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Transformers

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unrealdave
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Hi

I have decided to build a 6 volt, 2 amp min LT supply as a standalone unit. I picked up some transformers – the small one is 27.7 volts and 80va which means it will give me just under 3 amps. I have a voltage regulator that will handle 2 amps

http://www.ebay.co.uk/itm/321053982164? ... 1497.l2648

Could anyone help with some questions –

1. as the small one is 27.7 volts ac - over 4 times the 6 volts I need will it be wasting a load of power – i.e. will it cost me 4 times as much to run in electricity costs than if I got a 6 volt ac transformer and used that?

2. Also would I need a large heat sink on the regulator as its changing 27.7 v dc into 6 v dc?

3. Also would the amps it can give drop in line with the voltage the regulator is reducing?

4. The large transformer states ‘FLAME MACHINE’ P/N EMT-C01502, INPUT 240 OUTPUT 0-11.2. There is no mention of amps or VA but is much heavier than the other one. Is there any way to test the thing other than loading it till it heats up?

5. Even though it is bigger would it be cheaper to run than the small one as it would not need such a large reduction in voltage by the regulator?

I do appreciate any help given.

Dave

Maturity is overrated.

 
Posted : 09/02/2013 12:16 pm
peterscott
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Hi Dave,

The big one will be more suitable. If you plan on drawing 2 amps then you need to dissipate the extra voltage headroom somewhere. If you used the 27 volt transformer then you'd need to get rid of 40 watts somewhere. Your regulator certainly won't deal with that even if very well heat sinked. The big transformer will give you 10 watts to dispose of. You could just drop in a beefy series resistor on the way to your regulator.

Clearly you are better wasting 10 watts than 40 watts.

Peter

www.nostalgiatech.co.uk

 
Posted : 09/02/2013 12:30 pm
unrealdave
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cheers for that - how did you get 10 watts and 40 watts?

Maturity is overrated.

 
Posted : 09/02/2013 12:34 pm
peterscott
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Just approximate calculations:

Big transformer 11volts - 6volts = 5 volts headroom that you don't need so with the 2amps you plan to draw you need to dissipate 5 x 2 =10 watts, somewhere either in the regulator (with a big heat sink) or in a >10watt series resistor.

Likewise for the smaller transformer you have 27volts - 6volts = 21 volts headroom.
21 x 2 = 42 watts.

Peter

www.nostalgiatech.co.uk

 
Posted : 09/02/2013 12:42 pm
unrealdave
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Got that :D

thanks

Maturity is overrated.

 
Posted : 09/02/2013 12:50 pm
Valvebloke
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It might be even worse than this. If you were to use a full-wave rectifier then you'd get a DC output of 1.4 times the AC voltage - so about 40V for the small transformer. If you only want 6V then you'd need to dump 34V somewhere. At 2A that would be nearly 70W. What you would actually be doing would be building a small room heater which, as a side-effect, would generate 6V DC. You'd also have to check that your regulator could stand 40V in. It might not. So I think with that transformer you'd be forced to half-wave rectify.

Even the big transformer gives you a bit too much voltage. If you really want to do something useful with the small one you could connect the mains to its primary and then wire it's secondary in series with the mains supply to the primary of the big transformer. If you connect it the right way round it will subtract 28V or so from the mains and the big transformer will now deliver about 9.8V instead of 11V.

But best would probably be to sell both these transformers and use the money to buy the one you want - say 9V or so.

Whatever you do I don't think you have to worry about the cost. Even if you were to use the full 80VA capability of the small transformer it should still only cost you about a penny an hour to run.

VB

AmpRegen http://www.ampregen.com

 
Posted : 09/02/2013 1:02 pm
Valvebloke
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Too many edits caused a double-post.

AmpRegen http://www.ampregen.com

 
Posted : 09/02/2013 1:04 pm
Anonymous
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half wave gives same peak voltage as fullwave, so potentially x2 the average current in the rectifier.

For 6.3V LT, the simplest solution is a 6.3V AC transformer.

For DC, a bridge + 10,000uF and 2.5 Ohm load is average of about 6V and 800mV ripple.
At 12 Ohms load the ripple reduces to about 250mV and average volts increases to 6.75

Using 9V AC, Bridge and 6,800uF and 2.5 Ohm load the minimum volts is about 7.2V and max about 10.5, average about 9V, so an LDO (Low Drop Out Regulator) will work for 6.3V DC out.

If a 10,000 capacitor is used and load isn't as low as 2.5 Ohms, say 3.15 Ohms (2A), then minimum volts is 8.5V so a normal 5V 2A regulator with 1.2V Red LED fed by 680 Ohms from Regulator in and the Regulator OV pin connected to LED (decoupled with 220uF and 100nF) should be close enough to 6.3V.

Warning some Red LEDs are 2V or more.

Add a 100nF close to input and also a 2nd one close to output pin of regulator. 5V 2A regulators are cheap.
15V rated electrolytics.

At 2A the regulator needs to dissipate about 7W.

I've frequently used Red LED and diode in series for 12V 5A regulator's 0V to get 13.6V approx for gear designed to run of car battery (Transmitters lose some power at 12V vs 13.6).

 
Posted : 09/02/2013 4:32 pm
boyblue
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Toroidal transformers usually have the secondary wound on the outside so its easy to unwind as many turns as you need to reduce the voltage as far as you need. I hate to see regulators running hotter than needed.

Peter

 
Posted : 09/02/2013 4:58 pm
Anonymous
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From a data sheet;
"Note 2: These specifications are applicable for power dissipations up to 50W for the TO-3 (K) package and 25W for the TO-220 (T) package. Power dissipation is guaranteed at these values up to 15V input-output differential. Above 15V differential, power dissipation will be limited by internal protection circuitry. All limits (i.e., the numbers in the Min. and Max. columns) are guaranteed to National’s AOQL (Average Outgoing Quality Level)."

Make sure you have a good thermal contact between the device and heat sink. The T03 case ones are better.

Mike

 
Posted : 09/02/2013 5:19 pm
Anonymous
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Toroidal transformers usually have the secondary wound on the outside so its easy to unwind as many turns as you need to reduce the voltage as far as you need. I hate to see regulators running hotter than needed.

Peter

Yes, but a 2A 9V transformer isn't expensive.

Those are lovely transformers and more expensive.

 
Posted : 09/02/2013 5:23 pm
unrealdave
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Toroidal transformers usually have the secondary wound on the outside so its easy to unwind as many turns as you need to reduce the voltage as far as you need. I hate to see regulators running hotter than needed.

Peter

That is an interesting point - good to know for future reference :)

Maturity is overrated.

 
Posted : 09/02/2013 6:20 pm
unrealdave
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I've picked up a bit here so thanks to all

Only thing that remains is if the amps or VA is not given on the transformer is there a way to find out?

Maturity is overrated.

 
Posted : 09/02/2013 6:22 pm
Anonymous
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You can go roughly by size / weight and similar catalogue part of known wattage. Less accurate at higher powers. I.e. a 5W and 10W transformer are very much different in size than some 500W and 1kW models.

Higher power transformers are more tolerant of short term overloads too.

 
Posted : 09/02/2013 6:46 pm
unrealdave
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You can go roughly by size / weight and similar catalogue part of known wattage. Less accurate at higher powers. I.e. a 5W and 10W transformer are very much different in size than some 500W and 1kW models.

Higher power transformers are more tolerant of short term overloads too.

There's no wattage on the transformer to go by

Maturity is overrated.

 
Posted : 10/02/2013 12:10 am
Anonymous
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yes. So measure the SIZE and WEIGHT and compare with figures in catalogues.
The voltage isn't important.

The parts in catalogues DO have SIZE, WEIGHT and VA.

 
Posted : 10/02/2013 12:45 am
Refugee
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I would expect to get about 10 amps out of that big transformer going by the size of the smaller one that is quoted at 80VA next to it.
I had a good look at regulators and heat years ago after fitting a bigger capacitor upstream of one.
It got very hot and it took me a while to sort out what was going on.
The larger capacitor was filling in the gaps between the ripples and this lead to the regulator getting very hot bhbg
The silicon gain in regulators in ample to deal with any ripple that is a volt above the drop out rating given in the data sheets. More smoothing will give the regulator a hard time due to heat.
They have to dissipate more heat in the troughs between the ripples if the capacitor "fills them in".

 
Posted : 10/02/2013 2:10 am
unrealdave
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yes. So measure the SIZE and WEIGHT and compare with figures in catalogues.
The voltage isn't important.

The parts in catalogues DO have SIZE, WEIGHT and VA.

OK - will do

Maturity is overrated.

 
Posted : 10/02/2013 10:52 am
Anonymous
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I had a good look at regulators and heat years ago after fitting a bigger capacitor upstream of one.
It got very hot and it took me a while to sort out what was going on.
The larger capacitor was filling in the gaps between the ripples and this lead to the regulator getting very hot bhbg
The silicon gain in regulators in ample to deal with any ripple that is a volt above the drop out rating given in the data sheets. More smoothing will give the regulator a hard time due to heat.
They have to dissipate more heat in the troughs between the ripples if the capacitor "fills them in".

Yes, you first decide what the maximum load is and voltage, see what the minimum voltage between in and out on regulator is and then pick correct nearest AC voltage and then the size of cap.

The bridge drops about 1.8V as it's two diodes in series on each 1/2 cycle and only conducts at peaks. So larger the capacitor the higher the pulse current is in the bridge too and greater copper losses in transformer.

So at low currents you can actually use a 12 AC transformer for 12V regulator. But at higher currents you have to use 15 V AC transformer for 12V and it may save you on lower current PSU by letting you use a much smaller capacitor.

This is faster and easier to use than Spice for verifying your design
http://www.duncanamps.com/psud2/index.html
It doesn't actually design a PSU, just tells you if it makes sense.

You do a design "experimentally" by starting with what seems like reasonable values and adjusting them.
You put a resistor to simulate the load + regulator in series and "design" so bottom of ripple is 1V above minimum voltage the regulator needs.

At high currents and low voltages a pair of diodes and CT transformer winding means there is 1/2 the losses in the Rectifier bridge. So a really high power 3.3V or 5V (even in SMPSU) will use tapped winding and two diodes rather than 4 diode bridge and single winding.

Free better software (Spice) but needs more expertise and suitable models
http://www.linear.com/designtools/software/

 
Posted : 10/02/2013 11:13 am
Refugee
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The bridge drops about 1.8V as it's two diodes in series on each 1/2 cycle and only conducts at peaks. So larger the capacitor the higher the pulse current is in the bridge too and greater copper losses in transformer.

So at low currents you can actually use a 12 AC transformer for 12V regulator. But at higher currents you have to use 15 V AC transformer for 12V and it may save you on lower current PSU by letting you use a much smaller capacitor.

The copper losses apply more to squeezing a lot of power out of a smaller transformer.
I have a home brew project on my to do list that is looking like it needs a choke in the feed from the rectifier to tame the peak currant. Doing this on the output of the rectifier makes use of the 100Hz ripple and reduces the size of the choke required.
Whom wants a huge transformer in the ceiling running AM friendly LED lights?

 
Posted : 10/02/2013 12:28 pm
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