Revised Schematic

[-=CrAnSwIcK=-]

Hey guys, sorry for double posting...but no one seems to want to help with my project...I have revised my schematic and corrected some errors i think i had...now somebody told me this can be done this way...but everybody keeps telling me it's not a good idea, or that it won't work at all...but i know it's possible it's just a matter of getting it right...

I have: (what i was told i needed...if i'm missing something, speak up)

7.2v DC input
2.4v DC secondary power
general purpose pc board
50v 1A diode
100k potentiometer
5v mini spdt relay

I'm no expert so if my drawing is incorrect please let me know
**broken link removed**

 

[audioguru]

It looks like you are using a relay to switch the supply voltage for the motors from 2.4V to 7.2V when the pot is turned up.
But the 100k pot should be a 1k pot.
The pot might burn out when it is turned so that its resistance equals the relay coil's resistance.

 

[-=CrAnSwIcK=-]

i must have made an error...i'm trying to make it so the relay switches the circuit from 7.2v to 9.6v...if this can't be done, i can make the + side only going to the relay and just have a 9v battery as the secondary power source, and have the relay switch to the 9v, while cutting off the 7.2v source entirely, i was hoping the first way would work to save weight, as this project is for a remote controlled car...and i have a bunch of pots, but only one of them says 100k...the rest are little blue ones with various types of adjustment knobs...

 

[audioguru]

To save power and weight then replace the relay with a Mosfet to switch the 7.2V then use another Mosfet to add 2.4V for a total of 9.6V.

Use an electronic circuit instead of a pot to do the switch control.

 

[Leftyretro]

Well I was the one trying to help a little in your orginal posting. It's somewhat hard to help a raw beginner with circuit errors, but not because I don't want to help.

Present problems with your revised schematic:

You did correct the reversed polarity of the secondary battery, however the way the circuit is drawn all it does is switch on the low voltage battery such that it will be in parallel with the primary battery. This will NOT raise the total voltage as seen by the motor at all and has the risk of damaging the low voltage battery.

You need to wire the relay contacts such that battery #1 and #2 are series connected so the motor sees 9.6 volts (series total of batteries #1 and #2) when the relay is activated. The diode looking thing wired to the relay coil looks strange. A diode isn't required, but some kind of switch contacts are to turn the relay on and off.

This help any?

Lefty

 

[-=CrAnSwIcK=-]

yes it does, thank you...now the diode is another thing i wasn't sure of, the person that explained it to me told me it was supposed to keep the 2.4v boost from happening in reverse...though i wasn't sure exactly where to put it in the diagram ....

so now if i put the power sources in series, when the relay is switched, will the main circuit board of the car still be running on 7.2v will the motor is getting 9.6v?? and can i put the two power sources in parallel with a spdt relay?

and this part i didnt really understand..."A diode isn't required, but some kind of switch contacts are to turn the relay on and off." the relay is supposed to switch when a certain voltage is reached...so what are switch contacts??

could someone maybe redraw my schemtic to show how this can be done??

and another thing, how can i adjust when the switch is switched based on input voltage when using MOSFETs as switches??

 

[Leftyretro]

yes it does, thank you...now the diode is another thing i wasn't sure of, the person that explained it to me told me it was supposed to keep the 2.4v boost from happening in reverse...though i wasn't sure exactly where to put it in the diagram ....

so now if i put the power sources in series, when the relay is switched, will the main circuit board of the car still be running on 7.2v will the motor is getting 9.6v?? and put the two power sources in parallel with a spdt relay?

and this part i didnt really understand..."A diode isn't required, but some kind of switch contacts are to turn the relay on and off." the relay is supposed to switch when a certain voltage is reached...so what are switch contacts??

"...so what are switch contacts??[/QUOTE]"

What is going to tell the relay to turn on for 'turbo' mode ?

"so now if i put the power sources in series, when the relay is switched, will the main circuit board of the car still be running on 7.2v will the motor is getting 9.6v?? and put the two power sources in parallel with a spdt relay?"

Everything should stay wired to the +7.2 volt power except the + motor wire. The relay common contact will wire to the + motor lead and select the SERIES connected +9.6 tap or the 7.2 tap. The normally closed relay contact should wire to the +7.2V source and the normally open contact wires to the +9.6 source.

 

[-=CrAnSwIcK=-]

thanks for the quick reply, i think i undertand how the power sources should be wired, but i'm still a little confused about the switching, and the diode...where should the diode be to prevent the turbo from turning on in reverse...and also the speed of the car is proportional to pull of the trigger on the transmitter...so it starts at 0v and goes all the way up to 7.2v i'm assuming...i was under the impression that the relay would switch when relay's threshold voltage was reached, and that this threshold voltage could be adjusted via potentiometer, to switch at 5v, 6v or 7v, or whatever...am i missing something??

 

[Leftyretro]

thanks for the quick reply, i think i undertand how the power sources should be wired, but i'm still a little confused about the switching, and the diode...where should the diode be to prevent the turbo from turning on in reverse...and also the speed of the car is proportional to pull of the trigger on the transmitter...so it starts at 0v and goes all the way up to 7.2v i'm assuming...i was under the impression that the relay would switch when relay's threshold voltage was reached, and that this threshold voltage could be adjusted via potentiometer, to switch at 5v, 6v or 7v, or whatever...am i missing something??

Yes your are. You need to understand how the existing motor speed controller works, electronically. It sounds like you expect your new relay to be able to sense when the speed controller's output is full on and then switch in the 'turbo' battery, but there is no real easy way to do that without additional electronics. Trying to turn on the relay just by sensing full speed controller output voltage requires more then just a resistor, relay and diode.

I suggest you ask around your car friends and try to get a copy of an exisiting circuit that actual works. If you find that and can post it possibly we can help or comment on it. Otherwise we just are not going to make any progress proceeding like we have been.

Good luck
Lefty

 

[audioguru]

DC electric motors don't use a variable voltage for speed control, because the torque would change too much. They use pulse-width-modulation to control the speed of the motor.

 

[-=CrAnSwIcK=-]

yes, i know it's not variable...it goes in steps or stages, and the capacitors on the motor are to smooth out the transitions in between where there would otherwise be a instant of power loss to the motor...but i figured i could test the resistance with a multimeter and adjust the pot by trial and error till i got it where i wanted it...

 

[audioguru]

The voltage to the motor is not in steps. It is pulses of full power at a high frequency and the width of the pulses is changed to change the speed of the motor. Wide pulses is max speed and narrow pulses is slow speed without losing much torque.

A relay might activate when the pulses are wide and the motor is at max speed.

The tiny capacitors across the motor cannot smooth out the pulses to the motor because they have a very low capacitance. They are a radio filter to keep the motor from interfering with the RC radio receiver.

 

[-=CrAnSwIcK=-]

i have a larg capacity cap on my motor, it's supposed to do the smoothing thing....it's an upgrade...did i get gipped??

 

[audioguru]

i have a larg capacity cap on my motor, it's supposed to do the smoothing thing....it's an upgrade...did i get gipped??

The PWM frequency is high and the motor has inertia so the motor runs smoothly without a big capacitor. If the PWM circuit is any good then the motor gets the full supply voltage at max pulse widths.

Some cheap RC PWM circuits use a frequency of only 50Hz which causes the motor to jerk and get hot at low speeds.

 

[-=CrAnSwIcK=-]

ok, but can i adjust the relay to switch when i want it to, through trial and error...or is this method unlikely??