Well, there is laser trimming, and there is laser trimming.
For commodity resistors with leads and cylindrical bodies, a spiral cut is
fairly common. By varying the pitch of the spiral and number of turns, a
large number of values can be had from a single base part. This also
retains most of the resistive material so that usable area for heat
dissipation is not reduced a lot. The disadvantage is that the voltage from
turn to turn can be high enough (especially for impulsive waveforms) that
there can be arc over between the turns.
The other way of trimming is to take straight cuts along the axis of the
resistor. The voltage gradient along the resistor is not affected. However,
the base part needs to be close to the final value so that large amounts of
resistive material are not removed, causing the heat dissipation to
concentrate over a small area. This is more expensive to do, but yields a
more robust component, appropriate for high voltage use.
Donald Borowski
Schweitzer Engineering Labs
Pullman, WA, USA
"John McAuley"
<john.mcauley@cei
.ie> To
Sent by: "'Joe Randolph'"
[email protected] <[email protected]>, "'Ralph
McDiarmid'"
<[email protected]>,
10/08/2008 03:55 <[email protected]>
AM cc
Subject
Please respond to RE: IEC60225-5 surge test
<john.mcauley@cei impedance?
.ie>
High voltage resistors are becoming more readily available with thick film
technology.
See
http://radionics.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2509032292
A combination of these could meet the surge current requirement.
You need to be careful with some of these though, they use laser trimming
which results in affine line across the resistor that causes arcing. I
understand that the one listed above does not use laser trimming.
BR
John McAuley
www.cei.ie
[email protected]
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From: [email protected] [mailto:[email protected]] On Behalf Of Joe
Randolph
Sent: 08 October 2008 02:02
To: Ralph McDiarmid; [email protected]
Subject: RE: IEC60225-5 surge test impedance?
On 10/7/2008, Ralph McDiarmid wrote:
Is it possible to insert a resistor between surge generator and CDN to
provide the correct impedance? Here is one source of high-voltage,
power resistors.
http://www.caddock.com/Online_catalog/high_voltage/high_voltage.html
Hi Ralph:
It is common practice to insert an external resistor in series with the
output of a surge generator to reduce the short-circuit current. In fact,
some standards call out different values of external resistance in their
tests. The resistor will influence the short-circuit current waveform, so
you may need to pay attention to that.
You must be very careful about the power rating and construction of any
resistors you add. The resistors must be able to handle the surge power,
and they must be non-inductive (not wire wound). Interestingly, one of the
best types of construction for this application is the old
carbon-composition type, but these are getting hard to find.
As long as the resistor construction you use is non-inductive, you can
achieve whatever surge tolerance you need by connecting several resistors
in parallel.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected]
http://www.randolph-telecom.com
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