I am an engineering student and i am curious to know about circuit designing. Recently i have searched about transformerless power supply to step down to 9 to 12 volts but i am unsatisfied with the results. Please help me what are the elements to use and what is the maximum output current we get. Please provide me full details by describing the advantages and disadvantages of this circuit.

 

This App note from Microchip should answer technical specifics:

http://ww1.microchip.com/downloads/e...tes/00954A.pdf

(I noticed some errors in the index in the PDF document: It refers to "Figure 2: Vout at Startup with 10KW Load". This should be "10KOhm Load". The error is repeated in the index for Figures 2,3,4,6,7,and 8. The actual document Figures are correct.)

These power supplies are generally only used for low currents (ie <50mA).

The biggest disadvantage is "safety", because of the direction connection of the output, and whatever circuit you are powering, directly to the power mains. I have used these, but it is critical that no part of the electrical circuit be accessable to the user. If you have never designed or worked with mains-voltage circuits, I would strongly recommend against these.

Ken

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The advantage is low cost and small size. Disadvantages include low current and no isolation from mains.

 

your question is not clear.
1. is it that you want to convert main voltage to a DC between 9 to 12V?

2. or is it a DC to Dc converter not directly linked to mains.

If it is Mains derived, i would suggest that it is risky and even if you are using it, later on you are likely to forget that it is not isolated and likely to get shocked , if not electrocuted???
If some one else is the user the risk is STILL MORE as he would never know?

Let us not get the habit of designing non-isolated circuits.
After all an SMPS with Ferrite transformer will be small and cute for your needs. I strongly suggest this method.

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Regards,
Sarma.

 

Just for argument sake, couldn't one say that a standard PC SMPS is a "non-isolated" power supply. There are inductor/caps/switching devices, but not true isolation from meaning of the word as one gets with a ppower supply mains input transformer.

Lefty

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No, s PC power supply is isolated.

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In the past it was hard to find small, cheap(!) transformers for low voltage, low current supplies. Disemboweling wall-warts was a hobby option. I've since found transformers the are at a reasonable cost, and don't take any more foot-print space than the large'ish capacitors in the transformerless supplies. Again, unless I really have control of the end use, I'd stay away from the "transformerless mains power supplies".

Ken

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"To invent, you need a good imagination and a pile of junk."
Thomas A. Edison (1847 - 1931)

 

Except that you have not taken into account of small modern powers supplies for all the electronic gadgets. They are required to work correctly from 90V~ 260V AC, without any user intervention.

That said, a transformerless power supply is very rare except on products with totally enclosed design because of its associated danger. A switching power supply is not considered as a "tranformerless" design.

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L.Chung

 

Exactly, most SMPS for small gadgets such as mobile phone chargers are small and efficient and very usefull for hobby projects. Leftyretro is incorrect in stating such PSU are not isolated as the feed back is optocoupled and the transformer is probably better isolated in terms of insulation than a 'normal' mains transformer. Transformerless power supplies are well suited for applications involving mains control such as dimmers etc.

 

I agree with the above opinion BUT with changed comments from my side.
Even the ferrite Transformer is an isolation,
How ever the galvanic coupling capacitor shown between the output common and the -ve of the rectified energy of public mains (230/110 as the case may be ) will be the reason for my comment. we observe it in many such power supplies including the "light weight Cell Chargers". an accidental touch on the cellphone while its battery is getting charged,will give a minute shck even if we are well insulated.

Probably bodies of most electrical appliances are expected to be grounded before they are fit for use. hence we don't feel the shock.

Thus I agree to some extent that most SMPSs can not be called isolated.

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Regards,
Sarma.

 

I don't! - at all

It's isolated in exactly the same way as a conventional mains transformer, with a transformer providing isolation between primary and secondary. The components connecting across the isolation barrier (where fitted) are there for safety reasons, and are also present in units with conventional transformers for exactly the same reasons.

It all depends if the appliance is a Class I or Class II device, SMPS or conventional transformer makes no difference.

You can have SMPS's which aren't isolated, but you can also have conventional ones that aren't either.

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I am quoting the galvanic capacitor as a reason for my opinion. i do remember the non-isolated SMPS designs like few designs used by PowerIntegtation Inc, (www.powerint.com)

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Regards,
Sarma.

 

But that doesn't affect the isolation at all, along with the resistor in parallel it's there to prevent static build-up on the metal parts of a Class II device - a Class II device with a normal transformer often uses the same technique. A Class I device, such as a cmputer, has all external metal earthed anyway, so you can't get a shock from it.

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http://www.winpicprog.co.uk

 

I had mentioned that aspect, and an extract is here
"Probably bodies of most electrical appliances are expected to be grounded before they are fit for use. hence we don't feel the shock."

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Regards,
Sarma.

 

If there is double insulation between the primary and secondary side of a power supply (e.g. wall wart) then the secondary doesn't need to be earthed and it's considered to be a class 2 device.

If there is only single insulation between the primary and secondary side of a power supply (e.g, computer power supply) then the secondary needs to be earthed and it's considered to be a class 1 device.

Now there are power supplies that have metal frames which need to be earthed and are considered to be class 1 devices but the secondary side isn't earthed (e.g. bench top power supply); this is only the case when the secondary has double insulation from the primary side.

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