Need relay advice for proportional smoke...

[James-06scl]

Hi,
I would like to use a relay to turn on the fan in my smoke unit. Rather than wire the fan directly to the drive motors, I would like to have it come on at approx 1 to 2 volts at the main drive motors, but be fully powered by a separate power source.
From what I gather a 5v relay requires approximately 5 volts to energize the coil and switch on the load, so would a transistor be required to pick up the low voltage at say 2 volts to switch on the coil in the relay? Also the relay / transistor would have to energize at 2 volts but be capable of receiving up to 7.2 volts.
Is this possible?
Any help or advice would be greatly appreciated...
James
Tamiya Tiger1

[Saracen]

I wouldn't use a relay as the current needed to keep the coil operating is likely to be a significant drain on the battery.

I'd use a logic level FET - something like an IRL520 - http://www.vishay.com/docs/91298/91298.pdf

This should switch on at between 1V and 2V, and should be able to control the fan directly.

I don't know how the power is controlled to your motors, but I doubt it is a nice smooth rising voltage. If it is PWM, then you will probably need an RC network attached to the gate of the FET to smooth out the signal coming from the motors, so that the FET doesn't keep switching on and off.

Adrian.

[strmnd54]

If you want the fan to be " off" when the tank is idling , starts around 2.0 Volts and remains proportional to the voltage at the motor terminals ; a simple modification of the H.L. smoker proportional board might just do that ...
Furthermore ; the board is using a power transistor to drive to fan ; so the motors are not directly driving it ...
Here's a schematic of the board and the recommended mod...
Cutting off R2 10 KOhms will disable the fan at idling and the diodes in serial with the motors should raise the "turn on" of the fan around 2.0 Volts ....

[James-06scl]

Saracen, thanks for the link, this will come in handy!

Strmnd54, that very much describes what I would like to do.
I may opt for full power to the fan at initial acceleration, rather than having it run continuously faster with the drive motors. I personally find that a surge of smoke at start up and initial acceleration is more realistic than a continuous stream of smoke emitting from the exhaust. I've already rigged up a micro switch with the servo for my Tiger's machine gun to provide for a momentary power up of the smoker fan when desired. By switching on the smoke unit a minute or so prior to switching on the tank, I get a very realistic large puff of smoke in conjunction with the engine start sounds, simulating the same effect as a real cold start. Your excellent diagram will have me well on my way to getting the same effect for initial movement, gear changes, etc.

Thanks to both of you for taking the time to help me out!
James

[streetfighterjeff]

to damn clever by half the lot of you i cant read electrical schematics, when i have to do anything with electrics, my dad replaces all the leccy symbols with piping ones then i'm ok. i'm a pipefitter by the way
jeff

[James-06scl]

http://www.youtube.com/watch?v=vXr603aL ... e=g-crec-u