[Milsurplus] Wanted, plug-in gadget that powers AC outlet when current drawn by other load

[WA5CAB at cs.com]

Groups,

Unless anyone has a question or comment, or the unit experiences infant 
mortality, this should be the final comment on this subject.  And the first 
project (tomorrow/later today) with the new lashup will be a pair of adaptors 
to attach a pair of scissor-jacks to the legs of a PE-237 Vibrator Supply 
cover in order to remove it with hopefully far fewer hammer hits than the last 
time that I took the cover off.

First, I discovered why the (supposed) 12 amp breaker tripped every time I 
tried to start the saw.  A Simpson clamp-on AC ammeter adaptor revealed that 
the starting current of the Shopsmith when set to saw RPM is 48-52 amps for 
a full one second before it rapidly drops to about 6.5 amps no load.  When 
set to lower RPM the duration gets shorter but the peak current doesn't seem 
to decrease any.

Second, I determined that if I returned the iVac for credit I would still 
be out about a third of the total expenditure for shipping.  So I figured out 
how to remove (destructively) the plastic plug hiding one of the four rear 
cover retaining screws and opened the box up for inspection.

The AC wiring, including the two supply cords, was all #16 AWG (OK for the 
12 amp marked rating so no foul there).  The two breakers are actually 
marked 15 A/125 VAC/32 VDC.  The current sense device did turn out to be a 
current transformer (CT) but it would not be a trivial excercise to remove turns 
from the primary.  I managed to determine that the primary is probably 5 (or 
actually 4.75) turns of 2 x #18 bifilar wound, so equivalent to #15.  Mean 
turn length about 2".  The third (AUX) outlet was not in the CT primary 
circuit.  The dust collector outlet is controlled by a DC relay marked 120 VDC 
(which I assumed meant the coil rating).  There is a rectifier circuit across 
the main AC input and an ohmmeter check across the high current line cord 
plug will show about 50K ohms at one ohmmeter polarity.  I didn't attempt 
(bother) to trace out the rest of the low-level circuit, as it works.

To boost the current rating (and taking advantage of the fact that the 
circuit supplying the saw has only the one dual-duplex outlet on it which 
already has a 20 amp breaker) I jumpered out the breaker on the saw side of the 
iVac (the dust collector side was left as original).  The primary line cord 
was replaced with a short (18") #14-3 one (original #16 one was about 6' 
long).  I shunted the CT primary with 12" of #14 AWG (calculated to have 
approximately the same effect as removing 2-1/2 turns).  I put the AUX outlet in the 
CT primary circuit.

Then I reassembled and tested.  Results are that it works with the main 
Shopsmith unit plugged into the main load outlet or with the Shopsmith Power 
Station (currently carrying the belt sander) plugged into the AUX outlet.  It 
doesn't work with the 1/2 HP 12" Craftsman bandsaw plugged into the AUX 
outlet (meaning that when I start the bandsaw the dust collector does not 
start).  When I get a few minutes, I'll check the start and run current of the 
bandsaw and of the Power Station, which will give me some idea of the minimum 
operating current that my shunt established.  But that isn't a practical 
issue here as the bandsaw isn't located where it would be convenient to connect 
it to the Shopmate vacuum anyway.

Anyway, after a little file to fit I have the dust collector control gadget 
I was looking for installed and working.  Thanks to all for tips and 
comments.

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480