help please with toroidal transformers selection

[Robin Hewitt]

AC is forever bobbing up and down either side of zero volts.

Feed it through a transformer and you change that voltage in the exact proportion to the number of turns in the primary and secondary coils. It is still bobbing up and down.

Bridge rectify it and you mirror the volts below zero volts so they show above zero volts. It is still bobbing up and down but only one way.

Add a capacitor, any capacitor and the volts will stabilise at peak volts because there is no load to pull it down.

Add a load and discover what the problem with transformers is... to sustain the voltage at something useable you need a snogging girt capacitor. Any load will reveal the bobbing, the capacitor merely determines the degree.

1 Farad will give 1 Amp for 1 second. Unfortunately capacitors are usually rated in micro Farads, millionths of a Farad.

You can get 1F capacitors but they are either for memory back up purposes of they are the size of a house.

If your drivers don't care about the Volts bobbing up and down when you apply a load then transformers are for you

[Tom J]

Thanks Rob I know all that from college & university.
My power supply for drivers includes capacitors 2x10.000uF so can deal with back EMF well, that is why I am so concern about leaving spare 5V before max operating voltage.
I know some guys go for max like 1-2 volts off.
My 1st panel 3 years ago had 720VA trafo with soft start and stabilized 12v for BOB lpt1.



I am building another box 300x300 which is only High Voltage PSU and 24vDC

here are industrial PSU 24Vdc all stabilized



I need trafo to my 70VDC Leadshine EM705
60VAC after bridge and capacitors will be above 70VDC so have to get another one

I was asking for final dc voltage after rectifier and capacitors?
48VDC what ?VDC
50VAC what ?VDC
55VAC what ?VDC

Send the question to provider but no reply so far
Thanks

[Clive S]

I was asking for final dc voltage after rectifier and capacitors?
48VDC what ?VDC
50VAC what ?VDC
55VAC what ?VDC

Are you asking for the dc after caps if so its about AC V x 1.4

[Tom J]

Are you asking for the dc after caps if so its about AC V x 1.4

...yes Clive DC after cups but the real reading tbh.
my 30V trafo has 32VAC and after bridge and 2x10.000uF reads 41.5VDC unloaded
calculation gives 32V*1.4=44.8V 3.3V more than real test.

[Clive S]

...yes Clive DC after cups but the real reading tbh.
my 30V trafo has 32VAC and after bridge and 2x10.000uF reads 41.5VDC unloaded
calculation gives 32V*1.4=44.8V 3.3V more than real test.

The rectifier will drop a bit of volts.

But remember the AC mains fluctuates all the time

[Doddy]

...yes Clive DC after cups but the real reading tbh.
my 30V trafo has 32VAC and after bridge and 2x10.000uF reads 41.5VDC unloaded
calculation gives 32V*1.4=44.8V 3.3V more than real test.

Your off-load DC voltage is largely irrelevant, other than to consider any maximum voltage for component selection. The RMS voltage (i.e. 32V) represents the effective average voltage under the specified rating of the transformer. The problem you will have with an unregulated supply is exactly that - it's unregulated and with large value smoothing capacitors you can expect that off full-load that the capacitors can hold up the DC level as the AC supply transits through the zero-crossing point, and the effective average DC voltage will increase above the full-load value (but below the peak value).

Expect to lose 1.2-1.6V (typically) across a bridge rectifier. This affects the peak DC voltage directly.

When looking for variance between your measured vs theoretical, as already mentioned in the replies - consider the actual AC line voltage at the time of test, and the ratio of the primary and secondary windings (many transformers are rated with primaries at 220VAC - if fed with 240VAC then expect the secondaries to read 240/220 x the rated voltage).k

[m_c]

For a read of the how/why behind choosing a toroid based on steppers, have a look at my post in my Triac retrofit thread - http://www.mycncuk.com/threads/10344...5703#post85703

Although having just skimmed over the post, I've just realised I never mentioned about sizing the capacitor. It's a 100V 22'000uF from a reputable source.

[Tom J]

For a read of the how/why behind choosing a toroid based on steppers, have a look at my post in my Triac retrofit thread - http://www.mycncuk.com/threads/10344...5703#post85703

Although having just skimmed over the post, I've just realised I never mentioned about sizing the capacitor. It's a 100V 22'000uF from a reputable source.

That kind of answer I was looking for.
33VAC - 52VDC, big boost over calculation, any idea why was like that?

My both 420VA & 720VA has 30VAC and gave 41.5VDC (calibrated Fluke 87V and 1587)
Secondary coil was ~32VAC


picture shows Amp meter after calibration, voltage was set too

[Clive S]

My both 420VA & 720VA has 30VAC and gave 41.5VDC (calibrated Fluke 87V and 1587)
Primary coil was ~32VAC

Tom you are reading too much into all this. It has been done to death many many times on the forum.

Have a look at the quote and notice you are saying primary coil is 32 AC. The secondary would be the the 32 volt coil.
It is facts like these that can mess people up.

Also you are stating a 1.57 A load is that on the primary or secondary as they would be different.

All this can be very confusing for people seeing the thread title thinking this is the right way to pick a Tx.

[m_c]

You need to allow for mains voltage tolerance (230V -6/+10% or 216 to 253VAC), transformer load regulation (the toroid spec I just looked at was 5%), and a lower volt drop over the bridge rectifier when zero load.

So taking a 33VAC toroid, applying 10% more power (mains voltage here is at the higher end of tolerance) gives 36VAC (we'll round for simplicity) at rated load. Add 5% load regulation, which means we bump our basic unloaded voltage to 37.8AC.

Multiply by root 2 (or 1.41), gives us 53.3VDC. Take away a little bit volt drop for the bridge rectifier, and we have in the region of 52VDC.