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Old 18th Nov 2017, 3:59 pm   #1
kellys_eye
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Default Gardiner Transformer Ratings

Why does a Gardiner transformer show the secondary as 250-0-250 and 60mA 'DC' ??

Wouldn't the DC mA rating depend on the method used for rectification? i.e. capacitive input (1 x Iac) versus choke input (1.54 x Iac)?

So what, exactly, would be the rating for this transformer?
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Old 18th Nov 2017, 5:12 pm   #2
G8HQP Dave
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Default Re: Gardiner Transformer Ratings

If a DC rating is given for an HT transformer then you can assume this is for a capacitive input PSU - possibly with a valve rectifier. It is good that they specify DC, as some HT transformers uses a DC rating but don't say it is DC. Most other transformers (and any heater secondaries on an HT transformer) use RMS AC for current rating.
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Old 18th Nov 2017, 6:17 pm   #3
kellys_eye
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Default Re: Gardiner Transformer Ratings

Quote:
some HT transformers uses a DC rating
how does that equate with the different currents you can obtain depending on the type of filter? If the rating was shown as AC then you could determine the secondary current available according to how you design your rectification/filtering.

Is there an assumption towards a capacitive input filer in all circumstances?
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Old 18th Nov 2017, 10:30 pm   #4
G8HQP Dave
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Default Re: Gardiner Transformer Ratings

I can't remember what the actual factors are for a centre-tapped secondary, but I seem to recall that for a single secondary with bridge rectifier and capacitor input the DC current is around half the AC RMS. Choke input would be a lot nearer 1.
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Old 18th Nov 2017, 10:38 pm   #5
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Default Re: Gardiner Transformer Ratings

The 250-0-250 format strongly suggests what format of rectifier it is assumed to be used with. The capacitor input filter is by far the most common type, and it's harder on current ratings than a choke input type. So 60mA DC from either type would be fine. It's the output voltage that will be lower with a choke input filter AND you'll need to keep an eye on the minimum current consumption if you don't want the voltage to shoot up to the peak rectification mode voltage.

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Old 18th Nov 2017, 11:13 pm   #6
kalee20
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Default Re: Gardiner Transformer Ratings

Quote:
Originally Posted by kellys_eye View Post
Is there an assumption towards a capacitive input filer in all circumstances?
Not all circumstances, but almost always yes for transformers of this size.
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Old 18th Nov 2017, 11:27 pm   #7
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Default Re: Gardiner Transformer Ratings

The centre tapped transformer suggests use of a full wave valve rectifier with capacitive input filter. In this case Iac = Idc = 60mA. Vdc = 0.707 x Vac = 0.707 x 500 = 354v

Attached is a useful picture which shows this arrangement at top-right of the diagrams, albeit with solid state rectifiers.
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