Cap Drop VS Transformer power supply ?!

Hello,

I am designing a Electricity meter, for the power supply part both Transformer and Cap-drop power supply can be used.

i have seen both type products in different Meter manufacturers.
usually a cap drop power supply is lower price than transformer.

but why do some manufacturers use transformer then ?! i have seen this in a competitive market products that price is so important ...

what are technical differences between these two power supplies in life time or ... ?

which type usually gets more fail in function ?!

note the meter should work well for at least 20 years.

and any other reason which can encourage a designer to use transformers ... ?!

( for meter circuit something about 3.3V and 10-15mA is enough )

and my second question: in a chinese meter design, i saw they used 220V to 15V transformer then regulated it down to 3.3v by a zener diode !
what can be technical reason ?
according to IEC standard meter should function down to 80% of nominal voltage, so a lower output voltage such 7-9V can be still enough for 3.3V regulator safe working.
so the price of transformer can be less with lower voltage ( same current ).

Thanks & Regards.

When I belonged to a company that designed a 2 way AMR system back in the 90's they used a ceramic hybrid with hi-V SMD power resistors and 22V zener bridge off 240Vac /60Hz to charge a cap and provide enough power for the ISM radio that could broadcast any minute of the day on demand or by schedule. It must survive the meter arc gap voltage threshold of 6kV or under. 30mA max is doable.

A capacitor fed PSU would allow high voltage spikes through, and I have found to my cost that its the zener diode that gets blown up. For say a 6.KV spike and a .22MF feed cap, putting another cap to neutral of .22MF, would reduce it by 50% and reduce its source impedance by 50%, so you need a resistor as well. A transformer has an inherent LPF effect and what with core saturation provides a more reliable power source. It also has much better voltage regulation.
If you use a capacitor fed circuit with a heavy load that goes on and off (like a fan timer), it is advantageous to have a dummy load that is fed anti phase with the main signal (triac gate?), so as to keep the current consumption relatively stable (and not use a zener?)
Frank

Thanks all,

Do you have any idea why they used 220/15V transformer and then used zener to regulate voltage to 3.3V ?!
why didn't they use lower voltage transformer instead of 15V ? the higher voltage just makes much loss power on zener or the regulator !

and about life time ? which type usually has longer and stable life time ?

hm_fa_da said:

Thanks all,

Do you have any idea why they used 220/15V transformer and then used zener to regulate voltage to 3.3V ?!
why didn't they use lower voltage transformer instead of 15V ? the higher voltage just makes much loss power on zener or the regulator !

and about life time ? which type usually has longer and stable life time ?

Click to expand...

With a larger rectified voltage the RC time constant and thus the value of C can be reduced at the expense of more power wasted in R, but cheaper.

In that situation the unregulated discharge time is dependent on the load RL or current with Ic=C*dv/dt. The Zener must draw more current than the load does worst case , so when there is no load, so that it is always biased to offer regulation.