Why a 6.3vAC output on XP720 power supply? - Page 1
Forum section: electronics / projects

author: WacoAvionicsTech
author_rank: Newbie
timestamp: September 04, 2017, 09:02:08 pm
content: Im building the Xp720 p/s with my daughter. It has along with 15v dc vairable two 6.3vAC out puts fixed with a common. What use would one have for this?

author: Benta
author_rank: Super Contributor
timestamp: September 04, 2017, 09:17:37 pm
content: 6.3 VACis for vacuum tube filaments. Useful when testing old equipment.

author: floobydust
author_rank: Super Contributor
timestamp: September 04, 2017, 09:20:12 pm
content: It's a general purpose AC output, if you want to play with SCR/TRIAC circuits or rectifiers- anything needed a sine wave, for just the added cost of the binding posts.Educational if you have a scope to see the waveforms.In the old days it was to power vacuum tube filaments.

author: Zero999
author_rank: Super Contributor
timestamp: September 04, 2017, 09:58:15 pm
content: What you have is a split phase AC power supply. The common is the neutral and the 6.3V outputs are anti-phase, i.e. 180oout of phase. If you connect a 1k resistor to each 6.3V output and connect the resistors together, you see that the voltage where the two resistors join, is zero, demonstrating that opposite phases cancel.

author: Unknown
author_rank: Unknown
timestamp: September 04, 2017, 11:19:36 pm
content: Also, be aware that it's not floating - it's tapped from the main supply & shares the common with it, which makes it a bit less useful than it might seem.

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2017, 08:27:10 am
content: Quote from: Tac Eht Xilef on September 04, 2017, 11:19:36 pmGoing by the schematic, it is floating. There is no connection from the secondary to the mains supply, which would be dangerous and I doubt that's what you meant, or earth, which is probably what you meant.https://www.electronickits.com/wp-content/uploads/2015/01/xp720k.pdf

author: bd139
author_rank: Super Contributor
timestamp: September 05, 2017, 08:35:41 am
content: I can think of a ton of uses:1. Testing filaments.2. You can connect a 6.3v/110v transformer to it to step up to vacuum tube anode potentials.3. simple curve tracer4. Derive other voltages easily by adding a rectifier, smoothing cap, regulator, voltage doublers etc (you can get +/-15v out of that with 10 components)5. pure floating common mode voltage source for testing CMRR etc.6. driving straight voltage multipliers for electric death machines.7. blowing the shit out of electrolytic capacitors.I might actually just go and build one actually.

author: Benta
author_rank: Super Contributor
timestamp: September 05, 2017, 09:18:56 am
content: QuoteI think he meant it's not floating with regards to the DC outputs. I agree that this limits its usefulness.

author: Ian.M
author_rank: Super Contributor
timestamp: September 05, 2017, 09:37:24 am
content: A floating AC output would require an extra transformer winding, so extra cost.BD139 has already mentioned using it to derive other voltages - if you get into Arduinos, you typically need a supply in the 7V to 9V range to feed the Arduino Vin pin (as powering one from its +5V pin increases the risk of damage if you make any mistakes) and using a dual common cathode Schottky diode (or two individual Schottky diodes) with its anodes fed from the two 6.3V AC terminals, followed by a 1000uF reservoir capacitor would give you an unregulated +8.5V DC supply for Vin while still leaving the positive variable DC supply available. A 250mA fuse would be advisable. Suitable parts (apart from the fuse) could be found in any scrap PC PSU . . . .

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2017, 09:43:15 am
content: Quote from: Benta on September 05, 2017, 09:18:56 amYes, that makes much more sense. He did say "main supply" not mains. I must have misread it.

author: Unknown
author_rank: Unknown
timestamp: September 06, 2017, 05:23:41 am
content: Quote from: Hero999 on September 05, 2017, 09:43:15 amYep, sorry,my bad - I should've said something like "taps on the same secondary that supplies the main DC rails".