Resistor Advice

[DrTechnoid]

Greatings all!

I'm really hoping that one of you kind people will advise me on this matter!

I have been making a toy for my daughter and am struggling a little with what ohm resistor to use.

So, I have two circuits boards that run off one 7.4V/ 1300 mAh battery. One circuit board is designed to run with this battery but the second board is designed to run off x3 1.5V AA batteries. However, I want them both to run off the same power source. After some research, I eventually found a formula for calculating WATTS and ohms, and then from that, I further calculated that a 68 ohm resistor would be appropriate.


Is a 68 ohm resistor suitable or have I got it toally wrong? If wrong, which capacitor should I use?

Kindest regards and thanks in advance
Chris

 

[Knobhead]

Without the knowing loading of the secondary board it’s impossible to say. Any variance of that load will cause voltage variance.
You should rethink the idea.

 

[Richard Burns]

Not really the best forum for electronic questions, more electrical than electronic, however I would suggest that you would need to use a diode as a voltage regulator for the second boards supply.
There is a good article about this here.

 

[nickblake]

what i would be inclined to do is to use an adjustable voltage regulator , the size you require will be dependant on the load as tony has said ,this way you could have a 12 volt supply and adjust it using a multi meter to check the volatage places where you can get these in kit form are Ceebek ,velleman , CPC online ,unfortunatly without knowing the loadings im unable to advise you further

 

[i=p/u]

you need a cathode to keep one direction aswelll???? does this sound good????

 

[telectrix]

the easiest way would be to fit a second battery. as you require 4.5V, the old flat 4.5V cycle lamp battery would suit.

 

[DrTechnoid]

Thank you for the responses. The device is part of an RC helicopter, so more batteries or a change in battery is not practical. I have done tests on both circuit boards with the same battery and it works fine but as you will all know, the smaller board starts to over heat. I will look into using a Diode!

Also, you lot have answered me faster and better than any electronic site!

Thanks again.

 

[Knobhead]

A diode will not control voltage.

 

[telectrix]

try 4 or 5 diodes in series. sounds daft, but each diode will drop 0.6V.

 

[Silly Sausage]

A diode will not control voltage.

He (Richard) means 4.5V Zener diode.

 

[DPG]

Zener diode can be used to control voltage, but I would use an LM317 adjustable regulator. Datasheet widely available on web. Be sure to pick a version which can supply enough current. Daz

 

[Moses]

He (Richard) means 4.5V Zener diode.

You need a resistor( the value depends on the load current drawn), from the 7.4V Battery to a 4.7V zener, then the output from the zener goes to the load. The zener wattage depends on the resistor value.

Give us the details of the load current drawn and we will tell you the resistor value and wattage, and the zener wattage.

 

[nickblake]

Like i said adjustable voltage regulator 12 volt in and adjust to what you want out £5.00 at cpc job done and stable

 

[plugsandsparks]

You need a mA meter to measure the load off the second board. The meter goes in series for measuring amps. e.g. if the load is 50mA then the resistor needed is 58ohm and 0.25 watt. Whatever the resistor is you may need a bit of trial and error to get the right value, hence a potentiometer can be useful where you can adjust it to get the right voltage. A very small 100ohm pot , 0.25w is a couple of quid and dead light
cheers

 

[Moses]

OP

give us the load current and I will work out what values you need.