# Choose & find resistors for an Electricity Demo

Discussion in 'General Electronics Chat' started by Thender21, Aug 20, 2018.

1. ### Thender21 Thread Starter New Member

Jun 7, 2015
27
0
I'm trying to prepare to teach some basic Ohm's law concepts, applications, and diagnostics for automotive technicians. We work with a nominal 12 volt system, I might use a regulated 10 volt bench supply to simplify calculations.

I wanted to use a resistance decade box so I could manipulate circuit resistance very precisely at will, like this one:

looks like a clone of this one:

but it looks like the box is rated at .3 or .5 watts and I think I would toast that in short order. 12 volts, 1 ohm resistor = 1 amp. 1amp * 12 volts = 12watts. Correct me if I'm not calculating my power requirement right.

I will need to load my circuit with at least 5 amps, like a powerful headlamp bulb. That would be about 60 watts, and because of the initially low resistance there would be a high in-rush current, so to speak.

I want to add linear resistors to the circuit without burning them up. I might need to incorporate a circuit breaker to prevent excessive current flow in case circuit is assembled incorrectly as well.

Any advice on spec and availability of suitable components?

Thanks

2. ### LesJones Well-Known Member

Jan 8, 2017
1,917
535
You are not calculating you power correctly. I = V/R so I =12/1 = 12 amps. 12 amps at 12 volts is 144 watts. So the situation is worse than you calculated. I think you have just made a mistake writing it down as you are correct that a 60 watt 12 volt bulb takes 5 amps.

Les.

3. ### Thender21 Thread Starter New Member

Jun 7, 2015
27
0
yep I made a mistake in writing the first example. Still, I have to either limit the current by design or find higher wattage resistors, they would have to be large to dissipate something like 60 watts.

I am probably better off finding a device to load the circuit with or limiting the current.

any ideas?

Jan 15, 2015
4,055
1,845
Years ago when I was a kid in grade school I did a project to demonstrate Ohms Law. All I used was several fixed resistors, a bunch of knife switches, and a few old analog meters. The idea was I switched the resistors in and out of circuit and my current meter bounced around indicating current levels based on the voltage and resistances. Now obviously if you want to have higher currents you need higher power rated resistors. You also need a supply (battery) capable of supplying the current. Analog meters and analog current meters were nice as people like looking at the meter's pointers move. Use aluminum cased resistors and maybe toss in a pot. Inexpensive parts from China will do fine. Everything on a vertical board so people can see what's going on. Something along these lines is how I would likely approach it. I used analog meters simply because around 1959 there were no digital solutions.

Ron

5. ### Dodgydave AAC Fanatic!

Jun 22, 2012
7,715
1,277
Well why don't you use bulbs, a 21W bulb will give approx 1.8A , and these are easily available in a variety of wattages.

6. ### MrChips Moderator

Oct 2, 2009
17,761
5,546
Get a whole bunch of 10Ω 10W resistors. You can connect them in series or parallel for different kinds of experiments.

I = V / R = 10V / 10Ω = 1A
P = I x V = 1A x 10V = 10W

https://www.ebay.com/bhp/10-ohm-10w-resistor