Time delay relay switch

[fitzyno1]

Hi all, i was wondering if you could help me out. I need a relay time delay switch for a little 12v geared motor, it has to be as cheap as possible. Now the problem is, i have that motor hooked up to a on/on toggle switch, the motor will go forward until the toggle switch is activated, and in doing so, that reverses the polarity. I need something to slow down that quick change in polarity, because with the quick jerk in polarity it will ring the plastic cogs in the geared motor. I hope you understand what i need, i dont want to bore you by going into great detail. But if needs be i'll explain the hole device. T/Y

 

[jpanhalt]

How about using an On-Off-On DPDT toggle. That is, center off. Then you pass though off before reversing direction. A simple delay to actuate the relay probably won't help, unless it correspnds to "off".

John

 

[fitzyno1]

Thanks for the quick reply jp,
I can't see that working, the toggle switch is on the motor (5rpm), when that toggle switch hits a stationary object it will switch it off, because it will be travelling very slowly, and not make it past the off setting. If the motor itself was 5 rpm, i might make do with the toggle switch i've got, but the motor spins fast and the little plastic cog inside the gear box reduce the speed. If you know what i mean. T/Y.

 

[fitzyno1]

I know it may be a wild shot, but would something like this do...
**broken link removed**

 

[jpanhalt]

I think you could do it with an H-bridge and have a delay (resistor in series, capacitor to ground) for each signal line. You can read about H-bridges in many places, including Wikepedia.

That would be a bit more complicated. Is it something you would want to go into further. At a minimum, you will have 4 transistors, several resistors, 2 capacitors, and maybe 4 diodes.

John

 

[fitzyno1]

LOL John thats a bit too techanical for me, i'm just the type of person who buys a tiny plastic box with all that stuff in it, and solder a few wires. Is there nothing i could buy thats not a ridiculous price? T/Y

 

[jpanhalt]

It's the delay on change that creates the problem.

There might be such a relay, but I could not find it on a quick search. There are delay-on and delay-off relays, but I did not find any at DigiKey that were delay on change like what you would need to reverse. I suspect it is doable, but I believe the electronic version is easier and probably cheaper, once you get over the intimidation aspect. We are not talking speed control, just direction.

John

 

[fitzyno1]

John see the photo of the H-Bridge, that is in the Wikepedia link. There's one of those on Ebay for sale, could i use one of those in some way. There are around 16 pins, i'm lost there. T/Y

 

[jpanhalt]

In my experience, you will find one-of's electronic components at prices as high as or higher in ebay than from a reputable vendor.

Without regard to source of components, if you are willing to take it on, I am sure the members of this forum, including myself, will be anxious to help you. Don't jump on the ebay item. Figure out what you need to do first, then get the parts.

First, let's start with your motor. It is 12 Volts. Do you know how much current it draws or it's power rating? Any information in that regard will help, including just its physical dimensions.

John

 

[Mikebits]

For part vendors see my latest post.

 

[fitzyno1]

Mike thats great info. on finding the part a person requires. Thanx. I know what i want a part to do, but I don't know what i'm looking for. If that makes any sense.

John i think your right about Ebay, i just find that it's an easy place to buy stuff.
The motors sticker says...
D.C GEARED MOTOR. 12DCV
P/N:TG-38123000-900K

I think it's a 6amp, because i run it with a mains 6 amp power supply. The motor is 30mm long and 32mm dia. Gear box is 30mm long and 36mm dia. (60mm long altogether) It has a 6mm shaft. T/Y

 

[fitzyno1]

Has nobody got any ideas for me?
Would that speaker relay work? The link is in my 3rd post?
I just need something that will delay power to the motor when started up.
T/Y

 

[Torben]

Hi Fitzy,

Maybe it would be best if you could go ahead and describe the whole device. It's often helpful to know what the whole thing looks like; it can bring up other solutions to particular problems within the device which might be missed otherwise.


Torben

 

[jpanhalt]

Sorry for the delay. I was tied up on other matters. The attached schematic shows the direction I was thinking. S1 is the current toggle. R1C1 and R2C2 produce delays in bringing the enable pin high. EnA is for one direction; EnB is for the other. Since the capacitors are shorted to ground, the motor will stop until the appropriate enable pin crosses its threshold. The L293 is basically any appropriate H-bridge controller.

John

 

[fitzyno1]

John, that is perfect.
Yes, as soon as the toggle switch ingages the reverse current it delays the power to the motor.
Where do i get the R1C1 and the R2C2 delay circuits? Are they the capacitors?
Is there a need for a H-bridge controler, if so what is it needed for?

 

[jpanhalt]

R1 and R2 are resistors; C1 and C2 are capacitors. The product of R and C is the time constant

Similarly, in an RC circuit, the time constant τ (in seconds) is:
where R is the resistance (in ohms) and C is the capacitance (in farads).

You cannot ignore the input current to the controller in calculating the time to charge to threshold voltage. Try doing the calculation or look up one of the sites that does it for you. You will understand the circuit much better, if you do that. For starters, 100K resistor and 10 uF capacitor will have a time constant of 1 second. If you just want to go empirical, start with something like that. Then, reduce the capacitor or resistor to get the time you want. Maybe something like 10K and 10 uF will work.

You will need an interface between the RC delay and motor drive. The controller/bridge functions in that way. Without the interface, the resistor would have to pass the current to run the motor, so it would be very limited in size. That in turn would require a very big capacitor to produce any sort of reasonable delay. Moreover, when the big capacitor was grounded, you would get quite a spark at the switch contacts and wear them out very quickly.

John

Edit: I just looked at the L298. The enable pins look like they draw about 30uA. That would give a 3V drop with a 100K resistor. Go with resistors in the 10K to 33K range and see what happens. The device can output 2A DC, so you may not need a bridge, depending on what your motor draws. See the datasheet for how to connect.