# Need advice on circuit design?

Discussion in 'General Electronics Chat' started by electronice123, Feb 6, 2009.

1. ### electronice123 Thread Starter AAC Fanatic!

Oct 10, 2008
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I have been trying to learn circuit design over the last few months but I seem to be missing something.

I know the components I need, but when it comes to adding resistors between chips I have no idea how to calculate what I need.
I know ohms law and have used it alot in the past, but when designing analog and digital circuits which contain IC chips I start getting confused on where I need to have resiatances, op amps etc.

Also, when working with analog circuits, how do I figure out how much resistance will drop the voltage to a specific amount? Say I had 12V and I needed to drop the voltage to 9V, how would I determine the resistance needed to do that?

Can someone tell me how to design circuits correctly and find out where resistors and op amps would be required in a circuit?

Are there any books out there on circuit design you would recommend?

2. ### Alexei Smirnov Active Member

Jan 7, 2009
43
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That's exactly Ohms law, R = V / I. When selecting resistor, you have to know both voltage and current... Sometimes power too (P=V*I)...

3. ### mik3 Senior Member

Feb 4, 2008
4,846
70
Everything depends on the application and what the circuit is supposed to do. You have to approach different circuits in a different way depending on the accuracy you want, noise in the working environment and so on.

Better to post your circuit here and ask for help for specific designs.

4. ### beenthere Retired Moderator

Apr 20, 2004
15,808
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My guess is that you don't have a firm grounding in basic DC or AC electronics yet. Take some time to learn the basics before trying to design circuits.

5. ### Wendy Moderator

Mar 24, 2008
21,839
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That and put the schematic up here. We can help with specific issues.

6. ### electronice123 Thread Starter AAC Fanatic!

Oct 10, 2008
311
0
Here's the design so far.

The circuit is meant to do the following. The voltage regulator provides 10V to the 4046 VCO which produces a 50% duty cycle pulse and an adjustable frequency. The 4046 pulsed output signal drives the base of two NTE11 transistors, one of which drives a primary transformer coil, and the other drives 30 LED's which all pulse when the primary coil is in it's off time.

Where I get confused is how to figure out what resistances I need at the transistors base? How do you figure that? Also, should I have any op amps in there?

I have taken classes on electronic circuits, basic AC/DC theory and used Ohms law many many times over. But when it comes to designing circuits I start getting confused when IC chips are added. How do you design circuits with IC chips, do you just use the datasheets and go from there?

Maybe it's time to do more studying!!! I have one book I recently purchased "The Art Of Electronics" which has a lot of info. I guess I'll get to reading more!

In the mean time, is there anything wrong with my circuit design? How do I figure out the resistances needed at the transistor(s) base?

Last edited: Feb 6, 2009
7. ### mik3 Senior Member

Feb 4, 2008
4,846
70
When you use a chip you have to read the datasheet to see what each pin does and the electrical specifications of the chip.

The circuits look fine. Just some improvements:

1) Put a 2K resistor between the output of the LM317 to ensure that it regulates the voltage at 10V. If you read the datasheet is says that it regulates well if the output current is over 10mA. The 5mA will pass through the 240ohm resistor and the other 5mA through the 2K resistor you will add.

2) I don't know how a 4046 works but if you want to know more look at its datasheet:

http://bertrik.sikken.nl/bat/pdf/4046.pdf

To calculate the resistors values the forward voltage across the diodes and their working current is needed.

What is the maximum and average current through the primary?

8. ### electronice123 Thread Starter AAC Fanatic!

Oct 10, 2008
311
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The primary is made from 22 AWG wire so I don't expect to go over 5 amps. I also want current in the primary to be adjustable if possible from about 1A-5A. Average current is 3A.

9. ### mik3 Senior Member

Feb 4, 2008
4,846
70
The NTE11 is not suitable for your application then because its maximum continuous current is 5A and it is not good to drive it to the limit. Also, it has low gain and it will need considerable base current which the 4046 won't be able to provide (I think because I haven't read the datasheet). It would be better to use a MOSFET transistor to PWM the primary. Also, the current through the coil depends on the duty cycle of the output of the VCO.

10. ### electronice123 Thread Starter AAC Fanatic!

Oct 10, 2008
311
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After you mentioned that I realized that I could control the primary current by controlling the gate voltage with a voltage divider???

The datasheet says the output voltage of the 4046 at 10V input is 9.95V. Now I just don't know what kind of FET to use???

Is there a list of FET's somewhere?

Frequency 50Khz, 3-5A.

Last edited: Feb 6, 2009
11. ### mik3 Senior Member

Feb 4, 2008
4,846
70
I don't know a list but if you go to the website of Farnel, Digikey etc you can find many. Also, have a look in ebay, type MOSFET and you will see many.

12. ### electronice123 Thread Starter AAC Fanatic!

Oct 10, 2008
311
0
Thank you for all the help!!!