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DC heater large cap

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BobBeaston

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Joined 2010
#1
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/matchless_hotbox_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
 
#2
http://schematicheaven.com/effects/matchless_hotbox_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.
 
#4
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?
 
#5
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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#6
BobBeaston said:
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?
Click to expand...

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.
 
#7
Bigun said:
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.
Click to expand...

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 Drawings/Filters/Circuit Example.jpg

How to calculate the values and ratings for C2, C3 and C4? C1 is clear enough...
 
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#8
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...
--
 
#10
rongon said:
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?
Click to expand...

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.
 
#12
Should it be before or after all the B+ preamp filters?

SemperFi said:
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.
Click to expand...

Sorry I should have been more clear. I was asking where I should place the voltage divider in the circuit? Before the first cap? after the first rc filter? after the second rc rilter..last in line?
 
#14
Guys,
This from experience:
I ran out of heater current for an amp I built - I only needed supply for one extra 12AX7, that is 6.3V at 300mA.
I had a spare 5.0V @ 2 Amp Secondary winding on the power tranny (no doubt intended for a 5AR4 or 5Y3 tube rectifier).
I used 4 off 1N5822 Schottky diodes (40V @ 3A rated) as a bridge rectifier on that 5V winding into a 10,000uF 10V electrolytic capacitor. With the 12AX7 heater as a load (nominally 300mA) I got 5.95 Volts - That is about -5.5% which is close enough to the +/-5% specification for heater supplies.
Cheers,
ian
 
#15
Actually a bridge+C really is an RC filter -- the R is the DCR of the transformer windings...

If you want a common-mode choke, try to get one with bifilar windings. This ensures that the net flux really is zero, so it can do without the air gap, and it gives you much more inductance per volume.

Kenneth
 
#16
well I made a supply using CRC... the caps are both 16v 10,000uf (actually smaller than my B+ 47uf caps). The resistor is a 5W 2 ohm. I am getting 6.2 volts. This is not incorporated into my amplifier yet as I just used a breadboard. You guys see any problem with this heater supply?
 
#17
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.
Click to expand...

Exactly. I have succesfully used 1:1 power transformers for common mode chokes in the B+ supplies of tube amps. This only works where all of the current going out one winding comes back in through the other winding. In this case, or in a HV supply, wire the power transformer to a bridge rectifier (a schottkey in this case) a big cap goes across the output of the bridge with no connection to ground or anything else. Next the common mode choke followed by another big cap. The second cap becomes the ground and B+ points for the amp (or the heater supply).

Note, Isolation transformers are rarely 1:1 and don't work here. A power transformer with two primaries (for 120 or 140 V operation) works great.

Sometimes high current common mode chokes can be liberated from dead PC power supplies. These don't offer much help in reducing hum, but they kill the diode switching noise.
 
#18
Dumb question but this is an active thread. How do you calculate the dc voltage needed for a ac heater?

lets say its a standard 6.3vac heater. With a magic zero drop full bridge rectifier, thats 8.883 dc. If you include the drop of both diodes thats 7.5dc. If you include crc filtering its going to drop even more.

Do you just take the average power of a resistor into a 6.3vac signal?
 
#19
nightanole said:
Dumb question but this is an active thread. How do you calculate the dc voltage needed for a ac heater?

lets say its a standard 6.3vac heater. With a magic zero drop full bridge rectifier, thats 8.883 dc. If you include the drop of both diodes thats 7.5dc. If you include crc filtering its going to drop even more.

Do you just take the average power of a resistor into a 6.3vac signal?
Click to expand...

The 6.3V rating means ac (rms value) or dc.

The use of rms values is intended to express an ac-voltage that heats a resistance with the same power as a dc voltage of the same value.

So, you keep dropping your rectified voltage until 6.3V dc appears at the heater terminals.
 
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