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voltage and current reading

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Advanced Member level 4
Sep 4, 2005
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i've connected a 230 v powersupply through a triac (bta 41) to a circuit
i would like to change the firing angle according to the current or voltage variations to give the constant power to the can i find the variations of voltage and current .schematics also welcome


the schtematic given is only a block digram
for this purpose u can use feed back amplifier for given the constant input.
at load should have to connect the volt meter and make your desire output as a 0 volt so the variations occurs u should read directly, as same for the ampere meter.

my application is that
i will first make the triac close for some time
all the i/p voltage will be applied to load(230 v)
now i want to chop can i find out the firing angle/time period?
also i would like to varyfy the changes in current when i am choping 20v
i can use microcontrollers to read the values and set the time period
but what's the way to find them?

dear narccizzo
it's almost like that
bit i am not taking any data from the load
it's just like you will connect a remp sensor to the micro controller
when the temp is lesser,i will change the firing angle of the triac
i think 230 v means the rms value of the sine wave
if i want to give 210 v to the load,i have to remove 20 v
by changing the firing angle of the can i calculate the same.
ie out of 360 how much should i chop?
can you suggest a circuit by which i can measure the current.i think i can use a resisror bridge to ger the same(wheinstone bridge)
my system can measure up to 4v(the current will be converted in to voltage)
what should me the value of resistors that i should use
i am not so good in electrical parameters and calculations

thanking you


For current sensing with 4volt FSD it is better to use current transformer.
Then rectify its output and put one load resistor (paraleld with capacitor) such that when current reach to maximum value output voltage become 4Volt.

Sorry for the late..
Ok, the equation 1 of the picture is for a sinuidal wave where you can especify the time of trigger and you get the Vrms, so you cannot solve for t, you must make a table or a graph for that equation and get the angle that you need to get the desired voltage from there.
hope this helps, if this is not really usefull we are here to help us together..
best regards..

dear narccizzo
thank you for your mail
i am very weak in electrical calculations.that's why i'm going to ask you some more questions
i 've connected my microcontroller to controll the triac which is in serial with the load
i want to chop 20v out of 230v input.i must do this for bith the +ve and -ve the net o/p will be 210v.then i will give a delay and again chop 5 more
ie the output will be 105 v.this process i will continue until o/p=180v
the time period of a cycle is 20 i want to know what will be the time i should open the triac for choping the,25v,30v,35v,40v,45v and 50v
i am also trying to workout from your equations.if it's easy for you,pls send me the timings.i want it only for the +ve half cycle.the zero crossing and others i can findout with my circuit.
thanks again for your reply

mar narccizzo,
thank you for your was really great.
i am working on that
still there is a doubt
the maximum voltage you have specified is 218(rms)
it can vary from 180 to 230v
that i will measure with my circuit
if thevoltage varies,the triggering time period also will vary
what can i do then?
is there any equation so that i can find the triggering time?
with different rms voltages with the same time frequency? (50hz per second)
as i told you i am very weak in ac calculations
sorry for the disturbance if anything such caused by me.but i think i can learn more only after consulting peole like you
thank you very much


the table was made with the equation that I gave you before, as i told you you cannot find the triggering time for a specific Vrms, thats because i dont know how maybe it can if there is a some way to solve for t in the equation. I made a little mistake (sorry), these are the correct values for 50Hz..
as you can see the minimum value Vrms that you can get is 0 when you wait aprox 9.96ms after you detect the cross by zero, it must be exactly 10ms its like the triac goes off just when you want to trigger it, and you have the maximum voltage when you wait 0.0ms after you detect the zero cross, thats mean bla, bla bla, i think you should know.
if you want to calculate in the uC the triggering with a temperature measured value i think you should use a table conversion, because if there is a way to calculate that, i dont know it, this is not my speciality, this is only part of my power electronics basic knowledge.
if you finish your project, i'd like to make me know, or even if you have more doubts maybe i can help.
good luck..

hai narccizzo

i've tried this
but i couldm't find the value .
what's the value of E?
could you pls solve one equation so that i will come to know how to derive it from the equation

thank you


dear narccizzo
thank you for your reply
but i couldn't make it
could you pls give me the tine in milli seconds for the below given

1) 1.95E-03
2) 2.31E-03
3) 2.57E-03
4) 2.83E-03
5) 3.06E-03
6) 3.29E-03
7) 3.48E-03

hope tat u r not feeling bad
i think that this will solve my problem

once again thanke for your co-operation


1.95E-03 = 1.95millisec
2.31E-03 = 2.31millisec
2.57E-03 = 2.57millisec
2.83E-03 = 2.83millisec
1.95E-04 = 0.195ms
1.95E-05= 0.0195ms

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