Quick Circuit Help

0 6

Reply | New topic

#1 21667166 19 Mar 2013 06:15

George Capka George Capka

Anonymous

Post #1
21667166 19 Mar 2013 06:15

Hello guys… given the circuit below.. what is the voltage across the 1K resistor?
The input to the capacitor is AC 250 volts @60+ Hz……thanks for the help!!!
ADVERTISEMENT
#2 21667167 19 Mar 2013 07:20

David Adams David Adams

Anonymous

Post #2
21667167 19 Mar 2013 07:20

Without seeing the rest of the circuit and 250VAC applied to the cap - you ought to have about 227VAC which ought to cause the resistor to smoke rather quickly unless the resistor is rated for more than 50 watts. I am guessing that there is something upstream to limit the current.
ADVERTISEMENT
#3 21667168 19 Mar 2013 08:53

Steve Lawson Steve Lawson

Anonymous

Post #3
21667168 19 Mar 2013 08:53

This does a very good job of explaining how to calculate this (and why it's calculated that way):

* http://www.play-hookey.com/ac_theory/randr/ac_rc_series.html

[in their example the resistor is on top and the capacitor is on the bottom, but the same principles apply to your case, just flip it over ]
ADVERTISEMENT
#4 21667169 19 Mar 2013 09:02

Yoram Stein Yoram Stein

Anonymous

Post #4
21667169 19 Mar 2013 09:02

Xc=1/ 2 x pai x f x C=1000000 / 2x3.14x50x0.25= 12738.8Ohms
Z=Root ( R²+Xc²)=Root ( 1000²+12738²)=12776ohms
250VA/ 12776=0.01956 Amp= 19.56 mA
Vr=250VA : 12738Ohms x 1000= 19.62Volts
Pr= I² x r= 19.56mA² x 1000= 1.556Watts
So need only 2W resistor
So i think David Adams shoiuld check his calculations

See you all in Amsterdam Netherland
#5 21667170 19 Mar 2013 09:09

Yoram Stein Yoram Stein

Anonymous

Post #5
21667170 19 Mar 2013 09:09

Sorry
Z total=root (1000²+12738²)=12815Ohm
250VA: 12815Ohm= 0.019508388=19.5mA
Vr=I x R=0.0195 x 1000= 19.5Volts
ADVERTISEMENT
#6 21667171 19 Mar 2013 09:23

Steve Lawson Steve Lawson

Anonymous

Post #6
21667171 19 Mar 2013 09:23

Actually, it's more like 0.55 watts.

Two errors in your calculations:
1. It's 60 Hz not 50 Hz
2. 19.56mA² x 1000 ≠ 1.556Watts it equals: 0.38 Watts -- and because it's 60Hz, it's really: 23.5ma² * 1000 = 0.55Watts

BTW, the Vr value is wrong because it was calculated for 50Hz and not 60Hz (I leave the work for you
#7 21667172 19 Mar 2013 09:24

David Adams David Adams

Anonymous

Post #7
21667172 19 Mar 2013 09:24

Sorry - oops - forgot to do the vector sum

Xc = 1/(2*pi*60*0.25uF) = 10.6k ohms

so Z = (R^2 + Xc^2) = 10.65k ohms

Therefore i=250V / 10.65k = 23.5mA

so Vr = iR = 23.5ma*1000 = 23.5Vac

P= i^2*R = .55w so a 1 watter should work just fine

Yoram - the frequency was stated as 60 Hz not 50Hz
So You get an A- where as I flunked -

Another mistake I made is I did not see the decimal point in the 0.25uF cap - sorry
Create an account, log in and become active in a forum and ads will not appear. You will receive points by participating in discussions.
Join this discussion.

Install Elektroda application

Didn't find an answer? Ask Artificial Intelligence

*I agree to send the question to OpenAI, Anthropic PBC, Perplexity AI, Inc., Kagi Inc., Google LLC - owners of language models in order to prepare the best response. The companies may monitor and log information entered into the form.

*I agree to publicly display my question and answer. The question and answer will be publicly available to everyone. The process may take a few minutes. Upon completion, you will be redirected to the page with the answer.

Wait...(2min)

Reply | New topic

Report a violation of the law

Quick Circuit Help

Didn't find an answer? Ask Artificial Intelligence