RE: [TCML] Terry Filter Resistors

["Lau, Gary" <Gary.Lau@xxxxxx>]

It's not obvious, but the resistors in a Terry filter need to stand off a very high voltage.  The TO-247 case is definitely not up to the high voltage transients.  The fact that these units are non-inductive is of no value in this application.  Resistors should be large, with a ceramic (not metal) body, as a metal body would compromise its voltage rating.  Think of it this way:  For a 2-sided Terry filter and a 15kV NST, the tank cap charges to about 21kV.  Each of the two bypass caps charge to about 10.5kV.  When the main gap fires (shorts), the 21kV across the two bypass caps is discharged into the two resistors in series, so each one sees a 10.5kV transient.

If one were to build a resistor out of several smaller units, the resistors should be in series, not parallel, so that the voltage rating of each unit can be smaller.

Regards, Gary Lau
MA, USA

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of bartb
> Sent: Sunday, January 25, 2009 1:44 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Terry Filter Resistors
> 
> Simon,
> 
> The manufacturer PN is LTO100F1K and is a TO-247 package. In free air by
> itself it's rated at 3.5W. If sinked well (big with external cooling),
> it is claimed to handle 100W. That may be more trouble than it's worth
> and more expensive than using a standard ceramic power resistor.
> 
> If it were not for the cost and complexity, you could go with this type
> of package minus forced air cooling by putting say ten 1000 ohm
> resistors in parallel on a heatsink (100 ohm total). The current through
> each resistor would branch at 1/10 current per resistor. But due to the
> cost and complexity, not a good solution. The standard power resistor is
> the way to go.
> 
> Anyway, here's the datasheet.
> http://www.vishay.com/docs/50051/lto100.pdf
> 
> Take care,
> Bart
> 
> Simon Dodd wrote:
> > Would these resistors from RS 2509695294 be ok for the main power
> > resistors in a terry filter. The only reason I ask is that all the
> > terry filters I have seen have used much larger (in size) resistors
> > and I am worried that these will burn out too quickly.
> >
> > Thanks again
> >
> > Simon
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