I have a center tapped transformer that puts up about 32 volts and I want to use it with diodes and a CRC network for series connected 12.6 volt heaters. That's more voltage than I need.
I could: 1) use a CRC network with a big resistor or
2) put resistors in series with the diodes on the secondaries.
The first option wastes a lot of power and creates a lot of heat. Would the second option reduce the load on the transformer?
I could: 1) use a CRC network with a big resistor or
2) put resistors in series with the diodes on the secondaries.
The first option wastes a lot of power and creates a lot of heat. Would the second option reduce the load on the transformer?
You are going to have to lose the same amount of heat whatever you do. Whatever the current is through the heaters, multiply that by 32 - 12.6, and that is the watts you need to lose, which you will lose as heat.
Wait, I just noticed you said series connected. Then you'll have 12.6 x whatever of volts you need, and will have to lose the rest as heat.
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The load on the transformer is determined by the current requirements of the heaters, so won't change. It is voltage you need to drop. The duty cycle can be changed by putting resistors prior to the caps so the charging of the caps takes place over a larger part of the cycle with less current.
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May I make a suggestion? Choke I/P filter the O/P of the rectifier. Given that the critical current (in mA.) is approx. V/L, not much inductance is needed. A pure choke I/P filter yields about 90% of the RMS voltage as DC. If that's too low, install a "fudge factor" cap., which makes the filter cLC.
Less heat in I2R losses is (obviously) good. Because of heating effects in the power trafo, the available DC current is limited to about 50% of a winding's RMS capability, if a cap. I/P filter is used. 100% of that RMS capability is available, when choke I/P filtration is employed.
Less heat in I2R losses is (obviously) good. Because of heating effects in the power trafo, the available DC current is limited to about 50% of a winding's RMS capability, if a cap. I/P filter is used. 100% of that RMS capability is available, when choke I/P filtration is employed.
Can you be more specific about the series-connected 12.6V heaters? You mean a chain of 12.6V heaters connected in series? How many of them are you planning to connect in series in the chain?
Chris
A center tapped transformer with a full wave rectifier? Well, to avoid excess heat choke input is an option. Also, split voltage rails will give you +16, -16 but I don't know if that's a good option. Is there a problem if you leave the negative voltage unloaded? I don't know. I haven't seen people doing that so I guess it's not a good option. I'm curious to know anyway. I don't even know if you intend to power two 6.3V heaters or two 12.6V heaters. More info please.
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Yeah, more detail
It's a vintage preamp; a Fisher 400CX. There are eight 12.6v heaters. They are wired as 4 series pairs in parallel. So 25.2 VDC at 600ma. The rectified DC from the replacement PT (after diodes, before CRC) puts up about 36vdc rather than 28vdc as spec'd on the drawing.
To make things right I need 19R resistance in the CRC and that's way more than necessary and about 9watts more heat production than I'd like.
The object is to loose that voltage as efficiently as possible. Preferably in a way that reduces power transformer load.
And the rectification is full wave SS, a pair of diodes.
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What got me started on this thread was circuits I have seen that use resistors in series with the diodes. I can't at the moment cite a specific example, but IIRC I've seen this in at least one of Boskie's boards. I don't understand the purpose or benefit and was wondering if placing resistors in series with the diodes would be advantageous to the solution of my excess voltage problem.
What ever you put in (diodes or resistors) you will generate heat from them.
The simplest solution is 3 10 watt resistors to spread out the heat.
Otherwise you will have to consider a DC-DC convert which will be about 85% efficient. Personally I don't think the extra effort is worth it.
Just use resistors and ventilate your enclosure.
Saturday night thread.
Cassiel, would you like a sip of this really nice craft bourbon?
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