Reversable Motor with WC8, DPDT and STSP relays

[rossw]

do you believe it would be best to use spdt relays insteadnofnthe transistors? I do ave two TIP120s which I bought for another plan.

 
The TIP120 has an Hfe of 1000, so it has enough gain to do the job.
Maximum collector current is only 5A. If your motor takes 5A running, it will probably need more to start, and this transistor is already right at its limit.
It would work, but it really is pushing it.
 
I like the simplicity of two SPDT relays. It gives a whole slew of benefits over the more complex arrangement with a DPDT. It is also intrinsically "safer" in the event of a failure of the WC, because if both outputs are on OR off, the motor is off. It uses cheaper, more readily available relays, and if you use the inexpensive relay-driver boards, it is an "over-the-counter" replacement.
 
Where the DPDT relay for direction reversal, along with a series transistor or FET really comes into its own, is where you are using PWM for speed control.
I do this on my tracker controller -  fragment of the schematic produced below. It's more complex than you need, but quick explanation of some salient points.  D1 is the diode across the relay. The IRLZ14 is a logic-level power FET that is operated at several KHz to control the speed of the actuator from zero to full speed. D2 is across the actuator (motor), because without a relay to disconnect it, the back-EMF would destroy other parts.  R6 is a current-sense resistor, amplified by U2d, to provide feedback about the actuator current, it lets me detect high wind load, jammed actuator, etc.
 
In your case, the WC doesn't have PWM out and I really doubt the extra complexity is justified in the application.

 

[rfeyer]

Ok. Then Imwill return to using the spdt relays. I do have them already-A
arrived 2 days ago.
Just because the concept still bothers me: did i misunderstand the diode hacing to be placed across the coil, or is it just the wrong way around?

Rainer

 

[rossw]

Just because the concept still bothers me: did i misunderstand the diode hacing to be placed across the coil, or is it just the wrong way around?
 

 
The diode you indicated across the coil has the cathode (the end with the bar) to the negative side of the supply. That means when you turn the transistor on (which you couldn't actually do in the arrangement you indicated, with the collector to the +12V rail), the diode will start to conduct at about 0.6V, well below the relay coils operation voltage. Then, as there is no current limiting, either the diode, or the transistor, (whichever is lower rated) will let out the magic smoke.
 
Additionally, the diode indicated in the base of the lower transistor will prevent you getting any base current into the transistor, so you couldn't turn it on (even if it were connected correctly).
 
The 220R resistors are a bit low. They could allow 20mA output current from the WC, which might be ok for one, but is starting to push it for two. You'd be better served using signal transistors with an Hfe of 100 or better. (A BC337 is typically around 200), meaning to drive a relay coil that takes 100mA, will require less than 0.5mA of drive current, which is easily satisfied with even 4K7 base resistor.

 

[rfeyer]

Do you mean I should replace the Transistors or Resistors?
 
I looked up the BC337 at mouser and it describes it as a transistor, I think. Is that what I should exchange for the Tip220 transistors?
 
Rainer

 

[rossw]

Do you mean I should replace the Transistors or Resistors?
 
I looked up the BC337 at mouser and it describes it as a transistor, I think. Is that what I should exchange for the Tip220 transistors?

 
I mean all these things work in concert.
The TIP120 is a much higher current device (about 10 times the current a BC337 will carry), and a higher gain device (being a darlington pair that's to be expected).
The TIP120 is way way way overkill to operate a small relay.
The BC337 is way way way underkill to operate your motor.
 
If you used a BC337 to operate a relay, OR a TIP120 to drive a motor, the 220 ohm base resistor is too small to afford any real protection to either the transistors or the WC output. I would think a 1K resistor is the smallest you should be considering in this application.

 

[rfeyer]

The 220R resistors are a bit low. They could allow 20mA output current from the WC, which might be ok for one, but is starting to push it for two. You'd be better served using signal transistors with an Hfe of 100 or better. (A BC337 is typically around 200), meaning to drive a relay coil that takes 100mA, will require less than 0.5mA of drive current, which is easily satisfied with even 4K7 base resistor.

rossw,
I was going by what you had originally posted - so that means I am misunderstanding what you meant by '220R resistors are a bit low' - then I thought you suggested replacing them with a BC337, though that I found was a transistor.
 
Rainer

 

[smmaxxx]

rossw,
 
 I like your method of controlling direction with the SPDT relays and it's advantages. I have been using 1 DPDT and one other contact for power for the job but I will change over to your method. Thanks !
 
smmaxxx

 

[Tschmidt]

A little piece of engineering trivia to keep in mind when thinking about the pros and cons of using combo SPST/DPDT vs dual SPDT relays.

When using dual SPDT relays you are also dynamically braking the motor when you turn it off. This has the advantage of quickly bringing the motor to stop, very useful in robotic applications. However during breaking the motor is acting as a generator into a short circuit. Need to make sure the relay contacts are up to the task.

/tom

 

[rossw]

A little piece of engineering trivia to keep in mind when thinking about the pros and cons of using combo SPST/DPDT vs dual SPDT relays.

When using dual SPDT relays you are also dynamically braking the motor when you turn it off. This has the advantage of quickly bringing the motor to stop, very useful in robotic applications. However during breaking the motor is acting as a generator into a short circuit. Need to make sure the relay contacts are up to the task.

/tom

 
Two additional diodes and you can have 4 functions between the two:
Off (braked)
Drive fwd
Drive rev
Off (unbraked)
 
You can let a motor coast-down and stop (without the regenerative braking) for  a few seconds, then revert to "off/brake". I didn't draw it this way for simplicity, but it's easily done. (Diodes between +12V and the relay NO contact on each relay. Note, diodes must carry full motor current, so size appropriately)

 

[rfeyer]

Out of curiosity - what is the IRLZ-14 in your diagram? 
 
Rainer

 

[rossw]

Out of curiosity - what is the IRLZ-14 in your diagram? 

 
Logic-level (drive) power FET

 

[rfeyer]

TY!

 

[rfeyer]

Thank you ALL for your all your help!
I know it must have been trying at times, but I did learn the basics of how to read a schematic, some minor component understanding, and, soldering (and re-soldering) from you and some help from another board.
I finished the board and it actually works! Go figure (though it took 3 tries).
I have attached a pic of the board - again TY for hanging in there with me!

Rainer
 

 

[rossw]

Great news then. Always good when you can learn something AND have a working project!

 

[CAI_Support]

Have a lot of fun, too!