# electronics revision help

Discussion in 'Electronics Homework Help' started by tom6123, May 31, 2014.

1. ### tom6123

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May 31, 2014
Hello everyone,
Im trying to catch up on some revision but i have no clue how to do it, so ill just give the question.

A Circuit containing a transformer, Bridge rectifier, Reservoir capacitor and an additional resister/Capacitor (RC) filter circuit, is the be used to construct a dc power supply. the input voltage is [email protected] 50Hz.
The output voltage is to be 12V Dc and designed to drive a 1k Ohm load resistor.

Can anyone help me with this?

2. ### KrisBlueNZSadly passed away in 2015

8,393
1,270
Nov 28, 2011
Hi Tom, and welcome to Electronics Point.

I have moved your question to the homework help forum.

We don't give complete answers here. It's the old "teach a man to fish" thing. We encourage you to make an attempt, and we give suggestions and corrections. You need to make a start yourself.

If you've been taught this stuff, you should have notes. If you haven't, you shouldn't have been asked the question. Please have a go and post your circuit.

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May 31, 2014

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4. ### KrisBlueNZSadly passed away in 2015

8,393
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Nov 28, 2011
That's good!

C1 is the first smoothing capacitor, and R1 and C2 are the extra R-C filter. R1 is not the load resistor, and it probably shouldn't be 1k. Can you delete the 1k marking on R1?

You can add some information to the schematic. The transformer should probably be called T1, not T2. The bridge rectifier should be named BR1. We will also have to decide on some other information: the transformer's secondary voltage; the component type for BR1; the capacitances and voltage ratings for C1 and C2; the resistance and power rating for R1.

The circuit is supposed to provide 12V into a 1k resistor load. Can you add the load at the end of the circuit, across the output. Also, mark the +12V and 0V output connections for the power supply section.

You can also calculate the amount of output current that will flow into a 1k resistor at 12V using Ohm's Law. How much current will flow? Can you add an arrow on the schematic that shows this current flowing from the +12V output of the power supply and into the load resistor?

When you've made those changes, upload the diagram again and we can look into working out the component values.

5. ### tom6123

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May 31, 2014
Right sorry for the delay, this is what i produced. is this what you meant?

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6. ### KrisBlueNZSadly passed away in 2015

8,393
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Nov 28, 2011
That's good.

Bridge rectifiers are usually named BRx not Bx. Bx is not normally used.

R2's bottom end is shown connected to the GND symbol but there is no junction with the 0V rail. There should be a junction. Actually it's good practice to avoid drawing wires crossing each other like that when there is a junction, because it's easy to forget the junction. Better to stagger the connection to R2 and the connection to GND so the line does not cross straight throuch the 0V rail.

Also I would draw the +12V and 0V labels and dots to the left of R2 to indicate that it's not part of the power supply itself; it's an external load.

R2 should be called RL to indicate that it's a load resistor.

C2 should be a polarised capacitor like C1. Both of them will be electrolytics.

I don't have time now to cover the formulas for calculating C1, R1 and C2, and the secondary voltage, or the process for selecting a component for BR1.

Perhaps SOMEONE ELSE could do this. Otherwise I will reply again in a day or so.

7. ### tom6123

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May 31, 2014
i have tried to indicate that the rails do not cross by selecting different colours, ( i am not familiar with the newest software that i am using), but i am thankful for your help,
maybe when youre next available you can help me further into understanding the circuit more

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
Tom, tell me if any of these things ring any bells for you:

1. RMS voltage of an AC waveform
2. Peak voltage and it's relationship to RMS for a sine wave
3. Voltage drop across a rectifier
4. Low pass filter
5. RC time constant
6. Ohms law
7. Ripple
It's also a good time to tell you that your question does not have a single unique answer. In fact, the lack of detail about the tolerance or ripple on the output voltage means that it would be hard to ever get this question completely wrong (or right).

9. ### tom6123

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May 31, 2014
the Revision paper that this was from was about magnetism with motors and generators and it just askes to draw the appropriate circuit with relevant component values.

10. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
It's not fair to test you on stuff you haven't been taught...

11. ### tom6123

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May 31, 2014
this is the reason why i dont know how to draw the circuit for it.

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Dec 18, 2013

Hi Kris
The reservoir capacitor is calculated by C = I(Load)/(frequency of ripple*Voltage of ripple required), so for a ripple of 100mV at 100Hz and 12mA load approx. that's 10uF. You have to decide how much ripple you can allow for this I used 100mV but you might need less.

For the low pass I would say go for 50R series resistor to stop C from being too large and then calculate C for 50Hz so the ripple is further reduced when it's at 100Hz (UK). C = 1/2*pi*50Hz*50R = 63uF. I would go for 10 times that for even lower ripple and make it 630uF. The nearest value is 680uF.

Hope this helps

Last edited: Jun 1, 2014
13. ### tom6123

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May 31, 2014
What do i have to change on this circuit diagram? or am i way off base

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14. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
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Jan 21, 2010
The circuit seems to be what is asked for, however the values cannot be determined without more detailed requirements.

For a given set of requirements, there would likely not be a unique set of components that would provide a solution.

5,165
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Dec 18, 2013
Yes I was surprised there was not more detail. Maybe it is a test for the OP to have a think and make a reasonable decision himself. Rather than being given the exact requirements, this would show the ability to do a bit of research and also show he had the ability to possibly re-arrange formula and input values to get an answer.
Just a thought