Using half of a center tapped winding

[mcandmar]

Define continuous duty cycle. If a tube amp is going to draw X amount of current all the time, is that what you consider 100% duty cycle?

I was workong on the basis that the actual load will be ~80ma so i was specing 100ma to give a little headroom. Or should i jump to the next size up in transformer which is 160v @200ma? It would mean having to dump the additional 10v via resistors in the power supply.

 

[Elvee]

Or to put it another way, if i design an amp that creates a load of 100ma @200v, how do i calculate the size of transformer i will need? i know half the answer is 150v.

First thing to do is to define your needs exactly: OK, 100mA but is it a class A amp, AB ... ?
If your amp requires 20W DC, the transformer needs to be rated at >40VA but the datasheet is somewhat ambiguous and it doesn't seem to be suitable

 

[AndrewT]

Yes, continuous = 100% duty cycle.

 

[mcandmar]

I ended up ordering the next size up which is 160-0-160v @200ma so it should be more than capable for the job. Thanks for all the help folks, i even learned a bit along the way..

 

[mcandmar]

FYI - Got confirmation from Edcor that the ratings are per side of the secondary, i.e. the 150v-0-150v tap is actually 120ma each side, or 150v 240ma via a center tapped full wave rectifier.

 

[trobbins]

I think you will find that 120mArms rating on each 150V winding does not mean a "150-0-150 240mA rating for a CT full wave rectifier configuration". Was that your own assumption?

 

[AndrewT]

120mAac rating for a winding does not change.

 

[TonyTecson]

Excuse me if this is a dumb question, if i have a 320v center tapped secondary winding, i.e. 160v-0-160v is there any reason why i cant use half of it as a 160v winding and ignore the other side? Basically is there any downside, or side effect to having half the secondary floating?

a full wave bridge is better than a full wave center tap since in a fwb,
current is flowing in the traffo secondary for the full 360 degrees,
whereas the fct has half of the windings conducting at only 180 degrees.....
thus the fwb has better traffo utilization....more power is available this way,
because traffo heating is lesser....
but, the advantage may not be realized if you are not able to parallel the two 160 volt windings....
i have long ago used only the fwb or the full wave doubler and never looked back...

 

[mcandmar]

I think you will find that 120mArms rating on each 150V winding does not mean a "150-0-150 240mA rating for a CT full wave rectifier configuration". Was that your own assumption?

No, i specifically asked the question in relation to a CT rectifier configuration, and that was the answer.

The way he explained it was the wire used in each half of the winding is rated for 120ma, so you could draw that current from both sides. In fact he said the winding was actually rated for 150ma but that's beside the point.

 

[TonyTecson]

it is not a question of 120 or 150mA.....
you can draw more or less than these numbers,
it is the question of voltage drops since the resistance
of the winding is fixed......are they suitable for your needs?

 

[trobbins]

Power dissipation and its association with temperature rise in the winding is the fundamental restriction. Voltage regulation is, as AJT points out, the users issue to then manage.

Even a manufacturers rating of 120mA is still fuzzy just by itself, as the current waveform application should be appended - some manufacturers assist by giving a simple +25% current rating when using a choke input filter.

 

[TonyTecson]

and not to forget, is it the load that determines current flow
out of the traffo secondary.....
the figures cited by the manufacturers are guidelines....
as mentioned by N. Crowhurst, ussually a filament winding rated for 3 amps
can handle 1 amp more without problems...