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Resistive Power Supply

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Suraj143

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I need to build a resistive power supply to work from direct 230V AC power.This is for a PIC based lamp dimmer.
My load will be PIC & a 5mA gate drive TRIAC.

I want to calculate the resister value.Please Help me.
 

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MikeMl

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Most often done by using a capacitor in series with R1. Much less heat that way. Has to be a specially rated capacitor, though.
 

Suraj143

Active Member
Hi thanks for the input.

I want to get rid of capacitors, A good capacitor is very expensive & its bulky as well.

A resister is more reliable, only problem is getting a bit heat.But for a 10mA I doubt it wont generate heat much.
That is what i want to calculate the resister value & the wattage :)
 

MikeMl

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Here is my shot at it. Efficiency is abysmal; 3.6W in to get 45mW out, 99% of which winds up in R1. What would you rather have to buy, an X-rated capacitor, or a 10W resistor?

D1 needs to be 1000piV. C1 needs to a voltage rating of ~50V, higher if Rload is ever disconnected.

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alec_t

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Are you aware that, without an isolating transformer, your PIC circuit will be 'live' at mains potential?
 

Nigel Goodwin

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Hi thanks for the input.

I want to get rid of capacitors, A good capacitor is very expensive & its bulky as well.

A resister is more reliable, only problem is getting a bit heat.But for a 10mA I doubt it wont generate heat much.
That is what i want to calculate the resister value & the wattage :)
You've got it all wrong - the capacitor will be more reliable, probably no larger (as you need a BIG expensive resistor), and it won't be a 'bit heat', it will burn people and melt plastic.

However, as others have mentioned it makes the entire circuit live - you shouldn't mess with such designs unless you fully understand them.
 

unclejed613

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i remember my first attempt at working on "hot chassis" radios as a kid... line voltage through your arm is not a pleasant experience. "unsafe and cheap" is definitely something to avoid. the circuit in the OP has two major points of failure, the 22.5k resistor, and the zener diode. both of those will generate a lot of heat. using a capacitor will mitigate the bulk of the heat problem, but the zener will definitely still be a heat source. whatever device you are running on the DC side should have adequate insulation around it so nobody will touch any exposed metal on the device. such power supplies are simple and cheap, but very dangerous. i'm probably not the only one to call such devices "circuits of death". it's probably not as dangerous as a charged capacitor in a microwave oven, but it can kill you.
 
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