I am seeking advice on the replacement of the moving contact on a 2 amp
Variac.The original "Brush" appears to be made from Graphite and is about
4mm in diameter.
The makers were Zenith Electric London (RIP).
Thanks in advance geoff

"geoff smith" writes:

I am seeking advice on the replacement of the moving contact on a 2 amp
Variac.The original "Brush" appears to be made from Graphite and is about
4mm in diameter.
The makers were Zenith Electric London (RIP).

I'd simply file down a motor brush to fit. It's not critical.
Makes a mess but less time and effort than trying to find an exact
raplcement. One I did on a power supply 25 years ago is still going
strong.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
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I find that sandpaper between 100 and 200 grit works great for sanding
down brushes to fit. Wear rubber gloves if possible, and plan on
throwing the sandpaper away even if it is hardly worn. The black
carbon gets into EVERYTHING!.

"geoff smith" wrote in message
. uk...

I am seeking advice on the replacement of the moving contact on a 2 amp
Variac.The original "Brush" appears to be made from Graphite and is about
4mm in diameter.
The makers were Zenith Electric London (RIP).

Sometimes these were a critical design, to prevent shorting adjacent turns,
but it looks like yours isn't. Just find the nearest brush you can which is
bigger and plane it down. Use a coarse file, not any sort of sandpaper or
emery paper.

N

"NSM" writes:

"geoff smith" wrote in message
. uk...

I am seeking advice on the replacement of the moving contact on a 2 amp
Variac.The original "Brush" appears to be made from Graphite and is about
4mm in diameter.
The makers were Zenith Electric London (RIP).

Sometimes these were a critical design, to prevent shorting adjacent turns,
but it looks like yours isn't. Just find the nearest brush you can which is
bigger and plane it down. Use a coarse file, not any sort of sandpaper or
emery paper.

There's no way to avoid shorting adjacent turns. Else, there would be
gaps in the output voltage! However, the width could be important
to avoid shorting more than one adjacent turn.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
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"Sam Goldwasser" wrote in message
...

There's no way to avoid shorting adjacent turns. Else, there would be
gaps in the output voltage! However, the width could be important
to avoid shorting more than one adjacent turn.

ISTR some specification for resistivity for that purpose but these days you
have to use what you can get.

N

In article .com,
wrote:

I find that sandpaper between 100 and 200 grit works great for sanding
down brushes to fit. Wear rubber gloves if possible, and plan on
throwing the sandpaper away even if it is hardly worn. The black
carbon gets into EVERYTHING!.

Whether you use a file or sandpaper, do the shaping under a stream of
running water to avoid a terrific mess. Wet-or-dry carborundum paper is
your friend.

Isaac

"NSM" writes:

"Sam Goldwasser" wrote in message
...

There's no way to avoid shorting adjacent turns. Else, there would be
gaps in the output voltage! However, the width could be important
to avoid shorting more than one adjacent turn.

ISTR some specification for resistivity for that purpose but these days you
have to use what you can get.

Probably is and would matter for a high power Variac. I wonder what
the limiting factor is on current between adjacent turns. What other
factors will be important besides resistance of the winding and brush/
contact?

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
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| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
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"Sam Goldwasser" wrote in message
...

Probably is and would matter for a high power Variac. I wonder what
the limiting factor is on current between adjacent turns. What other
factors will be important besides resistance of the winding and brush/
contact?

Clearly these have a high volts per turn ratio. One shorted turn is limited
some by the impedance of the other windings, but really the resistivity of
the brush is critical.

N

On Tue, 05 Jul 2005 19:35:42 GMT, "geoff smith"
wrote:

I am seeking advice on the replacement of the moving contact on a 2 amp
Variac.The original "Brush" appears to be made from Graphite and is about
4mm in diameter.
The makers were Zenith Electric London (RIP).
Thanks in advance geoff



There are a few suppliers that can provide brushes for variable
transformers, such as
http://www.jenkins.com/jenkins/trans...able_trans.htm
http://www.ietlabs.com/Variac/Variac_Brushes.html

It appears that they are not cheap... roughly $20 USD for a 2A model.

A Google search revealed a variety of opinions on proper brush
replacement, from "any old motor brush" to "only the proper brush
specified for the particular model". Here's an excerpt from a web
site that gives a bit more info than most concerning brush
replacements:

"Easily replaceable carbon units are used, the resistance of the brush
being carefully calculated to limit the current circulating in the
bridged turns.
It is therefore most important that only the correct replacement brush
is used in any particular Brushes are mounted in robust brush carriers
insulated from the shaft, and designed so that metal parts cannot come
into contact with the winding in the event of brush fracture"

It makes sense that the composition of the brush be such that it
afford a higher resistance than normal motor brushes in order to limit
the circulating current in adjacent shorted turns.

If you can't find a suitable source for a replacement brush, your best
alternative is to buy a complete unit from ebay or find one at a swap
meet in your area. I've bought a number of variacs on Ebay for $15 -
$20 USD each, cheaper than a replacement brush.

Cheers!!!
==============

Dave M

Never take a laxative and a sleeping pill at the same time!!

"Dave M" wrote in message
...

If you can't find a suitable source for a replacement brush, your best
alternative is to buy a complete unit from ebay or find one at a swap
meet in your area. I've bought a number of variacs on Ebay for $15 -
$20 USD each, cheaper than a replacement brush.

I see some brushes are VERY expensive.

VBT-6 W50 3200-5906 $149
VBT-7 W50H 3200-5907 $149

N

Thanks to everyone for the very sound advice on replacement Variac brush.As
suggested I made one from an old brush removed from a scrap vacuum
cleaner.The results are very good indeed and the Variac has been operating
quite happily at its max current for several hours.I am now seeking advice
on the removal of carbon dust from various bodily crevices!
"Sam Goldwasser" wrote in message
...
"geoff smith" writes:

I am seeking advice on the replacement of the moving contact on a 2 amp
Variac.The original "Brush" appears to be made from Graphite and is about
4mm in diameter.
The makers were Zenith Electric London (RIP).

I'd simply file down a motor brush to fit. It's not critical.
Makes a mess but less time and effort than trying to find an exact
raplcement. One I did on a power supply 25 years ago is still going
strong.

--- sam | Sci.Electronics.Repair FAQ Mirror:
http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ:
http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites:
http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the
excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above
is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

"geoff smith" wrote in message
...

Thanks to everyone for the very sound advice on replacement Variac
brush.As
suggested I made one from an old brush removed from a scrap vacuum
cleaner.The results are very good indeed and the Variac has been operating
quite happily at its max current for several hours.I am now seeking advice
on the removal of carbon dust from various bodily crevices!

A nubile virgin with a flexible tongue? Or a cold shower.

N

On 05 Jul 2005 18:50:57 -0400 Sam Goldwasser
wrote:

"geoff smith" writes:

I am seeking advice on the replacement of the moving contact on a 2 amp
Variac.The original "Brush" appears to be made from Graphite and is about
4mm in diameter.
The makers were Zenith Electric London (RIP).

I'd simply file down a motor brush to fit. It's not critical.
Makes a mess but less time and effort than trying to find an exact
raplcement. One I did on a power supply 25 years ago is still going
strong.

I don't know about his model, but many of these use a brush which
consists of a carbon brush brazed to a brass holder, or a carbon brush
with a pigtail embedded.

The first is impossible to duplicate, while the second requires
slightly different starting materials.

Or does someone know how to braze to carbon?

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

On Wed, 06 Jul 2005 03:48:11 GMT "NSM" wrote:


"Sam Goldwasser" wrote in message
...

There's no way to avoid shorting adjacent turns. Else, there would be
gaps in the output voltage! However, the width could be important
to avoid shorting more than one adjacent turn.

ISTR some specification for resistivity for that purpose but these days you
have to use what you can get.

I once saw an ad for someone who was promoting the idea of a bifilar
wound variable autotransformer which was meant to "cure" this problem.
Personally, it didn't seem to be much of a solution, and I've never
seen them on the market. It sounds like an idea which was too hastily
presented, and it must have died as soon as someone thought more
seriously about it.

Anyone else remember seeing this?

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

"Jim Adney" wrote in message
...

Or does someone know how to braze to carbon?

It takes a lot of persistence or really strong spit.

N

"Jim Adney" wrote in message
...

I once saw an ad for someone who was promoting the idea of a bifilar
wound variable autotransformer which was meant to "cure" this problem.
Personally, it didn't seem to be much of a solution, and I've never
seen them on the market. It sounds like an idea which was too hastily
presented, and it must have died as soon as someone thought more
seriously about it.

Anyone else remember seeing this?

Nope. Sounds insane. After all, a Variac is really an autotransformer with a
LOT of taps. You don't want to short the taps together now, do you?

N

Jim Adney writes:

On 05 Jul 2005 18:50:57 -0400 Sam Goldwasser
wrote:

"geoff smith" writes:

I am seeking advice on the replacement of the moving contact on a 2 amp
Variac.The original "Brush" appears to be made from Graphite and is about
4mm in diameter.
The makers were Zenith Electric London (RIP).

I'd simply file down a motor brush to fit. It's not critical.
Makes a mess but less time and effort than trying to find an exact
raplcement. One I did on a power supply 25 years ago is still going
strong.

I don't know about his model, but many of these use a brush which
consists of a carbon brush brazed to a brass holder, or a carbon brush
with a pigtail embedded.

The low current ones I've seen usually just have a brush held in place
by spring pressure. Don't ask me how I know.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.