Matador

Center tapped transformer specs?
« on: September 13, 2019, 07:00:49 PM »
Sorry for the basic question: what does a 12-0-12 power transformer specification indicate?  I know it shows a center tap, but does it denote the peak voltage on the secondary, or the RMS voltage?  In other words, if I put a scope probe from the center tap to one outside, would I see a 12V peak voltage, or sqrt(2)*12V peak?


JohnRoberts

Re: Center tapped transformer specs?
« Reply #1 on: September 13, 2019, 07:20:46 PM »
Sounds like CT=0V, each winding will deliver 12VAC in opposite polarities. So 12V x1.4 peak voltage in each direction.

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

PRR

Re: Center tapped transformer specs?
« Reply #2 on: September 13, 2019, 08:27:38 PM »
Strictly, the transformer defines a *ratio*.

If you put 120Vrms in, you get 12Vrms out.

If you put 170V peak in, you get 17V peak out.

Since we "always" measure wall voltage as RMS (lamp brightness), we expect 12Vrms on secondary.

(SPICE does not know about wall-outlet conventions, is not limited to sine-line waves, so it thinks in Peak voltages.)

Matador

Re: Center tapped transformer specs?
« Reply #3 on: September 13, 2019, 08:42:29 PM »
Thanks, that makes sense.

So if used with a bridge rectifier in the following configuration:



'ea' would be 12VRMS, or ~17V peak (with respect to the 0V node), and V+ would charge to ~16.3VDC (assuming no load, steady state), and V- would charge to -16.3VDC (again, assuming no load).

Alternatively, if I declare -V to be 0V, then the '0V' node would sit at ~16.3VDC, and +V would sit at +32.6VDC.

Is this correct?

scott2000

Re: Center tapped transformer specs?
« Reply #4 on: September 13, 2019, 09:07:00 PM »
Does taking the diode drop before calculating make any difference?

PRR

Re: Center tapped transformer specs?
« Reply #5 on: September 14, 2019, 01:06:44 AM »
> So if used with a bridge rectifier

Do you have a 12V (or any) transformer around? And diode and cap?

Don't believe fools on interweb forums. Don't put total trust in dusty books (I have a Rufus here which is 90% right). EXPERIMENT! Know what you expect and document what you actually get.

So far this has been a numbers racket. When you have a REAL "12V" transformer, no-load, it is liable to really give 14VACrms, only sagging to 12V when fully loaded and warm. So "trust" the label only so far as to NOT pick a 600V winding, risking life and any low-volt cap you try. Measure what it really puts out.

JohnRoberts

Re: Center tapped transformer specs?
« Reply #6 on: September 14, 2019, 09:27:46 AM »
+1 to what Paul said..

Transformer voltages are nominal and transformer sag is called "regulation" while it isn't regulating anything. Regulation represents how much they sag under load. Transformers also sag more after they get hot (winding resistance increases).

JR 
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Matador

Re: Center tapped transformer specs?
« Reply #7 on: September 14, 2019, 11:26:46 AM »
Buy and measure becomes harder with $100 power transformers.  :)

Here's what prompted this question.  I need to make some high voltage supplies, looking to target around 450V pre-regulation/filtering, at about 100mA max sustained current (approximately 50W worst case).  Working backwards, allowing for about 50V of sag (e.g. a few hundred ohms of secondary resistance), means I need a 500V peak AC signal, or around 350VAC, or a transformer rated at 175-0-175.  This is out of a full-wave bridge rectifier posted above.

Going to Hammond's page, I see the 363CX, which is rated at 360V @ 288mA, labeled 180-0-180, so this seems reasonable.  However there are also a few discrepancies:  their own page says, "For a full wave rectifier with cap filtering, expect VDC(avg) = 0.9 * VAC rating of secondary".  I have no idea where this comes from.  In addition, a typical Marshall amplifier power transformer replacement is specified as 345-0-345 (or 690VAC), and it has the same 450VDC B+ operating point!

So I'm not sure if it's just a misunderstanding on my part about how these transformers are specified versus the actual operating conditions.

PRR

Re: Center tapped transformer specs?
« Reply #8 on: September 14, 2019, 12:05:57 PM »
> their own page says, "For a full wave rectifier with cap filtering, expect VDC(avg) = 0.9 * VAC rating of secondary".  I have no idea where this comes from. 

That's on Hammond's site but is not their own document, they "found it". It has several large errors.

The "Cap input" shows peak 1.414 and avg 0.9. But if the cap is sufficiently large these should be the SAME (nearly). Someone put the number for a NO-cap rectifier in the cap-input entry.

PSUD is more likely to give a right answer.

> 363CX ...  a few hundred ohms of secondary resistance"

Says right on the 363CX datasheet: 46.2 ohms red-red. The primary resistance (which reflects over as square of turns ratio) is two 6 ohm DCRs. Assuming 120V supply, 3r. Turns ratio is 1:3.2, so this reflects as 31 Ohms. 46+31= 78 Ohms (not "a few hundred"). PSUD gives a bit over 490V DC.
« Last Edit: September 14, 2019, 12:10:26 PM by PRR »

Matador

Re: Center tapped transformer specs?
« Reply #9 on: September 17, 2019, 04:28:46 PM »
I was racking my brain as to why PSUD was showing that a 550V RMS transformer swings from +775V to -775V....until I saw this note:

Quote
For full wave rectifiers, use the voltage for one side only.  For example, if the transformer off-load voltage is 350-0-350V (700V end to end), the correct value to enter is 350V.
>:(


 

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