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Old 21st October 2011, 01:14 PM   #21
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This is only a design at this point. I connected the transformer to the line and measured the voltage on the secondary. The circuit will only draw 90mA, so I can't imagine it's going to drop 20V. That would be a 12% drop.

I'm going to use an RCRC filter instead of the lamp, for reasons explained above. I'll measure everything once it's built and if the voltage is that far off, I'll change the 'R'.
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Old 21st October 2011, 01:32 PM   #22
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Quote:
Originally Posted by EddieRich View Post
This is only a design at this point. I connected the transformer to the line and measured the voltage on the secondary. The circuit will only draw 90mA, so I can't imagine it's going to drop 20V. That would be a 12% drop.

I'm going to use an RCRC filter instead of the lamp, for reasons explained above. I'll measure everything once it's built and if the voltage is that far off, I'll change the 'R'.
Measure the DC resistance of the windings and the unloaded secondary voltage and I can model it - so can you - download PSU Designer from Duncanamps.com. Voltage drop will be more than you think, since the transformer current has a high peak-to-average value. For instance, a 50VA isolation transformer (10 Ohm primary and secondary, good for 0.4A RMS) will have a drop of about 14V from no load to 90 mA DC. Transformer secondary current about 160 mA RMS, 550 mA peak.
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Old 21st October 2011, 02:01 PM   #23
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I agree. PSUD is a great tool. It will do a lot of the work for you before you start soldering or choosing values.
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