# Simple Circuit Problem(4.5v circuit) help needed for motor and led circuit

Discussion in 'General Electronics Chat' started by kissdom, Jan 8, 2012.

1. ### kissdom Thread Starter New Member

Jan 8, 2012
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hi everybody!

the "x" in the circuit is two motors connected in parallel and there's a off/on switch for them.

the triangle is 3x3V LED also connected in parallel and the voltage is step down using a 100ohm resistor else the led will blow.

the entire circuit is power by 2x3AA energise batteries connected in parallel.

The Problem:
the led haf no problem lighting up as long as the motor is off, and when i turn on the motor switch, the led gets really dim or doesnt light up, i need some enlightenment on what's wrong with the circuit.

thank you all

2. ### scottyjr New Member

Aug 12, 2011
12
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I'd say it's just a matter of not having enough power. Motors use alot of current. Most of the power that the batteries can output will be consumed there thus not leaving enough to power anything else. Depending on the size (current and voltage ratings) of the motors, this circuit may function if powered from a power source capable of providing enough power for the motors and the leds, such as a wallwart. - Scotty

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3. ### Adjuster Well-Known Member

Dec 26, 2010
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This may be a particular problem if you are using white, or blue LEDs. These need 3V or more to even begin to light. Green LEDs typically may need a little less, then yellow and red less voltage still - though there is some difference between different products.

You may find that this works better with the sort of red LED that runs on about 2V - the resistor might need to be rather lower.

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4. ### kissdom Thread Starter New Member

Jan 8, 2012
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hi scottyjr, thank you for your response.

I have a question, if i continue to add more sets of 4.5 voltage power source(3x1.5V AA battery) to the circuit, will my motor risk overheating and do you know how to calculate whats the maximum input for current my motor can withstand?

5. ### kissdom Thread Starter New Member

Jan 8, 2012
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Adjuster, you were right! im using white LED(3V).

I have since took out the resister, thus the 3 led in parallel is running on 4.5V, and they still dun light up.

However, i realized if I add more power source (4.5V) in parallel, i can see the a dim led glow.

My question is how much more power source do i need to add in, and do you have any recommendation as too how much is a safe limit so as not to blow the motors and leds?

I thank you all for the replies, its really helpful

6. ### Potato Pudding Well-Known Member

Jun 11, 2010
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Do not take the resistor out of series with the LEDs.

The suggestion was to switch to red LEDs which need less voltage.

Do not put LEDS in parallel. Each LED or string of series LEDs needs its own dropping resistor.

To make your circuit work better I would but the battery packs in series and use a chopper to drop the 9V voltage down to run the LEDs and motors.

Your white LEDs would work well enough as the voltage reference but I would use two Red LEDs in series instead to give about 4 volts.

I don't have time now to work out the circuit details.

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7. ### kissdom Thread Starter New Member

Jan 8, 2012
12
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Hi Potato Pudding, Im quite confuse with the suggestions to change to lower voltage LEDs? because the present circuit supplies 4.5V, and tt's sufficient to power the 3V white LEDs, m i missing smth?

and thank you a lot for yr reply
I will re-wire the LEDs and see how it goes - "LED or string of series LEDs needs its own dropping resistor."

8. ### Audioguru New Member

Dec 20, 2007
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White LEDs need about 3.5V, not 3V.
Alkaline battery cells are 1.5V when brand new then drop quickly to 1.2V as they are used. Then your 4.5V is actually only 3.6V which drops more when the motors run. Then the motors run slowly or will not start running and the LEDs will not light.
Some cheap battery holders reduce the voltage.

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9. ### kissdom Thread Starter New Member

Jan 8, 2012
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holy molly, so tt explains everything!! since tt's the case, i will be trying red LEDs, hopefully all goes well

thank you so much for the insights

10. ### kissdom Thread Starter New Member

Jan 8, 2012
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@Audioguru

does turning on the motor, reduces the voltage?
cose from my knowledge, voltage remains the same, its the current distribution tt's affected, i need some enlightenment!

11. ### Audioguru New Member

Dec 20, 2007
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896
A DC electric motor draws a very high current when it starts (it must overcome inertia) and when it works hard. The high current reduces the battery voltage because each battery cell has some series internal resistance. I think your battery holder contacts and wires also have series resistance that reduces the voltage.
Each time you use the battery its voltage becomes less.

12. ### kissdom Thread Starter New Member

Jan 8, 2012
12
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@Audioguru,

so tt's y!! and tt polly y the LED doesnt light up..

if this is the case, are there any component u noe of tt i can use to regulate the flow of current(ctrl the amt of current gg into the motors)?

13. ### Audioguru New Member

Dec 20, 2007
9,411
896
Please speak English. "tt's", "y", "polly", "u", "noe", "ctrl", "amt", and "gg" are not English.
Polly is my #1 dog's name. Polly has blinking LEDs on her collar.

The motors use only the amount of current that they need. If you regulate their current then they might not start running or might stall when they begin to work hard.

Your problem is that the voltage drops when the motors start or are working hard.

14. ### kissdom Thread Starter New Member

Jan 8, 2012
12
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ROFL(rolling on the floor laughing)! I tend to use short form when I get excited, sorry about that.

Now all of my doubts are cleared, I hope!
I will update you on the good news Audioguru.

Thank for your time, and thank you everyone who has replied!

Last edited: Jan 9, 2012
15. ### kissdom Thread Starter New Member

Jan 8, 2012
12
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Hi all! I haven encountered a another problem, I need some feedback on which circuit will overdrive my motor more.

The series or the parallel?

Picture shows circuit:

I have two circuits, both contains 6 batteries total as power source, they are to power two 3V motor connected in parallel.

I have in the picture two circuits:

Parallel - 3 AA batteries is connected series and then connect in parallel to another set of 3 AA batteries to power the motors.

Series - 6 AA batteries is connected in series to power the motors.

From my knowledge, power = current x voltage, and both circuits gives the same amount of power, however which circuit will overdrive my motor more? and is there a difference between this circuit?

Thank You

16. ### scottyjr New Member

Aug 12, 2011
12
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First what needs to be understood is whether you would intend on connecting additional battery sets in series or parallel. If you connect them in series then you would add an additional 4.5v for every set you add. Eventually adding enough sets would bring the voltage up to a point where it would damage the motor. Without data about the motor as to it's voltage and current requirements it wouldn't be easy to make an educated guess as what the voltage should be and what would damage the motor.

If you connect the sets up in parallel with the current set then you are safe. The voltage would remain at 4.5v but the current capacity would improve with every set you attach. How many sets you would need till you get the performance you seek from this circuit would again be determined by the motor current requirements usually gotten from a datasheet. Another way would be to use a multimeter and directly read the current passing through the motor under a no-load run condition and a stall condition. You could then calculate the batteries needed.

I think that your present situation is akin to what would happen if you tried to start your car with the headlights on and the battery not being up to snuff. When to turned the key to 'start' the headlights would dim. If the battery were low enough they might go out completely but would illuminate somewhat when the key was returned to the run position. The starter motor is consuming a monster amount of current from the battery leaving little for the headlights.

- Scotty

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17. ### Adjuster Well-Known Member

Dec 26, 2010
2,147
300
This will depend on how much current the motors will take, and on the battery internal resistances.

An alkaline AA cell has a nominal voltage of 1.5V, and may have about 0.2Ω internal resistance when fresh, but as the cell ages its voltage falls and its internal resistance rises. http://data.energizer.com/PDFs/BatteryIR.pdf

Different types and makes will of course have different characteristics, for example plain old zinc/carbon model might have more resistance, an alkaline D cell might have less

Voltages and resistances add in series. Equal voltages in parallel give an unchanged result.

Resistors in parallel are found from Rtotal = 1/((1/R1) + (1/R2) +... ...+(1/RN))

Thus the 6AAs in series give 6*1.5V = 9V open-circuit from an overall internal resistance of 6*0.2Ω = 1.2Ω

The two paralleled strings of 3AAs in series give 3*1.5V = 4.5V Each string has an internal resistance of 3*0.2Ω = 0.6Ω,

Two of these strings in parallel give an overall effective resistance of 1/(1/0.6Ω) + (1/0.6Ω)) = 0.3Ω

The cells in series might very well deliver more voltage to the motors when they were new, but as they wore down the rise in internal resistance could reduce the voltage a lot. The cells would also run down at a relatively fast rate because the full current would flow in each cell, and the higher voltage would increase the motor current.

The cells in series/parallel would give less voltage to start with, but the cells would last longer, with a lower internal resistance.

My guess is that the 6 in series would be faster to start with. Depending how the motor is rated, it might even be too fast - buning it out. The other version would start slower, but last far longer.

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18. ### kissdom Thread Starter New Member

Jan 8, 2012
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wow scotty, thanks for the reply!

my motor's only specs on the packaging is just a 3V requirement, in this case I would use your multimeter method to further investigate the best efficiency for my motor.

For the time being, i have connected another set of 4.5V in parallel.

However, I am in a dilemma as to whether connecting the addition set of 4.5V(3x1.5V AA Battery) in series or parallel would run my motor faster?

I calculated the power(watts) for this two circuit and they both give the same values.

Picture shows circuit:

19. ### kissdom Thread Starter New Member

Jan 8, 2012
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thanks for the engenizer link Adjuster, very much appreciated.

I have since tried connecting the 3 additional battery in parallel, the result wasn't unchanged though the voltage remains the same.

There was a significant boost in RPM, I reckon it's because of the additional flow of current, given energizer's 5mA output, a parallel circuit would have 10mA flowing compared to 5mA for series circuit.

The difference between Parallel vs Series so far(Ignore motor and wire resistance since they are constant):

The parallel circuit current: 10mA
Resistance: 0.3ohm
Voltage: 4.5V

Series circuit current: 5mA
Resistance: 1.2ohm
Voltage: 9V

However, now that you mention the 1.2ohm in the series and 0.3ohm in the parallel being the difference between the circuit, can say that the parallel circuit has more potential of over-driving the motor?

20. ### kissdom Thread Starter New Member

Jan 8, 2012
12
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Correction for the top statement, I interpreted the website wrongly, my bad (http://data.energizer.com/PDFs/BatteryIR.pdf).

the current output for Engienizer is not 5mA, but lets put it that the motor takes 5mA as a reference