Delay on a mains-powered relay?

[[email protected]]

Unless you have some overriding need for the delay when the mains come
back on again, why not just do something like this:

BATTERY
+-----+
| +|---+--------->VOUT+ TO EQPT
| | |
| -|---|-----+--->VOUT- TO EQPT
+-----+ | |
| |
+-----+ | |
MAINS>---|~ +|---+ |
| | |
MAINS>---|~ -|---------+
+-----+
CHARGER

This is a better idea than mine and avoids switching glitches. The only
thing I wonder about is the charger voltage. To float a 12 V gel cell
I need 13.65 V or so charging voltage. Right now, the cable modem has a
12 V, 1 A wall wart, most likely linear, probably unregulated, so the
"extra" 1.65 V may not be such a big deal. The VoIP box might have a
switcher, so its voltage input might be more critical. I could do
something like this:

MAINS>------------------------+
Relay |
MAINS>---------------======---+
+--/ --+
BATTERY | |
+-----+ | |
| +|---++---A--+->VOUT+ TO EQPT
| | |
| -|---|-----+--->VOUT- TO EQPT
+-----+ | |
| |
+-----+ | |
MAINS>---|~ +|---+ |
| | |
MAINS>---|~ -|---------+
+-----+
CHARGER

Component "A" limits the voltage when the mains are on. It might be
as simple as a diode or two in series, or as fancy as a voltage
regulator. The relay contacts are held open by the mains, to keep
the voltage to the equipment down. When the mains fail, the contact
opens and the equipment gets whatever the battery voltage is. (I am
less worried about what happens when the equipment has a lower-than-
spec input voltage, but I will test it.)

Thanks!

Matt Roberds

 

[Robert Baer]

This is a better idea than mine and avoids switching glitches. The only
thing I wonder about is the charger voltage. To float a 12 V gel cell
I need 13.65 V or so charging voltage. Right now, the cable modem has a
12 V, 1 A wall wart, most likely linear, probably unregulated, so the
"extra" 1.65 V may not be such a big deal. The VoIP box might have a
switcher, so its voltage input might be more critical. I could do
something like this:

MAINS>------------------------+
Relay |
MAINS>---------------======---+
+--/ --+
BATTERY | |
+-----+ | |
| +|---++---A--+->VOUT+ TO EQPT
| | |
| -|---|-----+--->VOUT- TO EQPT
+-----+ | |
| |
+-----+ | |
MAINS>---|~ +|---+ |
| | |
MAINS>---|~ -|---------+
+-----+
CHARGER

Component "A" limits the voltage when the mains are on. It might be
as simple as a diode or two in series, or as fancy as a voltage
regulator. The relay contacts are held open by the mains, to keep
the voltage to the equipment down. When the mains fail, the contact
opens and the equipment gets whatever the battery voltage is. (I am
less worried about what happens when the equipment has a lower-than-
spec input voltage, but I will test it.)

Thanks!

Matt Roberds

Please note that all unregulated wall warts have a higher output
voltage than the rated valus, even at *full rated current* (!!).
Therefore, the output is far in excess of "rating" at lesser loads.
In fact, to prevent damaging a radio i have, i use a "4.5V" wall wart
to supply the 6V the radio needs.