Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 28, 2024, 01:57:29 PM

Login with username, password and session length

Recent Posts

 

Can someone help me with this test question?

Started by Den., June 25, 2016, 06:03:52 PM

Previous topic - Next topic

Den.

I had this TRUE or FALSE question on a written test recently: You can change the resistance in a circuit that uses a 520ohm 1w resistor to 608ohms by adding four 22ohm 1/4w resistors in series to the existing 520ohm resistor.

I answered true but now I'm not sure of my answer.

phatt

22 div by 4 = 5.5
520 plus 5.5 = 525.5
does that help?
Phil.

J M Fahey

Sorry Phil, unless stated otherwise they are *all*  in series so 520 + (4*22) = 520+88 = 608 .

I don´t understand how Den can have doubts about this *very basic*  question .

Den.

Quote from: J M Fahey on June 25, 2016, 09:53:42 PM
Sorry Phil, unless stated otherwise they are *all*  in series so 520 + (4*22) = 520+88 = 608 .

I don´t understand how Den can have doubts about this *very basic*  question .

Yes, the way I read the question it seemed that they meant all five resistors wired in series. And I calculated that that would add up to 608. But, it was the power handling aspect I was unsure of; using four 1/4w resistors when one watt is required.

phatt

 whoops,,, to early over here only had one coffee :-[ :duh
Phil.

Enzo

1w was required of the 520 ohm resistor, that doesn't mean everything else will dissipate that much.

Use Ohm's Law.  1 watt from a 520 ohm resistor means about 44ma flowing through it, if my math is right.   Since the resistors are in series, and current is the same through a series circuit, I can assume each 22 ohm resistor has 44ma through it.  So calculate the dissipation of 44ma in a 22 ohm resistor.  I get about 43mw - 0.043 watts.  So 1/4w would be six times the dissipation of the resistor in this circuit.  More than enough.

Den.

Thank you. Might this mean then that you would be able to substitute two 29ohm with one 30ohm or maybe even just two 44ohm rated 1/4w and still be safe?

J M Fahey

Quote from: Den. on June 26, 2016, 09:05:31 AM
Thank you. Might this mean then that you would be able to substitute two 29ohm with one 30ohm or maybe even just two 44ohm rated 1/4w and still be safe?
WHICH 29/30/44 ohm resistors?
This question does not make sense since it´s missing data, besides never having heard about them before, certainly not in the above questions.

Den.

Quote from: J M Fahey on June 26, 2016, 02:23:23 PM
Quote from: Den. on June 26, 2016, 09:05:31 AM
Thank you. Might this mean then that you would be able to substitute two 29ohm with one 30ohm or maybe even just two 44ohm rated 1/4w and still be safe?
WHICH 29/30/44 ohm resistors?
This question does not make sense since it´s missing data, besides never having heard about them before, certainly not in the above questions.

Sorry, I moved off the test question and am now trying to understand substituting different numbers of resistors with their different watt handling.

J M Fahey

OK.
1) determine current I flowing through that circuit.
Apply Ohm´s Law.
2) power dissipated across any resistor is: P=I^2 * R

Do the Math and check whether dissipated power is less than resistor rating.

Loudthud

#10
In a series circuit, the resistor with the highest resistance will have the most dissipation. Calculate the current that will result in that resistor being at it's maximum, then check that the other resistors will be at or below their rating at that current.

In a parallel circuit, the resistor with the lowest resistance will dissipate the most power. Calculate the Voltage that will result in that resistor being at it's maximum, then check that the other resistors will be at or below their rating at that Voltage.

As a practical matter, running resistors at their maximum power rating is not a good idea. If they are on a PCB, it will usually make the board turn black. The resistor or it's solder joints will fail. A resistor's power rating is based on temperature. If they are in a warm environment, they must be de-rated. You can run resistors beyond their power rating for short amounts of time as long as the resistor does not get too hot. To be conservative, don't run resistors more than about half their power rating.

Den.

Quote from: Loudthud on June 27, 2016, 03:56:00 PM
In a series circuit, the resistor with the highest resistance will have the most dissipation. Calculate the current that will result in that resistor being at it's maximum, then check that the other resistors will be at or below their rating at that current.

In a parallel circuit, the resistor with the lowest resistance will dissipate the most power. Calculate the Voltage that will result in that resistor being at it's maximum, then check that the other resistors will be at or below their rating at that Voltage.

As a practical matter, running resistors at their maximum power rating is not a good idea. If they are on a PCB, it will usually make the board turn black. The resistor or it's solder joints will fail. A resistor's power rating is based on temperature. If they are in a warm environment, they must be de-rated. You can run resistors beyond their power rating for short amounts of time as long as the resistor does not get too hot. To be conservative, don't run resistors more than about half their power rating.

Thanks everyone.

.