Determining of resistor size

[LordJaivus]

Okay, I've never really been good at algebra. I know the purpose of a resistor but could someone explain to me, (in layman's terms,) how I'd figure out what size resistor I would need to step down the voltage to a hasbro sound board. If it will help, I'm running a lux V with a 3w TCSS driver and 6 NmHd AA's as power. I know the driver replaces the resistor for the lux, but I'm not sure if it does the same for the sound card. I really don't want to fry the darned thing and be out fifteen bucks. Also would It do any harm just get rid of the mom switch on the sound card, and run the wires there in a direct circuit?

[DizzyKungFu]

That's a good question. I don't know if what you are planning will work, but you can test the concept before attaching the Hasbro board if you have a voltage meter. I would say start with one of Tim's 4.7ohm/10w resistors going into where the sound board would be in the circuit and see what kind of voltage you get on the other side. You can then adjust the resistor type based on your findings. Your target for most Hasbro boards should be between 4.5v-6v.

[Darth Morbius]

http://www.hebeiltd.com.cn/?p=zz.led...tor.calculator

[neophyl]

Just stop. Right now. You DO NOT use a resistor to cut the voltage down to a safe level for a soundboard. A resistor is used to reduce CURRENT.
You need to reduce the voltage while still having enough current to run the soundboard correctly. Use a 1Amp 5v regulator, this will allow a maximum of 5volts through to your soundboard at anything up to 1 amps worth of current. With the amount of power that a hasbro draws just for the sound (if you arent powering your led from it too) then this is more than sufficient.

Also ignore what DizzyKungFu posted. The voltage measured depends on the load placed on the circuit, while measuring with the meter WILL give different voltages for different resistors, those values are only valid when the meter is the only thing in the circuit. Remove the meter or add an additional circuit and the load changes so the voltage will change too. Think of this too, as the sound that the hasbro board puts out varies, its load will change (drive the speaker harder and it requires more current). You will see that the load for most circuits is variable which is one reason why you cant really use a simple resistor to make the input power 'safe'.

Leds being a 'constant'* load type device (it wants as much as it can get, all the time) can use a resistor to throttle things back as its a mathematical constant so its possible to work out required values easily




*(yes I know Im not allowing for heat in the explanation, i was trying to keep it in laymans terms withput getting too complex)

[LordJaivus]

thank you very much, but since you happened to bring it up and it never occured to me, will heat be a potential problem when using the regulator with the hasbro board? Also, I imagine if I were to swap out the original speaker with something better in order to amplify the sound I would simply need to up the amperage on the regualtor instead of the voltage.

[neophyl]

It shouldnt for what youve described. The (rough) way these work as an example is say your sound board draws 200ma and your supplying 9v into the regulator, then the regulator as to dissipate 4v at 200ma (9-5 at the 200ma of current). While it will get slightly warm that wouldnt be a problem. However if you were supplying 1A then that would be a significant amount and extra heatsinking on the regulator would be required. The only way you would need that much current was if you used the sound board to supply power to the luxeon too. If all you are doing is supplying the speaker then thats LOTS less power and a normal 1A 5v regulator will be fine. Shouldnt need any heatsinking whatsoever, especially as your input voltage is around 7.2v.

Putting a bigger speaker onto your soundboard doesnt actually change the power that the soundboard uses. Speakers have a ohm rating (4, 8, 16, 64 and 100 ohm) being common values on small speakers, changing the type will alter how much power the sound board tries to supply. For example if it was designed to supply a 64ohm speaker then fitting an 8ohm speaker will be louder but can also potentially damage your sound board. The only way to effective increase the cheap sound boards power output is to either fit an extra amplifier circuit (usually not worth it in time, money and space) or to physically modifiy the soundboard to replace its exisiting transistor driver with one of a higher gain/power. Something else thats normally beyond the casual hobbyist unless they want to start learning more about electronics in general.

Fitting a bigger speaker with the same resistance is never a problem and usually is louder mainly from the fact that a bigger speaker can displace more air. Theres lots of other factors too like frequency response etc but proper physical design layout with reasonable space before and behind the speaker make alot more difference generally with the limited size speakers we can fit in our sabres.

thank you very much, but since you happened to bring it up and it never occured to me, will heat be a potential problem when using the regulator with the hasbro board? Also, I imagine if I were to swap out the original speaker with something better in order to amplify the sound I would simply need to up the amperage on the regualtor instead of the voltage.

[Novastar]

//begin comment
I say it too much, but... me = loves it when NeoPhyl posts.
//end comment & slight de-railment