No - in mine the primary side of the transformer would be on all the
time, and the battery is only used (and relay energised) while the
button is depressed. The idea is that pressing the button looks to the
fan like someone flicked the light on and off, and it then runs for the
duration of its timer.
Thanks. I was just concerned that this might be outlawed on the grounds
that a stray wire in the relay box could make the ultra-low-voltage side
live. Though the fact that the switch is actually mains-rated in this
case means you'd still need a second fault (or a lot of water) to get a
You can do. FWIW though 4x AA batteries cost about the same and hold a
lot more charge. PP3s are especialy bad deals in terms of energy per
cost. And C cells are much better than AAs...
You could use a switched mode wallwart, one of the small light ones
used for charging a mobile would be about right. No few watts wasted.
Pete Verdon wrote:
yes, it has to be that way
You probably wont want to, but you could use the fan rotor from a 1"
laptop fan as your press switch button. It would get the message
across in a cute way.
Re the proposed sign, 3d plastic turds are very easy to make. Let a
can of foam dribble the right amount out, it will fall/form like a
turd, and paint brown when set. :) Or not.
It could. That's true anywhere mains and LV coexist. A good method to
strongly discourage it is to have the 240v come in at a position where
the wires arent long enough to reach any low voltage part, make sure
theyre cord griped on entry, and cable tie L&N together so that if one
comes off it cant move far.
Grip & tie the LV wires similarly so they cant stray over to the mains
Then you can earth the secondary side as an added precaution.
As with many things there are 2 very different definitions of best...
Best in terms of run cost favours large expensive batteries that last
a very long time. 6v Lantern batteries are a good example.
Best in terms of lowest toy mfr cost and smallest size (cute factor)
favours small batteries that die quickly, often AA cells (or worse).
Re voltage, lower means shorter battery life and more trouble from
poor contacts. Higher means longer battery life but higher battery &
battery holder costs.
Some circuits are to some degree voltage specific, which overrides
other concerns. As to why they're voltage specific, thats getting into
circuit and IC design, which is a tad complex.
In general the smaller the battery the larger the cost per amp hour -
although economy of scale comes into this too. But perhaps the worst is
AAA against AA - they're often the same price despite being about half the
capacity. PP3s use 6 AAAA - not a common size on its own so difficult to
*El nino made me do it
Dave Plowman email@example.com London SW
Hmm, interesting. I don't know much about batteries, so I just picked
PP3 as the kind of thing that tends to hang around long-term in smoke
alarms and the like.
Unfortunately I've now built the box with a PP3-sized space in the
lower-voltage side, so I think I'll stick with it. If I find it needs
changing too often I can always stick a holder for four D cells to the
wall next to it, like an old-fashioned doorbell.
I've had a bash at actually making the thing, and what I've done is to
glue a plastic partition across the box - one half 9v, one half 240v.
The relay is glued into a hole in the partition, with the 9v half in the
9v cavity and vice versa. The connections to the relay are soldered, and
have then been covered in hot-melt so that any stray metal can't make
contact. These short connections go to terminal blocks for the in and
Interesting - not sure what you mean here. Just tie one leg of the
connection to earth, on the grounds that it won't make a complete
circuit with the battery? What about the other leg - I can't connect
them both to earth or I'd be connecting them together.
If you ever need to, a PJ996 might be easier to handle. 9v relays will
run on 6v.
sounds like a very effective solution
Just tie one of the transformer LV outputs to earth.
If mains meets that side of the LV supply, the fuse/mcb will trip.
If mains meets the other side of the LV circuit, the transformer
secondary will be low enough resistance to allow overcurrent, and
hopefully blow the fuse or trip the breaker, simultaneously frying the
You cant earth both of course :)
Someone else can hopefully remind us all of the regs on SELV, which
might say something else.
Um - what transformer?
Presumably the same would apply with the battery though?
I'd wonder about the current-carrying capacity of the wiring on that
side, but I guess it would last long enough to trip an RCD.
It might not, but thats still a big improvement on it definitely wont.
Mind you in this case if there's no exposure of the LV side it may be
better to leave it uneathed and just equibond the push button front.
(Mains to the battery would be fun)
Couple of practical points - don't place a push button within reach of 5 year
old sitting on toilet - pp3 will die quickly.
I found that my cheap fan only ran for about 10 minutes after "blipping" the
light on, even when the timer set for a nominal 20 minutes. It seems to require
a connection of about 4 seconds to charge the capacitor fully.
I went for 2 pull cords. One (with a shorter string) controls the shower
light and the fan, the other controls the rest of the lights. This
system has been in use now in both bathrooms for several years and has
been very successful.
On 10 Jan 2009 19:12:03 GMT, firstname.lastname@example.org (Andrew
To be honest I've had great success with these :
(A 3 AA battery operated PIR & Photocell controlled LED Nightlight)
No clicking of lightswitches in the middle of the night, great if you
have elderly relatives staying who have to visit the bathroom
frequently and don't know where the lightswitches are. Battery
operated so not constrained by the need for a mains supply. The 3 AA
batteries seem to last us about 6months. Plenty enough light to light
a whole bathroom, landing or staircase to complete the "intended
Only disadvantage I've found is that if you have them in the bedroom
in such a position that they can "see" head end of the bed, then they
come on everytime you scratch your nose during the night. However this
is easily avoided.
Simplest way would be with a timber fan such as you already have -
however disconnected from the light switch. Plus an additional external
humidistat. That way you can trigger it either from the push button
(which will cause it to run for the timer duration) or from the
humidistat which will cause it to run until the RH falls below threshold
+ an extra timer duration.
The main problem with a push button of inappropriate rating for the
location will be what effect the ingress of moisture will have on the
switch - regardless of any voltage exposure risk.
A momentary action pull switch may be better for the manual activation.
You can get momentary action rocker switches as well that look like
ordinary electrical accessories. These may prove somewhat more
Yep, that's almost exactly what I have in mind, except for using a flow
switch on the shower rather than a humidity sensor. Fan would be on
while showering, and for a timer-duration thereafter.
In this case I don't think it's likely to get water on it, and in any
case its design would discourage entry even though it hasn't actually
Probably, from a practical purpose, but I really don't like the things.
One of them (for the light) is just about acceptable since it's so
common, but multiple dangling strings (a mate's bathroom has three)
drive me nuts.
I'm not worried about maintainability, more whether the inside of the
boxing-in counts as part of the bathroom, since this alternative
approach would mean it contains mains equipment (and the connections to
the back of the button aren't IP-sealed). I know that the space under a
bath is non-zoned only if there is a panel whose removal requires tools
- to have my studwork treated the same way I want access to be more
difficult, not less :-)
oops make that a "timer fan" rather than a wood one!
The problem with that, is that the timer duration will probably not be
long enough to reduce the humidity completely. With a humidistat it may
need to run the fan for 40 mins or so to dry the room to normal levels.
Yup panels that need tools to get past count as if they are out of the
room. Obviously you don't want water getting under panels for all sorts
of reasons, so not getting your electrics wet is just another on the list.
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