[SOLVED] Capacitive power supply... why?

[kripacharya]

A typical "capacitive" power supply is capable of at best 10's of mA only. The capacitors used are physically large due to need for high voltage operation. Are there not miniature transformers which can do the same ?

Such a capacitive supply has the inherent problem that it is not isolated from the mains, making the rest of the circuit a death trap... and yet it is designed for and used all the time.

Why is this ? Is the fractional cost advantage really that great ? Is it an availability issue ?

I don't get it.

 

[andre_luis]

It is dictated by consumer market which admits low cost, low quality equipments.
The voltage transformer is a representative part of the cost.

 

[schmitt trigger]

Me neither.

This is my theory.....
Capacitive supplies are mostly used for low power, which were traditionally served by small iron-core transformers.

But iron-core transformers have been mostly superseded by switchmode power supplies, and the manufacturers have stopped producing the smaller size magnetic steel lamination, and thus the small transformers have either become unavailable or expensive.

Unfortunately, for an electronics novice a mains-powered SMPS is beyond the reach of their technical capabilities. Therefore, they consider capacitive power supplies.

But, they attempt to extrapolate those circuits into multi-watt supplies, powering all sorts of stuff, and that is where the problems begin.

 

[FvM]

I can't confirm the consideration. Small transformer are available at catalog distributors starting with 0.35 VA nominal power. Up to 1 or 2 W, transformer supplies are smaller and cheaper than SMPS. Their main disadvantage is fixed input voltage.

"Capacitive supplies" are used inside home appliances that don't need isolation, or for cheap LED light bulbs.

 

[kripacharya]

... Their main disadvantage is fixed input voltage.

"Capacitive supplies" are used inside home appliances that don't need isolation, or for cheap LED light bulbs.

Fixed input voltage is within a range, based on further voltage regulation circuitry surely. But in a sense you are right, since a capacitive supply delivers it's full (comparatively very high) voltage to a regulator which then must "manage" it. So typically any voltage from 50v upto 400v can be managed, and claimed to be a 'feature'. Good point.

However i cannot imagine a single home appliance which does NOT require isolation. It's just that the burden of isolation is transferred to a different place.

For example - a client wants a 'touch sensitive' switch to operate a load of N watts, and powered by a capacitive supply. And there are wayyy too many product of this nature.

Would you recommend a capacitive supply ? And specify for heavy isolation on the actual 'touch plate' ?

 

[dick_freebird]

I'm sure that in some way, or all ways, it's about cost.
Magnetics seem to be stuck with more machine- or
labor-intensive manufacturing than capacitors. And
especially if you're asking for isolation, let alone a
standards-compliant one with certifications, you can
expect that to cost you. If a pair of capacitors costs
less than a transformer, some person up the chain will
decide that's all that matters.

 

[andre_luis]

However i cannot imagine a single home appliance which does NOT require isolation. ?

I have a washing machine in home, made from a local manufacturer presumably reputable, which employes such capacitive-based power supply. The electrical isolation is accomplished by long rods connected to the front panel and the end result turns out to be the same in terms of personal safety.

 

[schmitt trigger]

Andre;
this is similar to the good old days where cheap "transformerless" vacuum-tube radios were sold.

The "hot" metal chassis would be enclosed in a wood cabinet, and plastic buttons were used for all its functions.

Kripcharya;
I also saw a modern, direct line powered device which used touch buttons. But the touch button's electrode was separated (and insulated) from the plastic overlay with a small glass window.

So...it can be done. But the question begs, does the cost of the isolation exceed any savings from the transformerless supply?

 

[FvM]

So...it can be done. But the question begs, does the cost of the isolation exceed any savings from the transformerless supply?

In case of home appliances, we usually have control elements (switches, displays, lamps) with sufficient isolation which is e.g recommended for ESD strength, so it doesn't involve extra costs. I don't know how common capacitive power supplies are in recent home appliances, like andre_teprom I noticed that they are used in a number of devices.

I didn't think particularly about touch switches, but they can be isolated with small "Y" capacitors (certified for enhanced insulation at mains voltage level). So I see that capacitive supplies can be used in this field.

Schmitt trigger mentioned the "historical" tube radios with non-isolated supply. Also tube TVs up to the early 70s have been designed this way. The need for video interfaces and lower supply voltages for the transistorized circuit parts with higher power consumption made an end to this technique.