Reversable Motor with WC8, DPDT and STSP relays

[rfeyer]

Well,
 
I am hoping that one day I will understand enough of this to actually make something with only little help.
This project is a 'Getting my Feet Wet' project. I have purchased several WC8's and would like to utilize them more and more.
 
So, if not too much bother, i will read an learn and ask - and hope my fingers will not be singed by the time it is all said and done.
 
I will give it a college try (college was 33 yrs ago) and start making a diagram - then post here for corrections, if that is OK?

Rainer

 

[rossw]

He who has made no mistakes....
 
.... has probably made nothing!
 
We look forward to your "college try", and to peer review your proposed diagram!

 

[Tschmidt]

I am hoping that one day I will understand enough of this to actually make something with only little help.

 
The best way to learn how do to do this stuff is exactly what you are doing. Try something, figure out why it doesn't work the way you expect, modify, rinse and repeat.
 
Can't speak for Ross but I was not born knowing how to do hardware and software design.
 
On occasion you will let out the magic smoke, it is good idea to have more parts on hand then you think you will need - just in case.
 
Enjoy your project.
 
/tom

 

[rfeyer]

Here it goes - first graphical, to see if it is close, then I will try actual diagram:
 
But, I do not understand the Diode across the core? The core is not really being used since the motor is supposedly getting feed from the COM?
 
 
 

 

[Tschmidt]

Here is a drawing of how to use a transistor to control a relay. Just happed to be the first one I found using Google search "transistor relay driver."
 
http://homemadecircuitsandschematics.blogspot.com/2012/01/how-to-make-relay-driver-stage-in.html
 
The diode is used to clamp the voltage generated by the relay coil when the transistor is switched off.  That action is just like what happens in the ignition coil in your car but on a much smaller scale. Energy is stored in the coil when it is switched on, it has to go somewhere when switched off.
 
/tom

 

[rfeyer]

Great!!
 
Looking at the link, which actually has tons of useful information, is my drawing above correct?
I am actually not sure why the relay does not need core activation, but going by previous instructions from here
 
Rainer

 

[Tschmidt]

Great!!
 
Looking at the link, which actually has tons of useful information, is my drawing above correct?
I am actually not sure why the relay does not need core activation, but going by previous instructions from here
 
Rainer

I  don't understand your drawing, it looks like you are using a transistor to switch the relay coil and another to switch the motor directly. 
 
If the symbol with the X is a DPDT relay used to reverse the motor, that part is fine. The upper transistor needs to drive a second relay that controls power to the motor, just like a light switch.
 
/tom

 

[rfeyer]

/tom and others,
 
this is what I thought I understood:
-the two SPDT relays are apparently not large enough; instead, I was to use Transistors.
-I thought the transistors are to act as a switch, allowing the small amp/V TTL from WC8 to allow current flow tot he motor and tot he relay.
 
But i certainly still do not understand the following:
Why can't the core of the main relay not be utilized? Wouldn't activation of the core not reverse the polarity in your H-Bridge instructions? And, if core is NOT utilized, why do we need a diode to stop reversal of flow?
 
Rainer

 

[Tschmidt]

Here is a quick chicken scratch (tomcad) schematic using two of the 12V 5A DPDT relays you have to control the 12V 5A motor.

Things you need to keep in mind when doing this kind of control:

1) What is the current and voltage capability of WC output bits?
2) What is the voltage and current required by the motor?
3) What is the voltage and current carrying capabilities of the relay contacts?
4) What is the voltage and current requirements of the relay coil.

Relay contacts need sufficient ratings to carry the motor load.
Since the WC is not capable of driving the relay coils directly need to buffer the outputs.

/tom

 

[rossw]

Here's my "chicken-scratchings" to do the same with two SPDT relays,
 
 

 

[rfeyer]

TY BOTH!!!
Just received the motors so will play a little with them, then try to disect the diagrams which look simple enough (but I can probably screw up). Since I have already ordered the SPDT which will arrive this week, I will try to use them if they work.
 
Still interested, though:
Why the diode across the main DPDT core if original instructions use COM pins, which means the relay core never gets activated (I am not asking to think I know more, I truly don't understand).
 
Again, TY both!!

Rainer

 

[rossw]

Still interested, though:

Why the diode across the main DPDT core

 
As someone said earlier (Tom?) - the "coil" of a relay is a lot of turns of fine wire, on an iron core.
When you apply power, you create a strong magnetic field (to pull in the relay mechanism and operate the contacts).
 
The damage is done when you turn OFF the power. The magnetic field collapses and as it does so, it creates a very short (but high voltage) spike.
This spike will (sooner or later) damage other electronic devices. Putting the diode *AS INDICATED* across the coil gives this spike somewhere to go. As the diode starts to conduct at about 0.6V (the spike is the opposite polarity to the power originally applied), it will in effect completely shunt this spike and prevent it doing any damage.
 
Diodes like this should go across any inductive load you are switching, which includes long wires to heating elements, and solenoids and motors etc.
 
In the two examples above, we don't put diodes across the motor - but the reasons are different.
In the first example (using a DPDT and SPST relay) - because we're using relay contacts that actually completely open-circuit the motor, the reverse spike has no path to get back to the rest of the circuit.
In the second example (using SPDT relays), when the motor is not running forwards, and not running backwards, it is actually short-circuited through both relay contacts. This not only prevents the high voltage spike (by shorting it), it also provides regenerative braking to the motor (to make it stop more quickly, and to make it resist turning due to mechanical input).
 
Please note, both examples assume a brushed DC motor. Neither is suitable for an ECM motor that has internal electronics to make it run. Applying reverse polarity to an ECM will generally destroy it instantly.

 

[kurtmccaslin]

If you are not a fan of soldering transistors (I am not) you might pick up an Elk 924 sensitive relay.    It has a circuit board with the transistors, diodes, and relays already mounted.   Ready to connect your motor.

 

[rossw]

Or, as I said on the previous page, for $6, you can get one of these... optical isolation, TTL-level drive, 12V operation, two SPDT relays with connectors and everything...

 

[rfeyer]

Yup, I appreciate those thoughts as it shows that i certainly do not know what I am doing, but I truly am looking at this as a learning experience - so, the soldering is no problem, I like to get my hands on this to see how diodes and transistors actually work, and so far I am getting good education thanks to you all!.
 
Of course, that said, /tom and RossW did most likely foresee that the motor may not be reversible - i was going on the fact that it was described as a car seat or window motor - which would indicate it's reversible, but so far it does not seem to go two directions but has 4 wires.
So, if I can not make it go backwards tonight, i will have to look for two new motors. Not a biggy, it's cheaper than college tuition for an electrical degree (don't worry though, i am not trying to go that far)
 
Rainer