Please help with power supply - Page 1
Forum section: electronics / testgear

author: samson
author_rank: Contributor
timestamp: January 14, 2024, 01:46:53 am
content: Hi, Could someone please advice were is the centre tap for this transformer. I need to tap 11v ac.Any help is appreciated.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 14, 2024, 01:59:50 am
content: you have 3 secondaries, one with a center tap who go on d208, but no where you get an 11vac ?? out of ityou already have 11vac form one secondarie, find it with an dmm ?? d201 d202 d203 d204 ??easy ?its between the yellow orange or the green, follow wich ones go the the 4x diode group d201 d202 d203 d204

author: Kean
author_rank: Unknown
timestamp: January 14, 2024, 10:40:24 am
content: It is kind of annoying that they numbers the transformer primary terminals on the schematic symbol, but not the secondary terminals.I think it is a fair assumption that the secondary terminals are in the same order as the schematic, and if so seeing the two wires that head off away from the PCB on terminals 11 & 13 make me believe those are the ones labelled "heater" on the schematic.That then leads to terminal 7 & 8 (orange & yellow) being the 11V AC winding, and terminals 9 & 10 (green and blue) being the 15V AC winding.This matches up with the PCB layout, with orange and yellow terminating near D201-204 as on the schematic, and green and blue near D205/207. Purple wire being the heater centre tap also makes an appearance in this area as you would expect from the schematic.Easy to confirm with with DMM, keeping in mind the AC voltage will vary with input voltage and load - but the difference between 11V and 15V winding should be quite obvious.

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 14, 2024, 10:53:51 am
content: Just a fair warning: Wherever you are going to find your 11V AC it will not be independent of the rest of the circuit, so depending on what you want to use it for think twice!

author: samson
author_rank: Contributor
timestamp: January 16, 2024, 07:22:26 am
content: Just a Note, With out the project hooked up to the transformer the voltage on terminal 9 and 10 on the transformer is 16.04vac. When I hook up my project which I need 15 volts ac and not 11volts ac. The voltage on terminal 9 and 10 drops to 15.87volts. Is this ok . Please advice.

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 16, 2024, 09:25:03 am
content: You are confusing, first you say you need 11Vac, now you say you need 15Vac and not 11Vac? Without knowing more about 'your project' most answers will be guesswork but the voltage drop you mention is probably as expected and nothing to worry about.

author: samson
author_rank: Contributor
timestamp: January 17, 2024, 02:16:58 am
content: Originally I needed 11 volts, but it turns out the Buck converter needs 15vac.

author: Kean
author_rank: Unknown
timestamp: January 17, 2024, 03:59:21 am
content: If you want any further assistance, you need to tell us a lot more. Apart from the photo and partial schematic, we have very little information.We don't even know what power supply that is, let alone what your "project" involves to be able to provide any valuable feedback.Quote from: samson on January 17, 2024, 02:16:58 amA buck converter generally needs a DC supply, not AC. Again, you provide too little information for us to be able to help.

author: samson
author_rank: Contributor
timestamp: January 17, 2024, 01:21:11 pm
content: The buck converter is ac to dc.

author: Kean
author_rank: Unknown
timestamp: January 17, 2024, 01:36:45 pm
content: Quote from: samson on January 17, 2024, 01:21:11 pmA buck converter is a DC to DC device by design, so if yours accepts an AC input then that means it has input rectification and likely needs an extra 1.2V headroom because of it.More information please. If you can't share more information then say so, but as already stated we cannot help you much with just these little tidbits and you are basically wasting everyone's time.

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 17, 2024, 01:41:56 pm
content: This is looking like a classic x-y problemhttps://xyproblem.info/Yes you need an ac voltage for a buck converter. Got it. But what is buck convertor going to be used for inside of an existing instrument? You’re telling us how the story ends and then asking us for the plot. Then we have to ask you dozens of questions to understand what you’re attempting to accomplish.This process (getting technical help on the internet) would work much better by you starting at the beginning with explaining what you what to do and why you want to do it rather than asking a question for which there is no simple, obvious answer.

author: samson
author_rank: Contributor
timestamp: January 17, 2024, 03:08:12 pm
content: The project is I have a frequency counter with a 10mhz crystal, I have put in a heater in for the crystal which needs 9volts dc. I use the buck converter to tap into the transformer rather than taking it from the main board. Now the frequency counter is stable.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 17, 2024, 03:21:18 pm
content: the best thing would have been to add an small other psu in the unit, not taking power on any section of your main instrumentyou may over time consume more current then normally provided and you may have problems in the future in your main unitit was not a good job ... even if you say it work

author: samson
author_rank: Contributor
timestamp: January 17, 2024, 03:33:52 pm
content: Not possible to put a psu in as there is no room. Thought of this first.

author: Kean
author_rank: Unknown
timestamp: January 17, 2024, 03:53:46 pm
content: A buck converter to power an OCXO? I would have used an LM7809 powered off the 11V AC tap (rectified and filtered of course). Avoids additional switching noise, and likely there is little lost in a linear regulator as the heater should reach equilibrium after a short time and not draw too much power. Assuming it is properly temperature controlled...A separate transformer would have been best, but I understand about the lack of space. As we have no idea of the design specs for that transformer or the rest of the circuit, I suggest you keep an eye on the transformer temperature to make sure it isn't overloaded under worst case conditions (cold environment).If you have an AC power meter, you could measure input power with and without the additional circuitry connected, again try to simulate worst conditions (A/C on full blast?). I would think an extra 10-20% is probably fine as that would be likely covered in the design margin, but much more could be a problem.

author: tunk
author_rank: Super Contributor
timestamp: January 17, 2024, 03:59:45 pm
content: What's the max power/current of the heater.And what's your buck regulator.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 17, 2024, 06:17:49 pm
content: you have lots of space in this loll can even put some cell phone chargers to give you some 5vdc to supply some circuityou have pretty good and small models