voltage dropper help

[aruna1]

Hi, I want to make a power supply that will reduce 230V AC voltage to 5V DC voltage. Do do this I want to use a buck converter. Easiest buck converter I can get is MC34063. it has input up to 40V.
what I want to know is how to reduce 230V rectified DC voltage to 30V DC voltage so I can use MC34063 to drop 30V to 5V.
I have added a diagram below. my question is to solve the middle box named as "voltage dropper". I'm looking for a simple circuit for this. since i dont know current requirement of the load I guess i cant use a resistive dropper. any idea would be great.

EDIT: cant use a transformer due to space constraints.

thank you

 

[dougy83]

You could use a buck converter. e.g. LNK304G POWER INTEGRATIONS|LNK304G|OFF LINE SWITCHER 120MA 170MA | element14 Australia
Depending on the output current you require, i.e. if <170mA, you may not even need the MC34063.

Note that the output voltage is not isolated from the mains.

Another way of dropping the AC voltage is to use a transformer, bridge and capacitor.

 

[aruna1]

You could use a buck converter. e.g. LNK304G POWER INTEGRATIONS|LNK304G|OFF LINE SWITCHER 120MA 170MA | element14 Australia
Depending on the output current you require, i.e. if <170mA, you may not even need the MC34063.

Note that the output voltage is not isolated from the mains.

Another way of dropping the AC voltage is to use a transformer, bridge and capacitor.

Hi
I'm trying to find a way to use MC34063 for this coz MC34063 is the only one I can find here.

and I dont want to use a transformer coz it makes the unit bulky. I want to finish it with minimum space requirement.

 

[dougy83]

You could try a capacitive power supply, e.g. https://www.electro-tech-online.com/custompdfs/2011/05/00954A-1.pdf

 

[aruna1]

@dougy83

Hi
capacitive power supplies can supply very low current right? so might not useful for this

 

[dougy83]

capacitive power supplies can supply very low current right? so might not useful for this

The current is proportional to the size of the capacitor.

Can you not just buy a premade converter for US$2.39: e,g. **broken link removed**

Another option is to make a circuit that only conducts when its input voltage is <30V - if you have this after the bridge rectifier, and before the filter cap, you can get the 30V DC (peak) from the 220V rectified AC.

 

[aruna1]

hi
I was thinking about MC34063 because it allows me to adjust output voltage. In future I might have to increase output voltage from 5V to 12V but if i buy something like that i will e stuck with 5V

 

[dougy83]

You can just adjust the output resistor divider in those things (and replace the capacitor with a higher voltage one) in up the output voltage.

 

[aruna1]

hmm
i checked a mobile charger circuit that gives 5V 500mA from 230V. it didnt has any resistive divider. output was simply taken from the transformer.

 

[aruna1]

how about this

I was running some simulations and thought this could do some thing. see below circuit.

230v to 7.5V transformer less. any ideas on this method? zener use is a 8.2V one.
you can see the voltage values on multimeters attached

 

[Nigel Goodwin]

I was running some simulations and thought this could do some thing. see below circuit.

230v to 7.5V transformer less. any ideas on this method? zener use is a 8.2V one.
you can see the voltage values on multimeters attached

Sorry, but a completely useless idea

It will be HUGE, use loads of electricity, and double as a toaster!.

You need a switchmode transformer based supply, small, efficient and provides mains isolation as well.

 

[dougy83]

hmm
i checked a mobile charger circuit that gives 5V 500mA from 230V. it didnt has any resistive divider. output was simply taken from the transformer.

The device I linked to is a SMPS; it has a SMPS controller IC, a tiny transformer & a normally they have an optocoupler providing feedback (related to the output voltage - which has either a regulator - e.g. TS431 - & resistor dividers or a zener diode) to the controller. You obviously checked the wrong sort of device.

Your regulator circuit won't work without a large heatsink and/or a cooling fan either - if you draw 747mA from that, the transistor has to dissipate >100W.

 

[aruna1]

@nigel and dougy
yep didnt thought that part.how stupid of me