12 to 5 AC Rectification

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woodrough

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woodrough

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Interesting thought, How in the world do AC light dimmer wall switches do it? They take 115VAC and can step it down to just about zero volts. I have seen some designs where aluminum heat fins are required and I also seen designs that have no heat dissipation spots at all… in fact, enclosed in a small plastic package that is all inside the wall. What is this all about?
 
Mooly

Mooly

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I think you've settled on the resistors :)

You want to look up "lamp dimmer circuits", not for this project but for interest. The variable speed on your electric drill works in a similar way using a thyristor or triac. They are electrically "noisy" though.

(You could always run a second similar valve in series to drop the heater voltage. Maybe they are a bit rare for that though :D )
 
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woodrough

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The problem is that (according to the schematics of the power supply I'm using) it says that with a full wave bridge, I can use 12 or 12.6 volts to run the heaters… but its gonna be at 2.5Amps. My transformer can only put out 2 Amps at 12Vac. So my next best bet was to voltage double 6.3 volts. 6.3 volts can run 3Amps. My original intention was to do just that. 12 for signal heaters and 6.3 stepped to 5 for rectifier.

I heard about using capacitive reactance to save wasted energy (I dig it). But it sounds like I would need giant film capacitors for that job in this scenario
 
Mooly

Mooly

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woodrough said:
Triac huh, minimal wasted heat? Also as far as noisy noise go, this is before any smoothing capacitor (being that this is for the main rectifier). Its notched ac is already noisy… what would a little ugly be with ugly, right?
Click to expand...

You don't want a triac to control the heater volts... honest :)

Getting it to trigger reliably on a 12 volt AC supply (such that the 5 volts across the heater stayed at 5 volts under all conditions/temperature/line voltage etc while probably do-able might be quite a challenge)

How about... nah, you'd laugh :D
 
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woodrough

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Here is my dilemma... and perhaps im looking at this wrong:

(using the PS-3 Supply schematic http://tubecad.com/Product_PDFs/PS-3.pdf)

edit: this is signal heaters:

-7-8Vacfor6.3Vdc(FWBridge)
-12-12.6Vac CT for 6.3Vdc (FW Bridge CT)
-12-12.6Vac @ 2.5A for 12Vdc or 12.6Vdc (FW -Bridge)
-24-28Vac CT for 12Vdc or 12.6Vdc (FW Bridge CT)
-6.3-8Vac for 12.6Vdc (Voltage Doubler)

This is my setup:
420V(210-0-210)@80mA, 12V@2A & 6.3V@3A
 
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DF96

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Elvee said:
That is a complete impossibility: no DC can go through a transformer.
The primary current waveform will be distorted, but will not have any DC component.
Click to expand...
The DC does not "go through" the transformer; it goes separately, but relatedly, through the primary and secondary. The flux generated by the primary current is almost exactly balanced by the flux generated by the secondary current.

6.3V at 3A is for a resistive load. You won't get 2.5A DC from that - you might get 1.5-2.0A DC.
 
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woodrough

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DF96 said:
6.3V at 3A is for a resistive load. You won't get 2.5A DC from that - you might get 1.5-2.0A DC.
Click to expand...

Wait... elaborate please, perhaps I do not and should not voltage double for the signal heaters. Right?

This design doesnt specify Current consumed by the signal heater circuit.
http://tubecad.com/Product_PDFs/PS-3.pdf

When he says "-12-12.6Vac @ 2.5A for 12Vdc or 12.6Vdc (FW -Bridge)" What does that mean in terms of my selected transformer?
 
Mooly

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The 6.3 volt 3 amp winding can't supply 12 volts at 2.5 amps with a doubler.

Is that what you meant earlier ?

So you have a 6.3 volt 3 amp winding available for the 5 volt heater of the rectifier ?
And a 12 volt 2 amp winding that really needs to supply 2.5 amps ?

Is that the long and short of it :)
 
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DF96

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Yes, I think so. 6.3V and 3A AC will give 1.5-2A DC, or about half that after voltage-doubling. The reason is that the charging pulses for a rectifier-capacitor have an RMS current which is significantly greater than the average current. Valve heaters run continuously so you can't have the duty cycle benefit which a Class B audio amp might get in an analogous situation.
 
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woodrough

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DF96 said:
Yes, I think so. 6.3V and 3A AC will give 1.5-2A DC, or about half that after voltage-doubling. The reason is that the charging pulses for a rectifier-capacitor have an RMS current which is significantly greater than the average current. Valve heaters run continuously so you can't have the duty cycle benefit which a Class B audio amp might get in an analogous situation.
Click to expand...

I am not quite sure I understand. When I rectify by voltage doubling, I drop voltage and amps? What does this mean for my setup?
 
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DF96

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When you double voltage, you can expect to get about half as much current. This means that a secondary winding which would not quite do what you want (with a normal full-wave bridge) will be nowhere near (with a voltage doubler). This is part of the fun of DC heaters! I suspect that many people do not realise this and go ahead anyway, but the result may be a hot transformer.
 
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