Transition to AC

Started by c04203420 October 12, 2005
Hopefully this time next week I will have got my hands on some nifty
little solid state relays to play with.

I have been reading through the physical computing book and Im about
to make the transition from working with low voltage dc to dangerous ac.

Any pointers would be reasuring before i embark upon such a potential
health risk. Could someone please give me some dooz and no noze before
i start.

Thanks. Marc.



--- In basicx@basi..., "c04203420" <c04203420@y...> wrote:
>
> Hopefully this time next week I will have got my hands on some nifty
> little solid state relays to play with.
>
> I have been reading through the physical computing book and Im about
> to make the transition from working with low voltage dc to dangerous ac.
>
> Any pointers would be reasuring before i embark upon such a potential
> health risk. Could someone please give me some dooz and no noze before
> i start.
>
> Thanks. > Marc.
>

Be careful.

I mount the solid state relay in a metal electrical box (covered)
using screws and run in my signal pair to control the relay.

When switching AC in the US, recall that Black is hot. Switch the hot
side to the lamps.

Best wishes.

P H Anderson, http://www.phanderson.com/basicx/



Marc.
I have used solid state relay's for years with no problems. Most of them
use very little current(about 1.5 to 5ma). Some are zero switching. Tip:
Just keep your hands clear of the high voltage and if you have expensive
front side electronics then I'd use an opto-isolator between to protect
yourself.
Harry

-----Original Message-----
From: basicx@basi... [mailto:basicx@basi...] On Behalf Of
pha555
Sent: Wednesday, October 12, 2005 9:11 AM
To: basicx@basi...
Subject: [BasicX] Re: Transition to AC

--- In basicx@basi..., "c04203420" <c04203420@y...> wrote:
>
> Hopefully this time next week I will have got my hands on some nifty
> little solid state relays to play with.
>
> I have been reading through the physical computing book and Im about
> to make the transition from working with low voltage dc to dangerous ac.
>
> Any pointers would be reasuring before i embark upon such a potential
> health risk. Could someone please give me some dooz and no noze before
> i start.
>
> Thanks. > Marc.
>

Be careful.

I mount the solid state relay in a metal electrical box (covered) using
screws and run in my signal pair to control the relay.

When switching AC in the US, recall that Black is hot. Switch the hot side
to the lamps.

Best wishes.

P H Anderson, http://www.phanderson.com/basicx/ ------------------------ Yahoo! Groups Sponsor --------------------~--> Get
Bzzzy! (real tools to help you find a job). Welcome to the Sweet Life.
http://us.click.yahoo.com/A77XvD/vlQLAA/TtwFAA/dN_tlB/TM
--------------------------------~- Yahoo! Groups Links



--- In basicx@basi..., "Harry J. White" <hwhite@d...> wrote:
>... I'd use an opto-isolator between to protect yourself.

Sometimes it is warranted to employ both a belt and suspenders.

An SSR generally (always?) has an optoisolator in it since the control
element is typically a light emitting device. Nonetheless, you could
use another optoisolator just to be extra safe.

Don


> ... you could use another optoisolator just to be extra safe.

Bordering on facetious, wouldn't an additional stage of isolation also
require an additional power supply? Tom



--- In basicx@basi..., Tom Becker <gtbecker@r...> wrote:
wouldn't an additional stage of isolation also
> require an additional power supply?
>
> Tom
>
Well, redunancy is a big factor in safety and reliablity. But it can
be overdone. At some point, the cost outweighs the benefit.
Complexity can add errors, too.

I seem to have opened a can of worms and I may have to eat them.
Yech!!!!!

Before I eat, let me say.....
Generally, isolation is more important as protection to the
microprocessor on the INPUTS where a wrong input will cause damage.

On the OUTPUTS, isolation serves primarily as a convience to allow
changes from one voltage to another and from DC to AC control. The
secondary function is safety to the user as a failure barrier.

The OUTPUTS are usually protected by limiting the amount of load via a
current limiting resistor or a buffer device[such as a driver IC
[74HC07} or a transistor]. These devices will suffer the abuse instead
of the microcontroller.

Mechanical Relays provide ISOLATION too. And, it is very tangible and
physical.

The whole point is [1] AC voltage above 48 volts is generally a
separate topic of design which calls for complete understanding Safety
and Hazards and [2]to design your safety features into the intial plan
of what you are building.

You need a whole approach, not piecemeal. For instance - If you are
using 'balanced' {American]220VAC, use two SSRs on one circuit with
proper fuses or circuit breakers which include mechanical shut off.

Leading edge technology while having many advantages inherently lacks
the test of time which is an important factor in designing Safety. Risk
should be weight against actual need.

[Though from what I read here, SSRs have passed being tested for years
in high power switching and with 3 phase motors; so I must reconsider
what I see on the web.]

Nonetheless, almost all of the electro-mechanical AC power switching
technology has been mature and tested for many years. It is easy to
buy and you can similarly interface whole systems with a
microcontroller via DC relays or SSRs to AC relays.