Need help building a proportional circuit for my RC Tank

[Se7eN]

Hello everybody, This is my first post.. So please be kind
I am a novice when It comes to building electronic circuits, but I do know the basics and know a few components used in circuits (Caps, resistors, etc..)

I need help in designing a circuit for my RC tank. I require a circuit that will allow me to run a 5 volt fan at 3.5 volts and have the ability to add voltage to the circuit to combine and increase the voltage to the fan via two variable voltage high speed DC motors. (both run at full 7.2volt) 7.2 volts would be what I would want the fan to run at when motors are running full speed then when the motors have stopped the fan returns back to 3.5 volts.

I hope I explained what I need as best as I can, any help would be appreciated

-x se7en x-

 

[(*steve*)]

Do you mean that you want to run a 5V fan from your 7.2V batteries, and vary the voltage the fan gets from around 3.5 to 5V?

A simple solution to this is an LM317 which makes about the simplest variable voltage regulator. The trickiest bit is calculating the value of a pair of resistors which will give you the voltages you require.

 

[Se7eN]

Do you mean that you want to run a 5V fan from your 7.2V batteries, and vary the voltage the fan gets from around 3.5 to 5V?

A simple solution to this is an LM317 which makes about the simplest variable voltage regulator. The trickiest bit is calculating the value of a pair of resistors which will give you the voltages you require.

Thanks for the reply Steve,

What I exactly need is this, I need to be able to drive the fan @7.2 volts so that the fan mimics the voltage that is given to the DC motors. However when there is no power going to the motors. I don't want the fan to stop rotating I want it to return to a voltage of around 3 ~ 4 volts and stay there until I power the motors again. I think this is called a proportional circuit.

I have been fooling around with a very good java applet that simulates circuits http://www.falstad.com/circuit/. I have created a circuit but I don't know if it is safe as I don't want to damage anything in my £200 Tiger tank hence why I am here. I have posted my question on another forum and did receive an answer, but it was vague and a bit cryptic to say the least
http://www.falstad.com/circuit/#$+3...352+1+0.805904783 o+11+64+0+35+20.0+0.4+0+-1

This rather huge link with take you to the site I mentioned so that you can see for yourself what I am trying to do

Thank for you help
PS.
On the simulation site there is no option to use motors or fans in the circuit you design, So I have had to change a few things. The lamp = Fan and the two variable voltage sources are the motors

-x se7en x-

 

[(*steve*)]

Nope, I still don't understand.

Lets start with the basics...

You have a fan. What voltage does this fan nominally operate from? What current is required?

What power source(s) do you have? (what voltage and what current can they supply?)

What do you want the fan to do?

 

[jackorocko]

So then you want to run the fan past it's max rating? Why is it so important to run the fan @7.2V?

 

[Se7eN]

Nope, I still don't understand.

Lets start with the basics...

You have a fan. What voltage does this fan nominally operate from? What current is required?

What power source(s) do you have? (what voltage and what current can they supply?)

What do you want the fan to do?

Ok, this is what I have:
Fan is a 5volt 40mm. It is know to be safe to overdrive the fan to 8volts. Current rating I'm unsure about maybe 400ma ~ 650ma? (could be more could be less) Power source is a 7.2 volt 3300mah battery pack.

So then you want to run the fan past it's max rating? Why is it so important to run the fan @7.2V?

The reason why I want to do this is because this whole thing is part of a device called a ''smoker'' or fogging unit that I have made for my RC tank. The main reason for running that fan at 7.2 volts is so that the air flow is increased in my smoke unit thus pushing more fog out of the exhaust system. but when the tank is moving slowly or is static, the airflow drops the fog/smoke output drops therefore creating a much more realistic effect.

So...
I want the fan to spin at 7.2 volts (the max allow by my power supply) when the tank is moving in any direction at full speed. when the tank slows down, the fan slows down.

so if motor = 5volts then fan = 5volts
But if motor voltage drops below 3 volts the fan stays at 3volts and does not decrease any further when motor voltage is less then 3 volts.
This is very important as I require the fan to spin at all times due to a small heating element that I have installed in my smoker box which is used to atomize the smoker fluid which in turn creates the fog (similar to party fog that DJ's use. Same principle, just on a smaller scale)

It really does sound more complicated than it is to be honest. But I know it can be done.
-x se7en x-

 

[jackorocko]

Well it goes against everything I was ever taught to overdrive a component out of spec, it will surely have a negative effect on the life of the fan. But, if it was me, I would find a way to sense the voltage and then have a uC or w/e to drive the fan with PWM based upon the sensed voltage to the motors.

Someone else will certainly chime in if there is a 'discrete' way of doing it.

edit: to expand upon my idea a little more. You have a comparator, ref V @ 3V. When the output of the comp. is negative or zero, the fan would be driven at 3V PWM. As the input voltage of the uC rises to a positive number the PWM output of the uC increases proportionally to the full load.

 

[(*steve*)]

I would look at something like a PICaxe. It can sense (via a low pass filter) the voltage across the motor. You write software to drive the fan by PWM, ensuring that the mark/space ratio does not fall below 3/7.2.

You would need a small 5V regulator to provide 5V for the PICaxe, some capacitors for filtering, and a logic level mosfet to switch the power to the fan.

If you're clever you could even detect changes in the power being delivered t the motor and increase the power to the fan when this happens so that it produces more smoke when starting than when travelling at a constant speed. You could do the opposite when it was slowing down.

 

[Se7eN]

Thanks for you replies guys, but It still doesn't really help me
I have managed to find the original diagram that was show to other RC enthusiasts regarding the problem I have. I have replicated the circuit but It doesn't seem to work the way I want it too. Please have a look at the diagram and see if you think its valid.

PS. disregard the heating element, That is powered by a seperate source and is not part of MY circuit.
-x se7en x-

 

[jackorocko]

Thanks for you replies guys, but It still doesn't really help me
I have managed to find the original diagram that was show to other RC enthusiasts regarding the problem I have. I have replicated the circuit but It doesn't seem to work the way I want it too. Please have a look at the diagram and see if you think its valid.

PS. disregard the heating element, That is powered by a seperate source and is not part of MY circuit.
-x se7en x-

But that design you just posted will not work the way you want it to. There is nothing variable about the design. You need to have some form of logic to do what you want. The easiest way is to use a microcontroller to wait for a specific condition and then act upon that condition accordingly.

Steve and I both guided you down the same path. His idea may be better. He has years on my experience, but they both conclude the same thing. You need some form of logic to do what you expect.

 

[duke37]

The regulator has given me an idea.
Use an LM317T set up to give 3V but instead of referencing it to ground, reference it to the motor voltage. When the motor voltage is zero, it will give out 3V, and when the motor voltage rises, it will rise also.

 

[Se7eN]

Thanks for all the help, I some how managed to get the circuit to work, I was able to add the voltage by routing the motor voltage directly.
Seems all I needed was a ground in the right place

Using my multi meter voltage at the regulator was : 4.96v.
Voltage at the fan was...4.96volts

Voltage at the fan with the motor running at full speed was 6.98. Exactly what I wanted

-x se7en x-