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Re: [Amps] SB220 power supply

To: "Pat Barthelow" <>, "jeremy-ca" <>, "Steve Flood" <>, <>
Subject: Re: [Amps] SB220 power supply
From: "David Cutter" <>
Date: Mon, 3 Mar 2008 07:14:25 -0000
List-post: <">>

There is a recognised way of measuring temperature rise to meet legal 
requirements, eg UL, CSA, BS etc:  the accent is on protection against short 
from the primary supply to anything else.
Leave the transformer off for a day to allow it to acquire its lowest 
temperature in your environment (which ought to be fairly constant, but any 
measure the primary dc resistance with a dmm
now apply the maximum load condition and then re-measure the primary dc 
resistance immediately.
Maximum load condition is a tricky one, but first you have apply the worst 
case line voltage, taken as 15% up on nominal.  Then you have to figure out 
what the worst case load is under the most extreme likely condition.
copper changes resistance by 0.39% per degree (Celsius scale) so you can 
work out the temperature rise of the primary.  There's a little formula to 
apply which I will look up.  The primary carries not only the power 
transferred to the secondary but also the magnetising power for the core and 
all losses.  From a legal point of view, only the primary matters because if 
this breaks down its insulation to the core or secondary then a danger 
ensues.  Here's the hard bit:  you now need to know what wire grade (not 
gauge) was used to wind the transformer primary and look up the limit 
allowed for that enamel grade and compare it to the rise + ambient 
temperature you calculated.  National standards give a table from which to 
determine the allowed rise.  If you don't know, you can only apply the 
lowest grade in the table.

I wouldn't advocate drilling the core.  You will short circuit some 
laminations and create a local hot spot, though on a big transformer I doubt 
if it will have much effect.  Try fitting your sensor somewhere near the 
centre of the winding wires as they exit the bobbin.  Ensure double 
insulation between the sensor and any primary wires.  There might be a 
difference of 20degrees or more to the hottest part of the winding.  The 
outside of the core might be 40degrees cooler.  Doubtless someone on this 
list will suggest a maximum temperature based on knowledge of the 

All my recent designs (20 years or so) have used a ptc protection resistor, 
which is a perfect solution to protecting mains supply windings.  Reverse 
engineering like this is interesting and educational but not sure if it will 
get you where you want to be.



> )
> Re the SB 220,
> If you had a fine carbide drillbit, would it be possible to drill cleanly 
> into the center region of the laminations of the transformer, without 
> doing functional damage to the tranformer?
> If you could, then,  slip a tiny, cylindrical RTD, or thermistor, to 
> measure temperature?   I dont know the hardness or drillability, of the 
> core laminations, so it might not be easy to drill....if you could place 
> the sensor in the middle of the laminations, center of the windings, 
> (which I would assume is the hottest spot) and  run the transformer hard , 
> what would be a reasonable upper limit temperature to look for as a 
> representative maximum running power/Duty cycle?
> All the best,
> Pat Barthelow    (916) 315-9271
> Project Manager,  Jamesburg Moonbounce Team
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