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Old 24th May 2006, 02:26 PM   #1
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Lightbulb Strange Transformer Question.....

Hi guys
I have been forced to use a seperate heater transformer in a preamp I'm making, because it can't supply the current I need.

Since I'm not using the heater winding, does this mean I can get away with drawing more current out of the HT winding, since the low voltage winding is not heating the core?
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Old 24th May 2006, 02:53 PM   #2
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It's not just the core, it's the gauge of wire used in the windings. I would never go over the ratings. My transformers seem to get warm enough even when running all windings well under the ratings!
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Old 24th May 2006, 03:22 PM   #3
vax9000 is offline vax9000  United States
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Since the core saturation is not a problem in your case, measure the DC resistance of the HT winding to come up with the amount of current it can handle. My transformer is 400V-0-400V and 49 Ohms per side, and I will leave along its 6.3V secondary winding. I think I can safely use it with 200mA DC output. 0.2*0.2*49 is abount 2W. It is a big 8LB transformer.
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Old 24th May 2006, 04:20 PM   #4
kevinkr is offline kevinkr  United States
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This isn't a good assumption as the currents flowing in the secondary windings are non-sinusoidal as a result of the rectifier. PK currents are a function of winding resistance, rectifier internal resistance and the capacitance of the first filter capacitor. Long term use at currents beyond the winding's continuous rating can be a problem due to thermal resistance between the windings and the core - although the overall power dissipation is within limits, the resulting temperature of that winding may not be depending on the current and the waveform present.

(To clarify note that the sawtooth rectifier current at a given dc load current may be far greater than the implied rms equivalent and hence there will be more winding heating as well.)

CCS (continuous commercial service) and ICAS (Intermittent commercial and amateur service) ratings took into account the long term and short term thermal rating of the transformer, operate one continuously based on the ICAS rating and it will probably fail. Obviously the core was rated for ICAS operation in order to avoid core saturation..

Winding current ratings are mA rms, whilst the actual current drawn is not, it is not bad practice to derate the transformer/rectifier dc load current to 75% or less of the rms rating - this incidentally can be more or less safely flaunted with a choke input.

While it is theoretically possible to do this also note that in the best case the additional power you can draw is limited to the heating power in watts rms that the original winding would have produced, and in terms of the high voltage secondary this might equate to just a few tens or less extra mA of load current.
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Old 25th May 2006, 12:32 AM   #5
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For the usual full-wave center-tap rectifier circuit with capacitor input, the RMS current in the secondary is almost exactly the same as the DC current - an easy relationship to remember. The RMS current is the important one for heating. There's a nice table of transformer current vs. load current at the Hammond site - worth printing out - http://www.hammondmfg.com/pdf/5c007.pdf For a better estimate, the PSU designer program from duncanamps.com is the way to go.

Core saturation won't be a problem unless your line voltage is VERY high or you're using a 60 Hz transformer on 50Hz.

As for the original question, you can't gain much HV capacity by not using a heater winding - the heating goes up by the square of the current - so 20% more current means 56% more heat generated.

Let's say you have a transformer with 600VCT @ 40 mA (24 VA) and 6,3V @ 2.5A (19 VA), with 10% power loss. If the heater winding isn't used, it's PROBABLY safe to allow 50 mA for the HV winding, as the total transformer heat (about 3.7W) will still be less than at full load (4.3W).
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