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Old 9th March 2013, 09:15 PM   #1
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Default Heater Transformer - too much voltage

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?
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Old 9th March 2013, 09:25 PM   #2
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Quote:
Originally Posted by Captn Dave View Post
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?
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.

Last edited by Robert Kesh; 9th March 2013 at 09:30 PM.
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Old 9th March 2013, 09:29 PM   #3
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I could have worded that question better.

I know that either way I produce close to the same amount of heat, but from the transformer's standpoint, is the second option going to reduce the load on the transformer?
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Old 9th March 2013, 09:31 PM   #4
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Quote:
Originally Posted by Captn Dave View Post
I could have worded that question better.

I know that either way I produce close to the same amount of heat, but from the transformer's standpoint, is the second option going to reduce the load on the transformer?
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.

Last edited by Robert Kesh; 9th March 2013 at 09:37 PM.
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Old 9th March 2013, 09:38 PM   #5
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By prior to the caps, do you mean in series with them?
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Old 9th March 2013, 09:42 PM   #6
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Quote:
Originally Posted by Captn Dave View Post
By prior to the caps, do you mean in series with them?
No. I mean between rectifier and first cap. An RC.
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Old 9th March 2013, 09:56 PM   #7
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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.
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Old 9th March 2013, 10:23 PM   #8
cnpope is offline cnpope  United States
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Quote:
Originally Posted by Captn Dave View Post
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?
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
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Old 9th March 2013, 10:53 PM   #9
Cassiel is offline Cassiel  Libya
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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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Last edited by Cassiel; 9th March 2013 at 10:59 PM.
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Old 9th March 2013, 11:04 PM   #10
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Wit ha linear supply you will always lose power in heat.
The only other was is a DC DC converter or SMPS.
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