P
Peter S. May
- Jan 1, 1970
- 0
(Almost certainly overthinking this one, but another case where the
wrong decision means more waiting for another mail order.)
A couple of projects I have in mind would benefit from
CMOS/microcontroller-controlled outlet strips, and I'm trying to make
some decisions about what sorts of relays this might require. I'm
assuming first that the contacts need to be rated for 125VAC (or more)
and for well over the current I intend to drive (does P=IV apply to AC;
e.g., is 10A right for 12500W @ 125VAC?). And I'm looking for either
12V or 5V coils, whichever comes easier.
The drive circuit I often see laid out is something like this:
+------+------> +12VDC
| |
- @
1N4001 ^ @ coil
| |
+------+
|
R |/ 2N2222
signal >--------/\/\-----|
|>---| GND
I would assume that I should select R to drive the transistor to
saturation. A relay I'm looking at has a coil rated for 120mA, 100
ohms. As far as I know, that ought to work, since even the TO-92
work-alike of the 2N2222 is rated for 500mA or so typically.
Any flaws in my logic yet?
Another matter concerns switch configuration. Would it be a better idea
to leave one blade always connected (single pole) or do I need to
connect and disconnect both at once (double pole)? I'm assuming that
the third grounding prong is not disconnected under any circumstance.
The IDEC RU2S-C-D12
(http://www.alliedelec.com/Search/ProductDetail.asp?SKU=814-0064) seems
to do what I want. Am I totally off?
One more thing: Say I'm running the microcontroller circuit and the
relay from the 5V and 12V pins, respectively, of an ATX-style SMPS. Are
there any special precautions that need to be taken other than the
typical reverse diode across the coil, decoupling caps on ICs, and so forth?
Thankses
PSM
wrong decision means more waiting for another mail order.)
A couple of projects I have in mind would benefit from
CMOS/microcontroller-controlled outlet strips, and I'm trying to make
some decisions about what sorts of relays this might require. I'm
assuming first that the contacts need to be rated for 125VAC (or more)
and for well over the current I intend to drive (does P=IV apply to AC;
e.g., is 10A right for 12500W @ 125VAC?). And I'm looking for either
12V or 5V coils, whichever comes easier.
The drive circuit I often see laid out is something like this:
+------+------> +12VDC
| |
- @
1N4001 ^ @ coil
| |
+------+
|
R |/ 2N2222
signal >--------/\/\-----|
|>---| GND
I would assume that I should select R to drive the transistor to
saturation. A relay I'm looking at has a coil rated for 120mA, 100
ohms. As far as I know, that ought to work, since even the TO-92
work-alike of the 2N2222 is rated for 500mA or so typically.
Any flaws in my logic yet?
Another matter concerns switch configuration. Would it be a better idea
to leave one blade always connected (single pole) or do I need to
connect and disconnect both at once (double pole)? I'm assuming that
the third grounding prong is not disconnected under any circumstance.
The IDEC RU2S-C-D12
(http://www.alliedelec.com/Search/ProductDetail.asp?SKU=814-0064) seems
to do what I want. Am I totally off?
One more thing: Say I'm running the microcontroller circuit and the
relay from the 5V and 12V pins, respectively, of an ATX-style SMPS. Are
there any special precautions that need to be taken other than the
typical reverse diode across the coil, decoupling caps on ICs, and so forth?
Thankses
PSM