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Old 8th March 2007, 06:00 PM   #1
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Default PSU for 12V DC heaters

Hello,

Would the following circuit work and give 12VDC? Excuse the schematic if some of the symbols are incorrect - I have limited software for doing this. The caps are non-polar electrolytic. The MC78T12CT is a Fairchild positive voltage regulator - 12V 3A.

Thank-you,
Charlie

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Old 8th March 2007, 06:02 PM   #2
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Yes, if you add also a bridge rectifier made from Shottky diodes. Also, the first capacitance have to be at least 10 times more.

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Old 8th March 2007, 06:31 PM   #3
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Sorry,

I meant to add a bridge rectifier made from 1N4001 diodes.


Charlie
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Old 8th March 2007, 06:49 PM   #4
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Quote:
Originally posted by cbutterworth
[B]I meant to add a bridge rectifier made from 1N4001 diodes./B]
Make sure they can carry enough current.
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Old 8th March 2007, 06:50 PM   #5
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Here is what I meant to post. So, I need something more like a 20mF cap for the first cap?

Charlie

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Old 8th March 2007, 06:50 PM   #6
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Quote:
Originally posted by cbutterworth
Sorry,

I meant to add a bridge rectifier made from 1N4001 diodes.


Charlie
Voltage drop on 4001 will be too high.

You may use Shottky diodes from salvaged computer power supplies. They are flat, have 3 legs like power transistors. You need 3 of them for a bridge rectifier.
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Old 8th March 2007, 06:51 PM   #7
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Do you have a schematic for the schottky dioides?

Charlie
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Old 8th March 2007, 06:57 PM   #8
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Quote:
Originally posted by cbutterworth
Do you have a schematic for the schottky dioides?

Charlie
Schematic is drawn on them: middle leg is a common cathodes connected together. Use one for a positive polarity (it has already 2 diodes inside), and couple for negative polarity (you may parallel both diodes on each).
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Old 8th March 2007, 07:43 PM   #9
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Quote:
Originally posted by Wavebourn


Voltage drop on 4001 will be too high.

You may use Shottky diodes from salvaged computer power supplies. They are flat, have 3 legs like power transistors. You need 3 of them for a bridge rectifier.
?

12.6VAC will rectify to 17.8V. The diodes will drop a couple of volts, and the regulator will drop a couple more. This still keeps you comfortable for 12.6VDC.

For schotkey diodes, just buy a couple from Digikey. Use their search function to find some with appropriate current ratings. They look like normal diodes with 2 legs -- you need 4. http://www.onsemi.com/pub/Collateral/1N5820-D.PDF Don't go pulling old $0.37 parts out of a computer supply unless you happen to have one you don't want and you need this today and you can't find any others anywhere else.

The only issue is how much current you need to draw from the heaters. If it is high, use a diode rated for higher current and larger cap, or a pair of 2200 caps. Also, for a regulator, look into the LN317 for low current (up to 1 amp or so) and the LM1085 for higher current. The LM1085 is a low dropout type, so you can worry less about the drop across the rectifiers if you use it.
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Old 8th March 2007, 09:45 PM   #10
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Default what's your problem?

I don't understand what's the story with Shottkys. Any bridge rectifier will work. Also, the cap value is not that critial. The larger the cap = the better the filtering, but you can get away with 10uF to 100uF because the regulator itself has some smoothing effect.

I've built a couple of exactly similar circuits and there's a diffrent problem that you might not be aware of.

The 12V/3A fairchild voltage regulator might shut down for some time if the current exeeds 1A (not 3A!). I don't know how this smart feature is called (protection?) but if you don't want do suddenly fry your tubes with zero filament and 300V on the plates, put at least two 3A regulators in parallel (I believe you want it for filaments?).

If you can tell me what exactly you want to power up with this circuit (how much current) I will be able to tell you what are minimum ratings for the transformer and if the fairchild will handle it. I've made many tests with all possible 12V trannies and generally you need at least a 4A transformer for 1.5A DC current.
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