On Tue, 25 Oct 2016 00:00:18 +0100, news wrote:

On Mon, 24 Oct 2016 11:18:20 GMT, Johnny B Good
wrote:


Looking at the picture, I can't see any signs of a triac (nor any place
on that PCB where one might lie hidden from view unless it's mounted on
the reverse side).

The black rectangle with a heat sink in the side above C2, what is it?

I can't see any evidence of a heatsink from that angle. However, it's
possible that what I thought was a small 100nF capacitor might well be a
plastic power transistor or triac with the heatsink tab chopped off flush
to the encapsulation.


The circled area marked "RV1" looks like this Variable Resistor
component may have been replaced by a couple of resistors located at the
switch (it's hard to tell, but it looks like the mounting holes are
wired back to the switch).

RV1 is the red blob

Ok, now you've given me another clue, I can see how that would fit. The
red blob, which I have to assume is a varistor in spite of it not being
blue, looks like it has been flipped up from its intended laid down flat
mounting position as implied by the silk screen print.


The two electrolytic caps at the opposite end of the board look
suspiciously like they may be a couple of 200vdc rated caps wired in
series to provide a smoothed 350v HT supply for a switching converter
(perhaps there *are* additional surface mount components on the
underside after all but the PCB's appearance suggests otherwise).

Apart from the 6 rectifier diodes and a small glass diode (probably a
zenner)

Yes I thought it was a zener too.

I will get some better photos up and see if I can trace the circuit and
also the other PCB in the motorhead but as I only want the brush to
work would there be any harm in abandoning the Dyson PCB and feeding it
with a simple 400V rated bridge rectifier off a separate switch?

Assuming that the "100nF capacitor" is actually a triac, I doubt you'll
get away with just a simple bridge rectifier. However, if you're going to
experiment, that 400v rating should be for the ac input voltage based on
the assumption that the output will be connected to a simple reservoir
smoothing capacitor ripple filter.

The rule of thumb in this case is to rate the diode PIV at 3 times the
maximum rms input voltage since each diode element in the rectifier
bridge will be subjected to twice the peak inverse voltage (1.414 times
the rms ac voltage) due to the peak voltage stored by the smoothing
capacitor. Assuming a perfect sine wave form on a 240v ac supply, each
diode will be subjected to a PIV of 2.828 * 240 volts, just under 680
volts! With an rms voltage at its upper 265v limit, the PIV will be a
whisker under 750 volts.

However, the Public Supply Utility voltage is far from a perfect sine
wave, typically looking like a slightly flat topped sine wave which will
reduce the theoretically calculated peak voltages by a few percent.

Even so, it's always best to use an even higher voltage rating, in this
case 800v PIV, to allow for spike voltages and sub station voltage
controller faults that allow the PSU to go above the 265v rms limit
(unless, of course you deliberately choose a 700 PIV rated diode to
protect the expensive HT switching transistors against such over-volting
events in an act of self sacrifice to blast the safety fuse to
smithereens - diodes and fuses are a damn sight cheaper and easier to
replace than HT switching transistors!).



Good luck & HTH

Yes you have been very helpful, thanks


The gratitude is appreciated. :-)

If you do decide to post any more pictures, try and compose the shots so
as to maximise clarity. Provide two or more angles of view, bending any
wires aside if necessary and pay attention to the lighting which can make
a huge difference to the clarity of the images.

Digital photography allows anyone to take as many photos as they fancy
of such a "Still Life" subject as this. You can easily fire off half a
dozen or more at various angles and lighting conditions from which to
pick out the best two or three to post up on a web site for us all to
peruse and admire.

Oh, and just one final point. As far as I can make out, there's no signs
of obvious component damage on that little PCB but, of course triacs and
diodes can develop faults without any obvious signs of failure. However,
when we're dealing with circuits carrying mains voltages, any such
internal failures tend to result in physical damage, either co-lateral or
self inflicted since the most likely failure mode tends not to be a fail
safe open circuit state but rather a dramatic pyrotechnical short
circuiting event.

If as you surmise, there's a triac on the circuit board, it must be
controlled from the switch using a couple of resistors to control the
triggering delay. Such resistors are typically high value resistors of a
low wattage rating. If they've been soldered straight onto the switch
contacts, it's possible they may have been damaged by the action of the
switch sending shockwaves into the contact tags. If the resistors have
been soldered to those contacts via very short leads, they may well have
failed as a result of the accumulated cycles of use over the years. IOW,
don't ignore such components which must exist somewhere external to that
PCB.

--
Johnny B Good