding another switching stage (e.g. transistor or another relay)
between the BX and what you're trying to control.

> The most inductive load i will have will be a 250Watt HPS lamp
> connected to a ballast.

Mixing inductive and non-inductive loads isn't a problem - just
treat it as an inductive load. An SSR with zero-crossing detection
may be just the thing. Switching the load on and off when the sine
wave is near zero makes a much "quieter" environment.