SoulFetish wrote:A different approach would be to add silicon diodes in series with the secondary heater taps. This would give you the flexiblity to determine the voltage drop by adding/subtracting diodes in series or increasing the current handling capacity by placing additional diodes in parallel. Here is a simple schematic illustrating the circuit.
Parallelling multiple diodes on each leg won't work the way you think. One diode will end up taking most, if not all of the current. Just use a diode with a higher current rating instead. (When you have a stack of UF4007 like I do that I'd love to use in parallel situations, that's not what you want to hear.)
Also, there's no real current flow through the CT (or artificial CT) of a heater circuit. It's just there as a DC voltage reference, either to ground or some elevated DC voltage. A zener there won't affect anything to do with the AC voltage.
To the OP, if you put in an anti-parallel pair of diodes of sufficient current rating on one of the heater legs, you'll be close to 6.3VAC.
If that's the case, I stand corrected. I appreciate the heads up. It seems counter-logical, though, could you explain the electro-mechanical reason why it wouldn't increase current capacity?
SoulFetish wrote:If that's the case, I stand corrected. I appreciate the heads up. It seems counter-logical, though, could you explain the electro-mechanical reason why it wouldn't increase current capacity?
If the diodes are exactly identical in all characteristics, it will _probably_ work. If not, one of the diodes will start conduction before the other(s). That diode will warm up as it is dissipating ~0.6V * current-draw. As the diode warms up, it becomes more "conductive" (can't remember the technical term) than it was when cold. When this starts to happen, that diode will attempt to conduct all available current until it fails. At that point, one of the other diodes will take over until it fails. With good, high-current diodes available that are more than capable of handling it, there's no reason to attempt to parallel them.
Just kiddin'. That's solid info; I really appreciate it. I'll fix the schematic and illustrate an alternative CT & artificial CT zener schematic as well.
Which zener do you recomend for this application?
Also, the UF5408-E3/73 (Ultra Fast recovery diode) is rated for 1KV/3A, whereas the UF4007 is rated 1KV/1A.
Any other ideas for capable current handling diodes?
SoulFetish wrote:... I'm sorry, I just don't buy it.
Just kiddin'. That's solid info; I really appreciate it. I'll fix the schematic and illustrate an alternative CT & artificial CT zener schematic as well.
Which zener do you recomend for this application?
Also, the UF5408-E3/73 (Ultra Fast recovery diode) is rated for 1KV/3A, whereas the UF4007 is rated 1KV/1A.
Any other ideas for capable current handling diodes?
It depends on the current draw. In the OP, it looks like 3A is the existing heater current, so something closer to 5A would be much better. I don't know the more normal designation, but an NTE577 (5A) or NTE572(6A) would work. It doesn't need to support a whole lot of voltage, but 1000V should be plenty.
As for the CT or artificial CT setup, it doesn't matter what you put in there, it won't drop heater voltage. The only reason it exists is for a DC voltage reference for the AC heaters to ride on top of to reduce hum. The reason it works on the B+ supply is that there is current flowing through the center tap. (There is a little bit of current going through the resistors for an artificial CT, but it's parallel to the heater supply so it doesn't matter what is going on there.)