Hi Frank,
I also turn speed to zero before using any other control. But I am
leery of a failure causing the chuck to move while I have a grip on
the key, unlikely as that may be. Note that when I inserted the
extra fuse and turned the power on with the speed control set to OFF
that the chuck went to full speed for a half second or so before the
fuse blew -- this is the type of thing I don't want to happen while
I've got a grip on the chuck or the work. Also, I sometimes use the
tailstock chuck to start taps and noted that the headstock chuck is
more difficult to turn if the power is applied even though the speed
control is set to zero -- the big red button fixes this too.
Like you, I turn the power completely off when I'm done using the
machine. This is good practice because otherwise the lathe's
electronics are exposed to line transients from passing T-storms,
etc. I wired my shop so all the wall outlets are on one wall switch
and the radio is a reminder that ensures I turn it off when leaving.
During the summer I will pull the plug as further transient
protection.
Some further news on the FET failure: I removed the shorted FET and
tried the unit with the single remaining FET in place -- it works
normally, at least for the short term. I assume it won't run too
long this way because the FET will over heat. I did add a number of
air holes to the case below the FET heat sink and will monitor the
temp while awaiting replacement parts.
As a guess about why some controllers last and others fail: if the
parallel FET's both conduct the same amount of current while ON then
the controller will last but if the FET's are grossly mismatched then
one will conduct the bulk of the current and eventually fail -- the
time to failure will depend on the degree of mismatch. The
replacement IRFP460 FET's have a lower ON resistance than the
original 2SK790's so they should dissipate less heat under all
circumstances (including mismatched device characteristics).
Probably, the 460 FET's would survive if used singly although I will
install two to ensure long term viability.
John
--- In 7x12minilathe@..., Frank Hoose <fhoose@y...> wrote:
I don't know for sure if it makes a difference, but I
always turn the speed down to zero before switching
direction or turning off the lathe. I have not had a
FET failure in over 2 years following this procedure
and I use my lathe about 10 hours per week or more. I
never use the big red button since I power off the
lathe from a power strip it is plugged into when I'm
done working with it.
Frank Hoose
--- John <moran03@e...> wrote:
Hi Chris,
Thanks for the info. Unfortunately I have already
ordered the
IRFP460 replacement FET's from an electronics
supplier -- they have a
minimum $ order so I'll wind up with a LOT of
transistors and fuses
(in case anyone else runs into the same problem,
contact me for a
good deal on these items...).
Homier is sending a replacement board but it may
take up to 2 weeks
so I'll probably try to repair the present board in
the interim.
I found a schematic for the Grizzly 7x12 in the 7x10
group photos
section in a folder titled ESC. This confirmed what
I had determined
from looking at the ckt board: the output FET's are
in parallel but
they do not use source resistors to ensure equal
current sharing. My
theory on why my unit failed is that this particular
board has 2SK790
FET's from different manufacturers; I suspect that
the device
characteristics were sufficiently different that
they didn't share
the current equally causing the device which took
the bulk of the
current to eventually fail. Others who experience a
similar failure
might check to see whether both devices have the
same manufacturer's
logo as a check on my supposition.
Also, I had gotten into the habit of putting the
fwd/rev switch into
the center=off position when fiddling with the
chuck, this to ensure
that the chuck wouldn't move in the unlikely event
of a failure
occurring while my hands were in harm's way. I can't
decide whether
this may have contributed to the failure. Based on
examining the
schematic I will in the future use the big red
button instead of the
fwd/rev switch to ensure the chuck can't move --
don't know whether
this will help but it certainly won't hurt.
Further, I intend to add some holes in the case
below the heat sink
and to add a spacer to ensure the clear plastic
shield is held away
from the heat sink so air can circulate past the
sink. These holes
will also help any metal chips which wander into the
electronics
enclosure find their way back out. Again, this may
not help but it
won't hurt.
Having had my favorite toy disabled by an
electronics failure I'm
trying to avoid a repetition.
John
--- In 7x12minilathe@..., Chris Wood
<chrisw@s...> wrote:
John:
It sounds like the MOSFETs have blown. We have
replacements that
are higher
capacity.
ProductID=1221
Regards,
Chris Wood
LittleMachineShop.com <www.littlemachineshop.com>
The premier source of parts and accessories for
mini lathes and
mini mills.
221 E. Walnut St. #110, Pasadena, CA 91101
(800)981-9663 * Fax (626)584-5844
-----Original Message-----
From: John [mailto:moran03@e...]
Sent: Sunday, March 23, 2003 8:32 AM
To: 7x12minilathe@...
Subject: [7x12minilathe] 7x12 Blows Fuses ?
My 7x12 which is now 2.5 months old popped its
fuse shortly after
startup Friday, just as I was beginning a parting
cut at low
speed.
I installed the replacement fuse and , with the
speed control in
the
OFF position and the tool backed away, the lathe
started as if it
was
at full speed and the fuse popped again in less
than a second.
This is a relatively new unit so I called Homier
and they promised
to
call me back on Monday. I'm just wondering if
this is a common
problem; my understanding is the Homiers use an
SCR based
controller
which is less prone to failure than the MOSFET
types. My ckt board
has a sticker with FC250J/110V and the large power
devices are
identified as K790 -- the board uses surface mount
devices except
for power handling which are thru-hole. Anybody
have more info on
the Homier controller?
Up until this happened I was very happy with the
controller since
it
allowed speed to be reduced to 12rpm, slower than
most references
in
the 7x10 group specify. I did notice that when
the speed control
was
in the OFF position that rotating the knob
slightly(as alowed while
OFF) made the chuck would move slightly and groan
a bit. Since it
seems to be live even while OFF I always turn the
wall outlet off
after use.
John
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