Transformer RMS value.

[davelectronic]

Finding it hard to get a definitive answers to this. I'm needing a high current transformer (100 VA plus) I can get 12 Volts RMS units reasonably priced, from previous unit they come out at about 13 Volts AC.

So going on the rectified and filtered value, could i achive a regulated 12 Volt output ? So worst case senario ( my working out) 12 Volts x 1.41 = 16.92 Volts peak value. Less 1 Volt for the bridge rectifier, less 3 Volts for the regulators overhead. As 12.92 Volts could i obtain a 12.20 - 12.50 Volt guarantee  output regulated voltage ?

I can get 15 Volt and 18 Volts transformers, but i only need 12 Volts final regulated output, and the higher voltage transformers are much more expensive for the same VA. So do you think the above is workable, or is the peak voltage just to low to get a 12 Volt regulated output.

Thanks for reading. Any advise appreciated.

[RoGeorge]

You need to take into account the ripple after filtering the rectified voltage, so you also need to know the maximum current drawn from the 12V regulated.

[Ian.M]

You also have to allow for the drop at full load.   If the transformer has 10% regulation and is 13V RMS at no load, it will be 11.7V at full load.

[snarkysparky]

I don't think you can depend on this to deliver 12V at full current through a regulator with 3V dropout.  It's just too close.

You can get another transformer with a 3V output and series this with the 12V one provided the phasing is correct.

[Ian.M]

Not a hope in hell.  Assuming 10% regulation, 13V RMS unloaded, and using the full wave bridge rectifier, capacitive load derating factor^* of 0.62, and the transformer's 100VA 12V rating to determine the maximum load current (5.16A), it needs at least 11,000uF of reservoir capacitance and a 25A Schottky diode bridge to keep the unreg rail above 12V.   Even the 15V RMS transformer is going to be marginal with 3V drop in the regulator.

* see: http://www.hammondmfg.com/pdf/5c007.pdf

[mariush]

You can use an ideal diode bridge controller like LT4320 (along with 4 mosfets) to convert your AC voltage to DC without losses in diodes of bridge rectifier (because you don't have diodes anymore).  So instead of losing about 0.6-0.8v per diode, or about 1.6v, you'll only lose about 0.1-0.2v (depending on your mosfet choice).
See LT4320 here: http://cds.linear.com/docs/en/datasheet/4320fb.pdf
The chip costs about 6 uk pounds in one piece, about 3.5-4 uk pounds if you buy at least 25 : http://uk.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=44&storeId=10151&categoryName=Semiconductors%20-%20ICs&selectedCategoryId=700000004166&gs=true&st=LT4320
The four mosfets would probably add at least half a pound to the cost.

So even if your transformer outputs only 11.5v AC at maximum usage, after rectification you'd have a peak DC voltage of about 11.5v x 1.414 - ~ 0.2v = ~16v
At 100VA and 12v AC you're looking at about 8.3 A ac , but using  the factor 0.62 we get only about 5.16A DC max ... let's just go with 5A peak.

So let's look at what we could use for linear regulator that takes in up to about 20v DC and can output 5A or more ...

* 7.5 gbp :  MIC29502WU  LDO Voltage Regulator, Adjustable, 2.5V to 26V in, 370mV drop, 1.25V to 26V/5A out, TO-263-5   
* 6.2 gbp : MIC29502WT  LDO Voltage Regulator, Adjustable, 2.5V to 26V input, 370 mV drop., 1.25V to 26V/5A out, TO-220-5
* 5.5 gbp : MIC29512WT  LDO Voltage Regulator, Adjustable, 2.4V to 16V in, 700mV drop, 1.25V to 16V/5A out, TO-220-5
* 2.28 gbp : LM1084IT-ADJ/NOPB  LDO Voltage Regulator, Adjustable, 2.6V to 27V in, 1.3V drop, 1.25V to 27.5V/1.5A out, TO-220-3
* 2.04 gbp : LM1084IS-ADJ/NOPB  LDO Voltage Regulator, Adjustable, 2.6V to 27V in, 1.3V drop, 1.25V to 27.5V/1.5A out, TO-263-3

So you either add capacitors to guarantee at least 12.4v..12.7v and use an expensive 5 uk pounds or more linear regulator, or you can use more capacitors to guarantee at least 13.5v DC but save on the linear regulator

C = I / [2 x mains frequency x (Vdc peak - Vdc min)]

= 5/100 *  (16-12.7) = 5/330 = 0.01515 Farads  or 15150 uF (so maybe 3 x 5600-6800 uF / 25v capacitors)
= 5/ 100 * (16-13.5) =  5/250 = 0.02 Farads or 20000 uF

So it's doable, but it's definitely much easier to just use a 15v AC rated transformer instead of a 12v AC transformer , as you won't spend 6 pounds on ideal bridge controller and mosfets... may as well spend those pounds on bigger transformer, unless you really don't want heat generated in the bridge rectifier or you have issues of product weight or volume (bigger transformer, more weight)

[Ian.M]

You've forgotten the mains voltage tolerance.  UK is 230V +10%,-6%, although the actual voltage in urban areas tends to be very close to the old pre-harmonisation 240V +/-6% standard.  However if you are getting 13V RMS for 240V in, at the lower legal limit of 216.2V, you'll only get 11.7V, which is likely to drop to 10.5V at full load.  I ran the numbers with ideal diodes, and for 20mF reservoir capacitance, got a peak of 14V and a trough of 12V for the rectified output, so there isn't enough headroom even for a good LDO

@O.P. - Why cant you use a switching supply?

[davelectronic]

Thanks to all for taking the time to reply here.
I had a feeling it was to close to be doable, i dont really want to get into fet's for this one, i can see there's just not enough overhead to manage it at 12 Volts RMS. Dam, its so bloody more expensive for 15 - 18 VA transformers. I won't even bother gambling on a chance..
So its either take the hit £££ Or wire up two in series and except a slight current drop. I can get a 100VA transformer at 24 Volts RMS, but then the heat creeps in with such a large differential.

[RoGeorge]

Then use a 24V RMS and a Buck DC-DC converter to avoid the heat dissipation. You still didn't tell what current you need for the 12V.

[davelectronic]

Yes sorry about that, about 10 Amps maximum. But typically around 8 Amps 50%  duty cycle. Noise would, or might be an issue with a converter.

I overlooked a really cheap option, shunts in or out, in i think to keep heat down. The MOT. Just watched this, i know its rough homebrew, but i was taken by his MOT rewind. Its a bit long, but i watched it. Its only an option to rewind a MOT at this time, i know all there  inefficiencies. Just a thought.
https://youtu.be/m-y0Cjx7KeE

[cs.dk]

If you just need regulated 12V and around 10A, the easiest and cheapest may be an off the shelf ATX PSU.

[davelectronic]

I've got usable ATX converted power supplys, quit a few come to think of it. Even some converted server units as well.

I'm trying to follow a linear scratch built idea for the moment, but thanks for the ATX idea, some make sound power supplys.

Going back to the original 12 Volt transformer ! It was mentioned peak load of 11.70 Volts RMS under load conditions. I've worked out i could take a hit down to a regulated voltage of 11.40 Volts minimum. So if i set an LM317K up with fixed resistors for bang on 12.00 Volts Regulated no load, and could afford the drop to 11.40 Volts...

With a very large, say 40000uf filter capacitor value, would the above be achievable with that 12 Volt transformer ? Unloaded this transformer comes out at just over 13 Volts RMS. Thoughts appreciated.

So say 11.70 Volts x 1.41, so 16.49 Volts, less 1.6 Volts for the bridge rectifier, and 2 Volts (Where I've read up) for the regulators head room.  So no load of 12.89 Volts. Is it achievable to set fixed value resistors for exactly 12.00 Volts ?

[Ian.M]

A LM317K is only a 1.5A regulator.  If you run the LTSPICE sim I posted earlier using silicon diodes (e.g. MUR460), and changing the load current to 1.5A, you'll find you have 2.35V headroom, which should meet your constraints if you can tolerate some ripple breakthrough on the output.  However if you use high current Schottky diodes you'll get >2.5V headroom with only 5000uF capacitance, and no dropout.

[davelectronic]

I'm planing on more current with a series pass transistor. The transformer is rated for 8 Amps, I'm looking for 5 Amps maximum, although 3 - 4 Amps would be closer to my needs. Given a 12 Volts RMS Voltage, that measures just over 13 Volts RMS, what Voltage would you set the LM317K up for ? Is 12 Volts set up with fixed value resistors a doable option ? I was thinking R1 120 ohms, and R2 1033 ohms. That's as close to 12 Volts i can get with fixed value resistors.

My maximum current would not exeed 5 Amps, and the transformer is rated at 12 Volts RMS at 8 Amps. But it does measure around 13.60 Volts RMS. What guaranteed regulated voltage would you aim for given those constraints ? I can operate the load at a minimum of 11.40 Volts. Could i achieve a no load Voltage of 12 Volts ? Is a shottky diode bridge an option worth thinking about.

[mariush]

The *1084 linear regulators have a dropout voltage of 1.3v at 5A (they're made by various) and they're not that expensive. At less than 5A the dropout voltage drops a bit.


At less maximum current (let's say 3A) you have quite a few options (more models of linear regulators with dropout voltage below 1v) ..
for example SC1592, LT1764, MIC29300 to MIC29303 (some models may be 16v max input voltage) , these are all with dropout voltage below 0.5v
for less than 1v you have for example LT1185 with about 0.7v dropout voltage

and i've only looked at Farnell in the LDO section, Digikey has other manufacturers and more chips to choose from

Anyway, the point is that most of these linear regulators can be connected in parallel, with a suitable resistor between their outputs to balance them ( in the datasheet for LT108* at page 15,  Linear recommends using a meter or so of AWG #18 wire as resistor, that's about 0.02 ohms  - instead of wire you could just use a proper resistor).
So if you can't find a cheap 5A linear regulator with less than 1v dropout voltage, you could opt for two 3A linear regulators in parallel that work with less than 1v dropout voltage.

I've looked through all your posts and you don't seem to post exactly how much current you want... so I can only give you general advice.

also..  for 3-5A of current you may be able to find separate Schottky diodes with low forward voltages in the range of 0.3-0.5v, so the voltage drop on a custom bridge rectifier could be less than 1.6v

ex
0.8 uk pounds each VISHAY  VBT4045BP-E3/8W has ~ 0.3v forward voltage at 10A , ~ 0.4v at 20A
0.7 uk pounds each  STPS20120D  Schottky Rectifier, 120 V, 20 A,  will give you ~ 0.6v at 10a, about 0.5v at around 3A
0.38 uk pounds NRVTS1545EMFST1G  Schottky Rectifier, 45 V, 15 A,   less than 0.4v at 10a (cheap but a bit harder to solder and would be harder to dissipate heat from them, compared to to-220 models that could be put on heatsink.

and many others.

Don't look just at diodes rated for peak current a bit higher than your maximum, and don't filter results based on the forward voltage.. because for example you may find a diode with forward voltage of 0.8v at 25-40A but the forward voltage would be much lower at 0.4-0.5v or even lower at 5-10A you may have.
You have a good example above, where the 40A diode is listed with ~ 0.67v forward voltage drop on the site, but in datasheet if you check the graph you can see it has much less, about 0.3v at 10A.

So if I had filtered to see only diodes with forward voltage less than 0.5v I would have missed this one.

also another note... if you could go with switching power supplies, there's also laptop adapters that output 12v (these are usually rated for 65w) and there's also adapters like xbox power supplies which output 12v and something like 120-160 watts (i don't know the exact figure).

Laptop adapters have the advantage of simplicity, passive cooling, already having wires and end connector that's convenient, no need to short ps-on wire to ground to turn them on , no 5v standby circuit always sucking power etc etc

There's also tiny ac-dc converters available at electronic distributors like Digikey and Farnell and others which are cheaper than atx power supplies and have either fixed 12v or adjustable output.
 
ps.   The majority of linear regulators won't just quit outputting something if the input voltage drops below the minimum the regulator wants. So you can configure the LDO or LDOs to 12v (or use a fixed output 12v regulator) and if it happens that input voltage is less than desired, the regulators will simply output less than 12v - the regulator will constantly try to reach the configured 12v but will keep working and give your circuit less than 12v.

[davelectronic]

The *1084 linear regulators have a dropout voltage of 1.3v at 5A (they're made by various) and they're not that expensive. At less than 5A the dropout voltage drops a bit.


At less maximum current (let's say 3A) you have quite a few options (more models of linear regulators with dropout voltage below 1v) ..
for example SC1592, LT1764, MIC29300 to MIC29303 (some models may be 16v max input voltage) , these are all with dropout voltage below 0.5v
for less than 1v you have for example LT1185 with about 0.7v dropout voltage

and i've only looked at Farnell in the LDO section, Digikey has other manufacturers and more chips to choose from

Anyway, the point is that most of these linear regulators can be connected in parallel, with a suitable resistor between their outputs to balance them ( in the datasheet for LT108* at page 15,  Linear recommends using a meter or so of AWG #18 wire as resistor, that's about 0.02 ohms  - instead of wire you could just use a proper resistor).
So if you can't find a cheap 5A linear regulator with less than 1v dropout voltage, you could opt for two 3A linear regulators in parallel that work with less than 1v dropout voltage.

I've looked through all your posts and you don't seem to post exactly how much current you want... so I can only give you general advice.

also..  for 3-5A of current you may be able to find separate Schottky diodes with low forward voltages in the range of 0.3-0.5v, so the voltage drop on a custom bridge rectifier could be less than 1.6v

ex
0.8 uk pounds each VISHAY  VBT4045BP-E3/8W has ~ 0.3v forward voltage at 10A , ~ 0.4v at 20A
0.7 uk pounds each  STPS20120D  Schottky Rectifier, 120 V, 20 A,  will give you ~ 0.6v at 10a, about 0.5v at around 3A
0.38 uk pounds NRVTS1545EMFST1G  Schottky Rectifier, 45 V, 15 A,   less than 0.4v at 10a (cheap but a bit harder to solder and would be harder to dissipate heat from them, compared to to-220 models that could be put on heatsink.

and many others.

Don't look just at diodes rated for peak current a bit higher than your maximum, and don't filter results based on the forward voltage.. because for example you may find a diode with forward voltage of 0.8v at 25-40A but the forward voltage would be much lower at 0.4-0.5v or even lower at 5-10A you may have.
You have a good example above, where the 40A diode is listed with ~ 0.67v forward voltage drop on the site, but in datasheet if you check the graph you can see it has much less, about 0.3v at 10A.

So if I had filtered to see only diodes with forward voltage less than 0.5v I would have missed this one.

also another note... if you could go with switching power supplies, there's also laptop adapters that output 12v (these are usually rated for 65w) and there's also adapters like xbox power supplies which output 12v and something like 120-160 watts (i don't know the exact figure).

Laptop adapters have the advantage of simplicity, passive cooling, already having wires and end connector that's convenient, no need to short ps-on wire to ground to turn them on , no 5v standby circuit always sucking power etc etc

There's also tiny ac-dc converters available at electronic distributors like Digikey and Farnell and others which are cheaper than atx power supplies and have either fixed 12v or adjustable output.
 
ps.   The majority of linear regulators won't just quit outputting something if the input voltage drops below the minimum the regulator wants. So you can configure the LDO or LDOs to 12v (or use a fixed output 12v regulator) and if it happens that input voltage is less than desired, the regulators will simply output less than 12v - the regulator will constantly try to reach the configured 12v but will keep working and give your circuit less than 12v.

Thank you for all the information, i will take a good look at alternatives as well.

[davelectronic]

Well i bit the bullet and bought a 300VA toroidal transformer, twin 115V primarys and twin 15V secondarys. Measured its no load voltage at 16.90 Volts RMS. Outputs of the secondarys in parallel for up to 20 Amps.
Maplins UK. £35 Not to bad i suppose. I might rewind a MOT if one comes up, just for the hell of it.