Puzzled About Use Of Centertapped Transformer - Page 1
Forum section: electronics / beginners

author: Sentrec
author_rank: Newbie
timestamp: July 02, 2022, 11:32:42 pm
content: Can anyone able to tell me why (and the purpose) a 470uF/35 volt Electrolytic Capacitor would be placed on the centertap of this simple regulator circuit? I am baffled as to why!Please see attached file...Thank you!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 03, 2022, 01:15:18 am
content: It does not look very sensible to me. It would just get charged to half the output voltage and then do nothing.Except maybe, when the momentary output voltage of the transformer is smaller then the rectified voltage the transformer windings would be "floating" because all diodes of the bridge rectifier are closed. That capacitor gives the transformer winding a defined voltage, but I do not see how that is significant in any way.If it was a small ceramic cap it maybe could have been part of a noise filtering feature.

author: David Hess
author_rank: Super Contributor
timestamp: July 03, 2022, 01:26:00 am
content: It is safe if that is what you are wondering. The bridge rectifier diodes only turn on at the peaks when the capacitor for the 317 is charging, and at that point the center tap is at half of the transformer secondary voltage and may be used as a DC output. At other times the diodes are off and the secondary is effectively disconnected and left floating.I thought I had seen that configuration before so I did a little searching and found it in the power supply of the Tektronix 475 oscilloscope shown below. It is exactly the same configuration with the secondary providing a +100 volt supply, and the center tap used to directly provide a +50 volt supply without the need for any additional rectifiers. Clever, eh?As to why that capacitor would be connected when the center tap is not used as an output, I do not know. The capacitor provides an AC ground to keep the transformer secondary from floating around which might suppress noise or other undesired behavior. I do not think it would suppress the switching noise from the rectifiers if present. Sometimes a small ceramic or film capacitor of 100s or 1000s of picofarads is placed directly across the secondary to do this, or small film or ceramic capacitors are placed across each diode with a value of 10s to 100s of picofarads.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 03, 2022, 02:50:16 pm
content: It definitely lets you use a filter capacitor with a smaller voltage rating.I also would think that a smaller cap would work better for switching noise, you can always add one in parallel to help with this.I wonder if there is anything about the phase charging and discharging the two caps that make the output smoother.Also there is reactance in the diodes and the transformer windings between the caps that affects their filtering (charging and discharging) I am not smart enough to figure this our mathematically.There is a formula that was used in RC filters and then in L C filters ( Power Supply LC filters in old radios had two caps to ground separated by a big choke) This worked better than just a cap to ground.Maybe the circuit electrically approximates one of these LC (big choke) filter types??I am unable to find anything on this particular configuration except something about a tube based audio amp application where it is said that the filter cap on the B+ of the output transformer to the speakers is helped in bass response by a big cap. (also filtering)Very Interesting.

author: fourfathom
author_rank: Super Contributor
timestamp: July 03, 2022, 06:19:18 pm
content: Quote from: vu2nan on July 03, 2022, 05:14:50 pmNo, look at the original schematic. The ground point is completely different. The center-tap capacitor will be sitting at half-supply voltage, but this point is not used and the capacitor seems to be unnecessary.

author: David Hess
author_rank: Super Contributor
timestamp: July 03, 2022, 06:36:40 pm
content: Quote from: fourfathom on July 03, 2022, 06:19:18 pmThey are the same thing, and moving the ground point does not change anything except the analysis.

author: IanB
author_rank: Super Contributor
timestamp: July 03, 2022, 06:56:42 pm
content: Quote from: David Hess on July 03, 2022, 06:36:40 pmI cannot see how to rearrange components to make the diagram in Reply #4 the same as the diagram in the first post.Isn't it the case that the 50 V smoothing capacitor is being charged to the full voltage of the transformer (terminals 3 and 5) and is presenting this to the regulator? That is the full supply, not half the supply.

author: bdunham7
author_rank: Super Contributor
timestamp: July 03, 2022, 07:04:01 pm
content: Quote from: vu2nan on July 03, 2022, 05:14:50 pmSort of, but not the way you've drawn it. The filter cap C7 goes from V+ to V-, the 'ground' is V-. This is not the same as a CT/bridge split supply where the total output voltage is across two filter caps the way you've drawn it.

author: mclute0
author_rank: Regular Contributor
timestamp: July 03, 2022, 08:18:42 pm
content: so, I have to say that schematic is a mess.... The parts are not numbered, not sure how someone can talk about C7 when all the Cs are labeled C?the capacitor you're asking about (the only one 35v) says 47Du and is using the EU capacitor symbol for a polarized cap, the other capacitors are using the US symbols. How does one get 470u from 47Du, D is not a common typo for 0.My simple understanding is...When the opposing ends of a center tapped transformer are used, the center tap is the balance point and should be approximately zero volts relative to the two ends. V++ V-=V0(source:https://www.physicsforums.com/threads/understanding-output-voltages-for-a-center-tapped-transformer.984018/)The reason for the capacitor is to filter noise with a cutoff frequency based onthe Cutoff Frequency Formula 1/(2πRC). Without a resister in-line with the capacitor the cutoff will be based on the capacitors ESR and any trace resistance, so in this case around ~1.5khz if the ESR is around .22ohms?

author: mclute0
author_rank: Regular Contributor
timestamp: July 03, 2022, 08:23:13 pm
content: Quote from: bdunham7 on July 03, 2022, 07:04:01 pmI don't think yours is quite right either, since it is possible to trace from a path from V-output to the center tap without going through the (or any) capacitor the OP is asking about...

author: bdunham7
author_rank: Super Contributor
timestamp: July 03, 2022, 08:27:34 pm
content: Quote from: mclute0 on July 03, 2022, 08:23:13 pmI guess the tiny picture is hard to see, but I put an 'X' below the top capacitor indicating a break or disconnect. The top capacitor is only connected from V+ to V-.

author: fourfathom
author_rank: Super Contributor
timestamp: July 03, 2022, 08:36:29 pm
content: Quote from: David Hess on July 03, 2022, 06:36:40 pmDoesn't change anything except:QuoteWhich I was commenting on. And regardless of the ground point the circuit topologies are different, so it actually changes a lot..

author: mclute0
author_rank: Regular Contributor
timestamp: July 03, 2022, 08:37:07 pm
content: Quote from: bdunham7 on July 03, 2022, 08:27:34 pmopps, sorry..

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 03, 2022, 08:55:43 pm
content: The center tap filter filter cap is only encountering 1/2 the voltage, not the total voltage, and has a large inductance with it. The inductance is 1/2 of the transformer, which is substantialThis makes the capacitor arraignment look like a Pi network, if you imagine part of the transformer being the choke.I do not remember seeing this particular arrangement before.You could also put a cap between center tap and POS, with the appropriate pos and neg cap leads being at pos for for lead 5 and neg for center tap.Pi networks were commonly used in Center tap full wave tube rectification systems. When tubes were more expensive than copper. Of course with these the center tap was ground and not available for this circuit.Maybe that is what this really is?? Only without the center tap being GND??EDIT; The voltages seen by the cap are unfiltered DC.

author: Sentrec
author_rank: Newbie
timestamp: July 03, 2022, 09:39:41 pm
content: Hello All,Thank you for your posts! Very helpful in shedding a new light to my circuit analysis skills! I'm sorry about the confusion about the component nomenclature. I went ahead and annotated the components and increased the font size in hopes to reduce further confusion.This 17.5v regulator section is only meant to provide 17.5vdc, nothing else that I can tell. This circuit came from a power supply board of which there was no schematic so the schematic presented is an "AS BUILT" representation of the circuit card assembly. Capacitor C1 is simply attached to the centertap of T2, it is not used anywhere else - there is no load connected to it. As pointed out first by Doctorandus_P, I did measure 20vdc across C1 when the power is on. That said, it's also worth mentioning that there is also 40 volts DC across pins 2 & 4 of the diode bridge, D2.I might add that a Physicist designed this circuit so I found the analysis reference by mclute0 an interesting call... : )Thank you all once again for sharing your wisdom with me, very much appreciated!