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Old 23rd June 2010, 04:37 PM   #1
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Default DC heater large cap

I am looking for a way to run DC heaters off a 6.3v supply. In this schematic there is only a large cap after a bridge rectifier.

http://schematicheaven.com/effects/m..._overdrive.pdf

I thought you needed a full CRC.. wouldn't there be DC ripple without a resistor?

If I used a CRC what value caps and resistor should a use (just an example, I know the resistor will vary depending on transformer and heater current draw)

I know there are several threads on this but I couldn't find exactly what I was looking for. Thanks
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Old 23rd June 2010, 05:13 PM   #2
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http://schematicheaven.com/effects/m..._overdrive.pdf

I thought you needed a full CRC..

Well, the problem with putting an "R" in there is that you can't afford to lose any voltage. That 6.3V needs to be within about 5%. Note that one side of the filaments is grounded, and the value of the "C" is pretty darn big. So it probally works well enough. If you are building this specific device you could do it that way, but in general I would catagorize this aproach as quick and dirty.
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Old 23rd June 2010, 05:22 PM   #3
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If you are not going to build from the afomentioned schematic, but just want to know about DC supplies and filtering/regulation ect. Check out the thread in the power supply section. I think you can start here.

Help Calculating Voltage after Rectification

Bob
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Old 30th June 2010, 01:48 PM   #4
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thanks..I'm thinking I may run the heaters AC and reference an artificial center tap to 30-40 volts above ground. I know you are supposed to use a voltage divider and a cap from that junction to ground with large value resistors but where should this be in the power supply?

Should it be before or after all the B+ preamp filters?
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Old 30th June 2010, 02:43 PM   #5
Bigun is offline Bigun  Canada
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I would suggest you include a common mode choke to the heater dc supply. It will help reduce ripple further and in particular it will reduce the higher frequency noise generated by the diode rectifier. Since the choke has a low dc resistance you'll minimize any voltage drops.

Here's an example from Panasonic ELF-22V030A, costs $5 from digikey.
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Old 30th June 2010, 02:50 PM   #6
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Originally Posted by BobBeaston View Post
thanks..I'm thinking I may run the heaters AC and reference an artificial center tap to 30-40 volts above ground. I know you are supposed to use a voltage divider and a cap from that junction to ground with large value resistors but where should this be in the power supply?

Should it be before or after all the B+ preamp filters?
That cap is only to reduce the impedance of the voltage divider, no real currents go there, so it's not critical. I'd connect it to the same place as the first B+ filter cap.
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Old 30th June 2010, 07:23 PM   #7
rongon is offline rongon  United States
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Quote:
Originally Posted by Bigun View Post
I would suggest you include a common mode choke to the heater dc supply. It will help reduce ripple further and in particular it will reduce the higher frequency noise generated by the diode rectifier. Since the choke has a low dc resistance you'll minimize any voltage drops.

Here's an example from Panasonic ELF-22V030A, costs $5 from digikey.
At the risk of hijacking the thread... I'd like to know more about common mode chokes used in heater supplies.

- When you choose a common mode choke, what characteristics should one look for?

- Does the inductance or DCR need to be varied depending on the filament voltage, current draw?

- Is this the type of choke seen in Morgan Jones' Valve Amplifiers book, in an AC filament supply? How would one adapt this to a DC filament supply? Before the rectifiers, I'd assume...

Thanks.

[edit] - I found this application example - http://media.digikey.com/pdf/Catalog...%20Example.jpg

How to calculate the values and ratings for C2, C3 and C4? C1 is clear enough...

Last edited by rongon; 30th June 2010 at 07:35 PM.
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Old 30th June 2010, 07:28 PM   #8
rongon is offline rongon  United States
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To the OP... I've used this one cap DC supply from time to time. It works OK, but the ripple is still a bit high.

I have a single-tube preamp where the tube filament is drawing 900mA, but the 6.3VCT winding is rated for 2A, so the DC after rectification and a 4700uF cap was something like 6.8VDC. In this case, I could afford to add a 0.47 ohm 5W resistor and another 4700uF cap for a CRC filter, which got the filament supply down to 6.3VDC on the nose.

I'd like to add a common mode choke to that scheme, though...
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Old 30th June 2010, 08:36 PM   #9
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Quote:
Originally Posted by Bigun View Post
Here's an example from Panasonic ELF-22V030A, costs $5 from digikey.
AFAIK that's completely useless for this; it's intended for switching PS and would be saturated in a DC application.
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Old 30th June 2010, 08:37 PM   #10
Bigun is offline Bigun  Canada
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Originally Posted by rongon View Post
At the risk of hijacking the thread... I'd like to know more about common mode chokes used in heater supplies.

- When you choose a common mode choke, what characteristics should one look for?

- Does the inductance or DCR need to be varied depending on the filament voltage, current draw?
I'd be looking for low DCR and if the resulting voltage is a little high you can pull it down with some additional serial power resistor.

I'd want to ensure that the choke can handle the power without overheating, which means that the rms current flow through it should be within the current rating for the choke. The example I showed has a current rating of 3A, I'd want to be operating at half that to be safe.

Leadbelly - the nice thing about common mode chokes is that they are balanced - they can handle DC current rather like a push-pull output transformer. In a heater circuit all the current going out through one leg of the choke comes back through the other leg.
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