# Safety in simple circuits

Discussion in 'General Electronics Chat' started by Shmurk, Mar 3, 2009.

1. ### Shmurk Thread Starter Member

Aug 13, 2007
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I'm currently reading the chapter 10.3 of the book, the one about the Mesh current method to analyze DC circuits with a few resistors and multiple voltage sources.

Everything looks good on paper but I want to try it in real life, and I'm wondering if these simple circuits are "safe." I'd like to test my calculated results with a multimeter (it would last just a few minutes) and I'd like to know if there is any chance that my resistors can overheat or explode (or empty my fridge without my permission ), or if my batteries can die really faster because of the "uselessness" of the circuit.

Yes, it's a rather stupid question but, well, I wonder...

2. ### russ_hensel Distinguished Member

Jan 11, 2009
820
47
Good chance to use ohms law to compute expected current and power. That said, use a current limited power supply ( a small 9 volt battery for example ) and I doubt you could blow anything except the current range on your multimeter. ( do not use the current function, measure the voltage across a resistor and ohms law to get the current )

3. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
It's not a stupid question at all!

The low resistances specified in those circuits would result in high currents, and large power dissipation in the resistors. Real batteries would rapidly become discharged. The resistors for some of the circuits would need to have a high Wattage rating; for example in the first schematic, R1 should be rated for 200 Watts!
Power(Watts) = I^2 R (Current Squared times Resistance), in this case 5^2*4=100 Watts; you always double the wattage requirement for reliability and lower heat.

Rather than build the actual circuits, I suggest you use a PSpice simulator.
Linear Technology's LTSpice is good, and free.
http://www.linear.com/designtools/software/

4. ### Audioguru New Member

Dec 20, 2007
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I have always calculated the max current and max power properly so i have never had anything blow up.

Last edited by a moderator: Mar 3, 2009
5. ### mik3 Senior Member

Feb 4, 2008
4,846
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If you use voltages below 50V then you will be safe.

As for the resistors if they are not of the proper wattage they will overheat and burn.

6. ### Shmurk Thread Starter Member

Aug 13, 2007
24
0
Thank you for all the answers, this really gives me the incentive to keep on studying electronics!

Here is what I'll do:
• use one 9V battery, one 1.5V battery, and a few 1/4W resistors
• compute expected current and power by hand
• check my results with Spice
• build the actual circuit and check with a multimeter

Thanks again!

7. ### leftyretro Active Member

Nov 25, 2008
394
2
Good plan. Keep in mind that calculated results and measured results will differ somewhat as components have manufacturing value tolerances differing from their marked value.

Lefty

8. ### Shmurk Thread Starter Member

Aug 13, 2007
24
0
I'll have to explain why I insist on building the circuit with real components:

I'm a C/C++ programmer, and when I began, I read programming books for 6 months without writing real programs. At the end of these 6 months, I knew... nothing! It was a disaster. Programming was harder than I thought, and I really learned and understood what was in the books when I wrote my own programs. That's why I want to do it for real right now, I don't want to wait till I've read the book to the end.

I've already learned a lot about electronics by building simple circuits and comparing my breadboard circuit with what's written on paper. It seems to work the same way as computer programming.

Thanks again for all your support

9. ### leftyretro Active Member

Nov 25, 2008
394
2
I agree that real world experience cannot be accomplished solely on simulations or book reading. Both have there place under the sun these days. While I'm pretty old school (and just old also ) So simulators don't work well for me, but I certainly appreciate the power these tools have with the ever increasing complexities of designs these days.

10. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
The AAC book has a good volume of hand on type experiments. Volume VI, on the top of the main forum page.

11. ### russ_hensel Distinguished Member

Jan 11, 2009
820
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No simulator can give you the thrill of putting a heavy wrench across a car battery and watching it glow. Do not try it at home.