I have a workshop vac with power take-off (ie. plug tool into socket
on vac, vac operates when tool trigger pulled). I plugged a 200W
sander into it, it fired up for a second and then the vac shut down -
although the sander continued to run.
I assumed the vac was knackered (it was cheap) but I plugged an 800W
tool in to check and it worked fine. A 250W sander worked fine too.
Then I plugged an extension lead into the vac with no tool connected
to the extension - and it fired up for a fraction of a sec and then
I'm assuming there's a minimum load for the power take-off to work
properly and I've never noticed before because I've never connected
such a low-wattage tool. Is that right?
But what's the deal with the extension leads?
Thanks for any help.....
It would seem to be the case. It might use a current sensor to detect
when the tool is in use and the small tool was just not drawing enough
continuous current to keep it activated.
It is a so called "Transmission Line" effect. You can think of the
length of wire as a long chain of very tiny inductors and capacitors
that can store a small amount of energy. When connected to the power you
will get a very small current flow into the lead that will in effect
cause it to charge and discharge in sync with the AC supply. However
since you don't control exactly where you are in the AC cycle when you
connect the lead, you may happen to pick a point where the voltage is
close to its peek value (340V) - this can result in a very brief but
substantial current flow - possibly enough to trigger the current
sensor. Since there is no actual load other than the wire this current
flow is not sustained and hence it shuts off straight away after.
John's answer appears admirable and I've made a note of it, but just to put
my mind at rest....
You said "the VAC shut down"
If that was a slip then I can relax, but why should a sensor close down the
vac rather than the take off, specially if it doesn't shut down the take
Please put me out of my misery
On Fri, 30 Jan 2004 19:52:34 +0000 (UTC), mike ring
Because the power to the take off is there as long as the vac is
switched on, how else would it be able to sense when there is being
current drawn if an appliance is plugged into it and switched on?
Because the current drawn by the sander is below the minimum amount
required to activate the vac then it continues to supply power to the
take off waiting for the current to rise above the threshold required
to operate the vac.
IIUC what is being described here then it does actually make sense that
way round. The purpose of the power take off on the vac is to trigger
automatic starting of the vac when you turn the tool on, not to turn on
the tool when you fire up the vac (which could be a bit dangerous!).
[posted and mailed]
On 30 Jan 2004, you wrote in uk.d-i-y:
Got it now John, and Lurch.
My elderly Miele has a takeoff for a turbo brush which is live when the
cleaner is running, and dead when it isn't; I didn't cotton on to the power
tool take off notion
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