230v to 12v convertion

[rajesh1987]

i need to connect 5 small bulb parlely . rating of one bulb is 12-16 v and 5W. I need to connet it in normal house supply of 230v. tell me how choose the transformer rating for this using formula so that it will be useful for me to connect more bulbs by parallel. Also suggest me circuit to convert 230v to 12v dc with out transformer, so that it will be useful for me.


regards

 

[betwixt]

Unless you are absolutely sure that none of the wiring or bulbs can be touched by anyone, including the possibility of a bulb being changed when the power is turned on, do not consider a transformerless supply. They have no isolation between the load (bulbs) and the incoming AC so touching any part of the circuit could be lethal. A transformer provides a safety barrier and also drops the voltage reliably and efficiently.

The rating you want is 230V primary, 12V secondary and the VA rating is the total of all the light bulb wattage ratings. For example, for a single 5W bulb a 5VA transformer would be adequate, for two bulbs it should be 10VA and so on. It would be a good idea to use a transformer with a slightly higher VA rating than the bulbs to add a safety margin, a transformer with a rating higher than the total bulb ratings can be used but not one with a smaller rating.

Brian.

 

[rajesh1987]

how to calculate VA rating and current rating for bulbs of above mentioned ratings. tell me formula because i need to connect around 50 bulbs parallely.

regards

 

[betwixt]

I did.

the VA rating is the total of all the light bulb wattage ratings

1 bulb at 5W = 5VA
2 bulbs at 5W = 10VA
3 bulbs at 5W = 15VA
.
.
50 bulbs at 5W = 250VA

The current is the bulb power divided by it's voltage ( I = W/V ) so it's 5/12 = 0.42 Amps per bulb.

Brian.

 

[patelhirenh]

Watts to VA calculation
The apparent power S in volt-amps (VA) is equal to the real power P in watts (W), divided by the power factor PF:
S(VA) = P(W) / PF

VA=5/0.9
VA=5.5

VA=50/0.9
VA=55.55

 

[betwixt]

Where did the PF of 0.9 come from?

Brian.

 

[patelhirenh]

Tell me the equation of VA???

- - - Updated - - -

Visit below link for transformer less power supply may be useful.


**broken link removed**



**broken link removed**

 

[BradtheRad]

Are these incandescent bulbs? Is it satisfactory to string them in series?

You can assemble a string of 19 bulbs (based on each bulb needing 12 V). This does not require a transformer.

It will present a shock hazard (as post #2 states).

 

[betwixt]

@Patelhirenh, I can't open the first link but the second one is not transformerless, it just doesn't show it on the schematic. It needs a transformer to drop the mains to 19.5 V before using the schematic shown.
You already gave the formula for VA calculation but these lamps are essentially resistive so there is no phase difference between the voltage and current so the PF is 1.

Brian.

 

[patelhirenh]

find the attached circuit. but i found 30V 1A maximum



If we put P.F -1 then
VA=50/1
VA=50

so we required 50VA transformer. is it correct?

 

[betwixt]

That schematic is a controlled SCR rectifier, it WILL produce a fairly stable 30V DC and you can adjust the voltage by changing the values of the Zener diodes. You can also increase the current rating by using a larger SCR but please treat it with caution. It offers NO isolation from the incoming AC and should any component break down, the chances are it will produce over 300V at it's output. When I say you should use a transformer, I mean that if anyone touches any part of the circuit, including bulb holders with the bulbs removed it can kill them instantly, you do not get a second chance at life.

You should note that when that kind of schematic is used, there is normally an over-voltage detector connected to it's output which is designed to instantly blow fuses if the voltage starts to rise. This protects against component failure but still does not make it safe to touch. It should really have an interference filter at it's input as well, this kind of circuit produces considerable radio interference and in some countries is illegal because it draws asymetric current from the mains supply.

Brian.

 

[tpetar]

i need to connect 5 small bulb parlely . rating of one bulb is 12-16 v and 5W. I need to connet it in normal house supply of 230v. tell me how choose the transformer rating for this using formula so that it will be useful for me to connect more bulbs by parallel. Also suggest me circuit to convert 230v to 12v dc with out transformer, so that it will be useful for me.

You can extract small good SMPS from some cheap reliable adapter or to make small DIY SMPS, here is simple example :

http://danyk.cz/izled_en.html



Best regards,
Peter

 

[patelhirenh]

Right... step down transformer is the best option.

 

[tpetar]

SMPS of 5W at 12V have max around 0,42A, and this SMPS is very small, size is not bigger then dangerous transformeless design. Just open one phone charger adapter, which make 5V 0,5A from 220V/110V and see PCB size and weight, SMPS for 12V is the same size.

Small SMPS extracted from some wall adapter will be cheaper than to make transformeless psu. :smile:



Best regards,
Peter

:wink: