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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Why are some power supplies rated in VA and some in Watts and some in amperes?
Why are some transformers and power supplies, rated in VA and some in
amperes, and some, I think, in watts? For example, 1) I have a doorbell transformer whose secondary is rated AC 10V 5VA. 2) And I have a plug-in little-black-box transformer about the same physical size that is rated AC 9.5V 750mA. I guess for the first one, 5VA is the same as 5 watts?, and the second one is .75 x 9.5 = 7.125 watts. Is it that simple, or am I missing something? It surprises me a little because the first one is heavier, but what really confuses is me is their rating one as VA and the other as mA. Meirman -- If emailing, please let me know whether or not you are posting the same letter. Change domain to erols.com, if necessary. |
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"meirman" wrote in message ... Why are some transformers and power supplies, rated in VA and some in amperes, and some, I think, in watts? For example, 1) I have a doorbell transformer whose secondary is rated AC 10V 5VA. 2) And I have a plug-in little-black-box transformer about the same physical size that is rated AC 9.5V 750mA. I guess for the first one, 5VA is the same as 5 watts?, and the second one is .75 x 9.5 = 7.125 watts. Is it that simple, or am I missing something? It surprises me a little because the first one is heavier, but what really confuses is me is their rating one as VA and the other as mA. VA is only equal to watts if the power factor is 1 (resistive load) but if you have an inductive load (such as a doorbell) or a capacitive load, then the watts will be lower than you would expect by measuring the current. |
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"James Sweet" wrote in message news:86IWd.80130$wc.50373@trnddc07... VA is only equal to watts if the power factor is 1 (resistive load) but if you have an inductive load (such as a doorbell) or a capacitive load, then the watts will be lower than you would expect by measuring the current. Agreed. But for doorbell usage the rating no matter how stated does not necessarily seem to be for continuous power. Domestic 'bell transformers' in my experience rarely fail. But we had a case where front door located 'push' button stuck 'in', in the permanently 'on' position. The homeowner complained that door bell didn't work and that the chime unit had 'hummed' (i.e. it was permanently energised) for a few months before that. The 'bell transformer' had burnt out. On other hand I've abused those bell transformers, putting the low volt secondaries in series with the primary to step up or step down the 115 volts supply (the insulation held OK), testing for TVs that might shut down etc. with low mains voltage. One and/or two such transformer arranged in a 'voltage bucking' manner with switches would step down the nominal 115 volt supply by some 9% or by some 18%. Useful for testing radios and TVs for rural low voltage conditions. Many years later I finally acquired (could afford!) a variac. |
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