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Old 24th May 2011, 04:17 PM   #1
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Default Power supply for heater

I know this is a bit unusual, but since i have all these parts available from were i work i thought i would give it a shot. Do you guys think this would work for a heater DC power supply? Input is 24V from a trafo i have (2A)...

Thanks
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File Type: pdf Heater PS Board (Schematic).pdf (16.9 KB, 94 views)
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Old 24th May 2011, 04:31 PM   #2
tomchr is offline tomchr  United States
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I see two issues with it:

1) The voltage at the inputs of the 7806'es will be very close to the maximum rating of 35 V. Recall that 24 V AC when rectified will result in sqrt(2)*24 = 34 V unloaded and roughly 1.25*24 = 30 V under load. If that 24 V deviates even a little bit, you risk breaking the 7806.

2) At 2 A out (1 A per regulator), roughly 24 W - or nearly 50 W total for the two regulators - will be dissipated in each regulator. That will require a heat sink about the size of a 500-sheet ream of copier paper to dissipate safely. -- Or if you don't mind the noise, you can use one of those CPU coolers with a big fan on it. That should be able to get rid of 50 W.

If you insist on using the 24 V input supply, I'd probably be looking at a switchmode supply. Many of the ICs available require very few external components and are easy to get going. Check out the SimpleSwitchers from National for example.

Another approach would be to find a transformer with a more reasonable output voltage. A 10 V transformer would probably be fine.

Aside from these two issues, I don't see anything wrong with the schematic. I am assuming that you'll be feeding two separate heater strings with the two regulators. Don't connect them in parallel. They don't play nice if you do that...

~Tom
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Old 24th May 2011, 04:32 PM   #3
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That schematic has numerous issues going on. 24 volts in should be the maximum, not the normal for a 6 volt regulated supply. The capacitors say 100p which can't be right. 1000uF is too little. 24 volts AC in would give you around 33 after the filter caps. Even a small load would generate tons of heat on the regulators.
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Old 24th May 2011, 04:37 PM   #4
SY is offline SY  United States
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Here's a better approach- if you're trying to heat several 6V heaters with the same current rating, configure the reg as a current source and put the heaters in series. You've got voltage to burn (literally).

Yeah, MUCH more filter capacitance is needed to knock down the ripple to something acceptable.
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Old 24th May 2011, 05:15 PM   #5
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How to i calculate how much current should i have if i connect them in series?
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Old 24th May 2011, 05:57 PM   #6
SY is offline SY  United States
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Get the currents off the tube datasheets.

Current in a series string is the same for each element (Kirchoff's Law), so this only works if all heaters have the same current rating.
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Old 24th May 2011, 05:58 PM   #7
DF96 is offline DF96  England
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How much current does the valve heater take? Look at the datasheet. For small signal valves it will often be 150mA or 300mA. All the valves in one series string should take the same current, otherwise you have to use balancing resistors.

Is this for your guitar amp, or something different? DC heaters for a guitar amp would be unusual. Only really necessary for a phono stage, and not even there with good valves and good wiring.
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Old 24th May 2011, 06:00 PM   #8
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that's why i asked, i thought about that, but the heater do now have the same current, so i thought maybe there was a magical way to do that...
I guess i my best option is to get some monster capacitors and go with that...
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Old 24th May 2011, 06:08 PM   #9
SY is offline SY  United States
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Well, if they have the same current, a CCS feed can have some advantages. How many tubes are we talking about?
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Old 24th May 2011, 06:10 PM   #10
tomchr is offline tomchr  United States
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Quote:
Originally Posted by astouffer View Post
The capacitors say 100p which can't be right.
I read the capacitance as 108 = 1000 uF. But whatever. It's hard to tell given the image quality. Depending on the load current, that might be OK, though, for a heater supply, I'd expect capacitances of at least 4700 uF if not larger. I generally don't like more than 0.5~1 V of ripple before the regulator.

~Tom
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