Circuit Lab B/C

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mdv2o5
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Re: Circuit Lab B/C

Postby mdv2o5 » January 9th, 2019, 8:55 am

jaggie34 wrote:
knightmoves wrote:As drawn, the 12 and 6 ohm resistors are in parallel, so can be replaced by a single 4 ohm resistor.

The right hand loop on the picture is irrelevant, as the diode is reverse biased and does not conduct (proof left as an exercise for the reader).

So you have a simple loop circuit with a diode and 7 ohms of resistance, so i = 11.25/7 = 1.607A.

Now consider the 6 and 12 ohm resistors separately again. 1/3 of the current goes through the 12 ohm resistor (1/3 of the conductance), so answer = 0.536 A.


Why would you ignore the right side, isn't the diode not in reverse bias in respect to the 4V source?


To determine whether or not the diode is forwards or reverse biased, you need to look at the voltage across the diode. Here, the anode is indeed at 4V, but we have no idea what voltage the cathode is. Thus we can't find the voltage across the diode. If the cathode is at a lower voltage, then the diode is forwards biased and conducts. Otherwise, it's reverse biased. The problem is we don't know the cathode voltage. The way I initially solved the problem is to just assume that the diode is forward biased and is dropping 0.75 V and apply KVL/KCL. But then, I ended up with current flowing backwards through the right diode. This suggests that the diode might actually be reverse biased. Using this assumption, we try again and ignore the right half (since no current flows). As it turns out, the voltage at the cathode using this assumption is 6.432 V which is consistent with the assumption that the right diode is reverse biased since the voltage across the diode is now -2.432 V.

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Re: Circuit Lab B/C

Postby UTF-8 U+6211 U+662F » January 9th, 2019, 12:47 pm

jaggie34 wrote:
knightmoves wrote:As drawn, the 12 and 6 ohm resistors are in parallel, so can be replaced by a single 4 ohm resistor.

The right hand loop on the picture is irrelevant, as the diode is reverse biased and does not conduct (proof left as an exercise for the reader).

So you have a simple loop circuit with a diode and 7 ohms of resistance, so i = 11.25/7 = 1.607A.

Now consider the 6 and 12 ohm resistors separately again. 1/3 of the current goes through the 12 ohm resistor (1/3 of the conductance), so answer = 0.536 A.


Why would you ignore the right side, isn't the diode not in reverse bias in respect to the 4V source?

The 12 volt battery source overpowers the 4 volt source.

Edit: Whoops, I see a more detailed explanation has already been posted...

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Re: Circuit Lab B/C

Postby DocThorium » January 10th, 2019, 2:24 pm

I have a question about the rules regarding a possible contradiction.

3. c. The test will consist of at least one question from each of the following areas:
xiii. Division C only - Electrical characteristics of a silicon PN junction

3. d. Topics not included in the competition are: semiconductors, AC circuit theory, inductance, non-linear
devices, three-state logic gates, sequential logic, and oscilloscopes.

Aren't silicon PN junctions a semiconductor topic? If so, doesn't this mean that the rules contradict themselves, since they require a topic explicitly stated not to be in the test to be in the test?

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Re: Circuit Lab B/C

Postby UTF-8 U+6211 U+662F » January 10th, 2019, 4:29 pm

DocThorium wrote:I have a question about the rules regarding a possible contradiction.

3. c. The test will consist of at least one question from each of the following areas:
xiii. Division C only - Electrical characteristics of a silicon PN junction

3. d. Topics not included in the competition are: semiconductors, AC circuit theory, inductance, non-linear
devices, three-state logic gates, sequential logic, and oscilloscopes.

Aren't silicon PN junctions a semiconductor topic? If so, doesn't this mean that the rules contradict themselves, since they require a topic explicitly stated not to be in the test to be in the test?

Yes, silicon PN junctions are semiconductors. My guess is that Division C includes silicon PN junctions but no other semiconductor topics.

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Re: Circuit Lab B/C

Postby DocThorium » January 11th, 2019, 9:13 pm

UTF-8 U+6211 U+662F wrote:
DocThorium wrote:I have a question about the rules regarding a possible contradiction.

3. c. The test will consist of at least one question from each of the following areas:
xiii. Division C only - Electrical characteristics of a silicon PN junction

3. d. Topics not included in the competition are: semiconductors, AC circuit theory, inductance, non-linear
devices, three-state logic gates, sequential logic, and oscilloscopes.

Aren't silicon PN junctions a semiconductor topic? If so, doesn't this mean that the rules contradict themselves, since they require a topic explicitly stated not to be in the test to be in the test?

Yes, silicon PN junctions are semiconductors. My guess is that Division C includes silicon PN junctions but no other semiconductor topics.


Wouldn't that still be a contradiction in the rules, though?

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Re: Circuit Lab B/C

Postby Things2do » January 12th, 2019, 1:15 pm

DocThorium wrote:
UTF-8 U+6211 U+662F wrote:
DocThorium wrote:I have a question about the rules regarding a possible contradiction.

3. c. The test will consist of at least one question from each of the following areas:
xiii. Division C only - Electrical characteristics of a silicon PN junction

3. d. Topics not included in the competition are: semiconductors, AC circuit theory, inductance, non-linear
devices, three-state logic gates, sequential logic, and oscilloscopes.

Aren't silicon PN junctions a semiconductor topic? If so, doesn't this mean that the rules contradict themselves, since they require a topic explicitly stated not to be in the test to be in the test?

Yes, silicon PN junctions are semiconductors. My guess is that Division C includes silicon PN junctions but no other semiconductor topics.


Wouldn't that still be a contradiction in the rules, though?

To me, it sounds more like a exception rule 3.d. than a contradiction of it.
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Re: Circuit Lab B/C

Postby Creationist127 » January 12th, 2019, 3:26 pm

Things2do wrote:
DocThorium wrote:
UTF-8 U+6211 U+662F wrote:Yes, silicon PN junctions are semiconductors. My guess is that Division C includes silicon PN junctions but no other semiconductor topics.


Wouldn't that still be a contradiction in the rules, though?

To me, it sounds more like a exception rule 3.d. than a contradiction of it.

If anyone's really confused, an FAQ is probably the best route... but it can't be that difficult to study one extra topic, even if you don't need to.
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Re: Circuit Lab B/C

Postby SciolyHarsh » January 13th, 2019, 8:35 am

Tfw you go to an invitational feeling ready and the circuit test whips out gauss's law. Thank god I take E&M, but that test had more magnetism than AC/DC circuits questions. Only the lab was truly circuits
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Re: Circuit Lab B/C

Postby neerja.shah » January 14th, 2019, 6:02 am

When it says on the slideshows that some things are divison C only...does that mean that division b will not cover it at all? Such as op amps?

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Re: Circuit Lab B/C

Postby Creationist127 » January 14th, 2019, 6:57 am

neerja.shah wrote:When it says on the slideshows that some things are divison C only...does that mean that division b will not cover it at all? Such as op amps?

Yes, Division B is not supposed to cover things labeled "Division C only" at all. However, especially in invite-level settings, there are often cases when these topics are included on Div B tests, due to ignorance or laziness of the ES. So, it wouldn't hurt to study the 'Div C only' topics, even if you are in Div B.
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Re: Circuit Lab B/C

Postby jaggie34 » Yesterday, 9:21 am

Does anyone know what a placing score would be at MIT?
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