Transformer Theory Questions

[mr.duck]

If you connect a transformer to the mains, but do not connect the secondries to anything, will the transformer draw any power?

If the transformer is unloaded, will the secondary voltages be exactly as rated for the transformer? And then the more the current you draw, the more the voltage will drop? Or does the voltage only drop after you exceed the VA rating?

Would you say 25v capacitors are ok for smoothing DC in an unregulated PSU when using transformer with 15v secondries? (15 * 1.414 = 21.2)

Cheers.

[BWRX]

If a transformer is connected to the mains but is unloaded, it will still dissipate some power due to the resistive losses in the primary.

If the transformer is unloaded the secondary voltages will not be exactly as rated. Usually manufacturers rate the secondary voltage at a certain current. The less current you draw the higher the voltage will be and vice versa. There is also a % regulation spec which tells you how much the secondary voltage will deviate when unloaded. Higher VA rated transformers will have tighter regulation and lower VA rated ones will have more variation in their output voltage depending on the current.

25VDC rated caps should be ok to use with your 15VAC transformer. I would get some caps with a 35VDC rating if you don't already have the 25V ones.

[FastEddy]

BWXR: " ... 25VDC rated caps should be ok to use with your 15VAC transformer. I would get some caps with a 35VDC rating if you don't already have the 25V ones. ..."

... Careful here: A transformer output voltage rated at +/- 15 Volts AC is capable of easily resulting in +/- 25 Volts DC ... so if you are using 25 VDC rated capacitors, you are riding right on top of the absolute maximum voltage rating that the manufacturer suggests = which could also easily be +/- 10% of the "real" maximums = or as low as a "true" rating of ~< 22 Volts DC = Oops = zzzzztttt ... smoke ...

FYI: Voltage AC through a common, full wave, bridge diode rectifier produces 1.707 Times the Voltage AC across the well regulated DC rails = the peak voltage ... that's what they mean by RMS = Root Mean Square = the peak voltage of the AC sign wave representated by the AC voltage .... and it can be mean and nasty.

I always use capacitor ratings at least 1.5 times the expected DC voltage ... 'cause some of them component manufacturers lie about their ratings, no polliter way to put it.

[BWRX]

Originally posted by FastEddy
Careful here: A transformer output voltage rated at +/- 15 Volts AC is capable of easily resulting in +/- 25 Volts DC ...

Easily capable of 25VDC? Not easily, but it could happen. If the transformer has 15% regulation and the line voltage is a bit higher than normal then yes, the peak of the AC voltage on the secondary could reach 25V, but there will also be two diode drops if a full wave bridge rectifier is used. That will knock that 25V down a volt or so to 24V which is still within the rating of the cap. For use at room temp. I think 25V rated caps will be fine, but like I said I'd still go with 35V rated caps if I had the choice.

Originally posted by FastEddy
I always use capacitor ratings at least 1.5 times the expected DC voltage ... 'cause some of them component manufacturers lie about their ratings

I always try to use caps rated at twice the expected voltage but that's not always economical. The solution? Get caps from a reputable manufacturer that doesn't lie about their ratings.

[FastEddy]

" ... Easily capable of 25VDC? Not easily, but it could happen ..."

Well, every time I use +/- 15 VAC (center tapped or not) and connect that to any diode bridge and parallel filter capacitor, I get a reliable volt meter reading of 25.605 Volts DC ... every time! ... unless there is something wrong with my meter.

Even if you onle have a half wave rectifier (two diodes) the results above will be the same, every time .... or else Maxwell was wrong ...

[FastEddy]

" ... I always try to use caps rated at twice the expected voltage but that's not always economical. ..."

Yes, I suppose so ... In the case of smaller caps, I double the expected DC voltage for the cap rating as well.

There are several inexpensive ways to "secure" very large capacitors from voltage spikes, or "near death" experiences ... But I will stick to what I said: 1.5 X expected DC voltage as a minimum reliable, workable, safe spec.

[BWRX]

Originally posted by FastEddy
Well, every time I use +/- 15 VAC (center tapped or not) and connect that to any diode bridge and parallel filter capacitor, I get a reliable volt meter reading of 25.605 Volts DC ... every time! ... unless there is something wrong with my meter.

What is the primary/secondary voltage rating and the % regulation of your transformer?

I have a 120V 120VA Plitron toroidal with 15VAC secondaries @ 4A and 11% regulation. With a full wave bridge rectifier, a 22,000uF cap, and a bleeder resistor I have seen up to 24VDC unloaded with 121VAC line voltage. If your transformer has worse regulation it's possible to see higher like you're seeing. An easy solution is to use a lower value bleeder resistor to draw more current and pull the voltage down a bit, or you could get a better transformer

[AKN]

Originally posted by FastEddy

FYI: Voltage AC through a common, full wave, bridge diode rectifier produces 1.707 Times the Voltage AC

FastEddy,

Careful here...

Correct would be VPeak=VRMS*Sqrt2 sqrt2=1.414....
So Vpeak=VRMS*1,414....

If you want to go back: VRMS=VPeak/(1/sqrt2)
1/sqrt2 = 0,707......So VRMS=Vpeak/0,707....

[FastEddy]

" ... I have seen up to 24V unloaded with 121VAC line voltage. ..."

You are probably quite correct. I used a multiplier of 1.707 instead of the (correct) multiplier of 1.414 .... make 24 VDC closer the the upper limit on the rails.

Looks like You are right on this one ... and so is Maxwell.

But my statement still stands = 1.5 times expected (peak or DC) voltage for a well behaved capacitor = no fried weenies ...

[AKN]

Originally posted by FastEddy

But my statement still stands = 1.5 times expected (peak or DC) voltage for a well behaved capacitor = no fried weenies ...

But then you have to tell why.

Probably you mean that unloaded tranny put out some more voltage. Value can found in transformer specs so you can make more precise calculations.

Regarding voltage, ceep in mind that line voltage can vary approx +/-10% and so on both primary and secondary.