# [Urgent] Problems with transformers.

Discussion in 'Homework Help' started by EOT, Aug 26, 2009.

1. ### EOTNew Member

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So, first post on the site and already i'm crying out for help, sorry about that. Very rude of me. Anyway, here's my speil:

So...i'm training to be an Engineer. But one of the classes i was required to take last year was a Module in Electrical Engineering (very basic electrical engineering at that). Now, my problem is, i suck, is suck at all electrical stuff so effing hard. I always have. Never quite got it through my thick skull. Thermodynamics, trig, algebra, mechanics, all that good stuff. That i can do. Ask me anything past Ohm's or Kirchoff's law and i'm buggered.

Basically, even though i went to all bar two of my lectures last year, i still managed to fail the exam and had it handed back to me as a referral. So basically i have until friday to complete it and i've been slacking (as per at the moment, it's hard to keep motivated when you're jobless i find).

Anyway, now we come to the crux of the problem. I'm stuck. Stuck on something probably really simple to you. But it's giving me problems. Anyway. Here's the question:

A transformer has a primary to secondary turns ratio pf 20:1. If the secondary voltage is v(t) = 20sin(2π3000t) volts. Determine the:

1. expression for the primary voltage. (I assume Vprimary = 20xVsecondary = 400V)

2. peak voltage across the resistor (well for Vsecondary = PV = 20V but...i dunno)

3. Peak current through the resistor

4. peak power dissapated by the resistor

5. peak power dissapated by the diodes

With the diagram below:

Anyway, if someone could give me a quick walkthrough of the solution, or link me to a site where something similar is provided it would be very much appreciated.

EDIT: and don't be surprised if i end up posting almost the entire paper up here. I really am just that bad at this stuff.

Last edited: Aug 26, 2009
2. ### audioguruWell-Known MemberMost Helpful Member

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Why do you have two rectifiers? The first one rectifies the input and the second one does nothing, or is a backup.
Since you are not using a full-wave rectifier (yours is half-wave) then your ripple will be horrible.

3. ### EOTNew Member

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Thats the transformer on the left (Primary on the left hand side, secondary on the right). My own lazyness has lead to that not beeing made very clear in the diagram.

Last edited: Aug 26, 2009

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5. ### RoffWell-Known Member

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It's the conduction path for a half cycle through a full wave rectifier.

6. ### RoffWell-Known Member

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Don't forget to include the sin term.

If you assume the forward voltage of each diode is 0.7V, and there are two of them, what does this leave for the resistor?

Use Ohm's law in conjunction with the previous answer.

What is the equation for power?

What is the equation for power?

7. ### EOTNew Member

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Thank you very much. I thought it would be something simple like that. Thanks for giving me pointers instead of just answering it too (as much as part of me would have just liked someone to answer it for me ).

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