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Re: [Amps] SB-200 circuit breakers

To: Tom Osborne <>,
Subject: Re: [Amps] SB-200 circuit breakers
Date: Sun, 26 Aug 2007 16:07:20 -0700 (PDT)
List-post: <>
The breakers are thermally activated - the heat being generated by the current 
passing through the mechanism. The trip time is related to the current AND the 
time - with a big current, the breaker will trip quickly. With a lower current 
it will take longer. With a 'right on the edge' current it can take 10's of 

What you describe is quite consistent with one of the breakers getting a little 
'soft' with age so the current trip point has drifted down. Actually these type 
of breakers do tend to degrade (usually the current threshold decreases) as 
they get older and you probably should just go ahead and replace them both with 
new ones.  

If you are interested in more details, do a little prowling on the internet via 
a search for 'thermal circuit breaker' or something like that. The 
manufacturers have some good web sites and you can look at their data sheets: 
for example:

Plus a bunch more. You will find curves that relate the amount of time it takes 
for the breaker to 'break' versus the current. Pretty interesting stuff (for an 

Bob Groh, WA2CKY

----- Original Message ----
From: Tom Osborne <>
Sent: Sunday, August 26, 2007 5:16:42 PM
Subject: [Amps] SB-200 circuit breakers

Hi All

On my SB-200, the front circuit breaker trips after half hour or so of 
operation on RTTY.  If running RTTY, I usually keep it down to 200-250 
watts.  It is the same breaker, the front one, that always trips.  After it 
cools down a bit, it will come back on. The plate current is only running 
about 300 ma or so.

The tubes are fairly new (4 months) and don't run red when operating RTTY at 
200-250 watts.  I also have some muffin fans on top with the lid open to 
take out some of the heat.

In looking at the schematic, the breakers are in the primary of the power 
transformer.  What causes them to trip, heat?  There doesn't seem to be that 
much with the extra fans on top.  73 and thanks

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