How Do I Convert DC Motor To Reverse With SPDT Switch?

[snub]

Here is my problem. I'm working on my '66 Corvette which uses two 12vdc electric motors to open and close the headlight buckets. These motors are 45 years old and parts are hard to come by. Someone has started reproducing these motors but they are $400.00 each. So I decided to reinvent the wheel, err, the system. My plan is to use a single motor mounted between the buckets with a driveshaft  running to each one.

I would like to keep the orignal 'up/down' switch as it is very classy looking. The only problem is that is  a single pole double throw switch which won't work with any motors I have come across. On the original motor (which is much like any power window motor of past or present), the case is grounded and their are two 'positive' terminals. Power to one terminal is CW, power to the other is CCW.

I have no idea how they wired these. I'm wondering if the wiring on a new motor can be changed to work like the old one. Or, if there is such a thing as a relay that would reverse the polarity of the wiring to the motor?


Plan B is to simply hide a DPDT switch under the dash near the old one.

Here is a diagram I made showing my current wiring. Please feel free to fill in the missing links on the bottom part.

[russ57]

i reckon it would be fairly straightforward, you'll need to use 2 dpst relays. One is wired to switch  positive and negative from the battery  'direct' to the motor, and the other to switch them 'reversed' . Use the SPDT switch to apply power to either relay as required.

I assume the spdt switch is momentary? how do you know when to stop applying power? the original motor may have had some end of travel detection. you may need to consider some sort of limit switch, so that power to the relay is removed when it reaches the end of its travel.

i could probably whip a simple diagram if you need.

regards

russell

[andyf]

Snub, DPDT relays are common, and reading down this page may give you a start:
http://www.distel.co.uk/DC_MOT_CON1.htm
It shows a DPDT relay with its coil energised/de-energised by a simple SPST push button switch. Your existing SPDT switch could be converted by using its common terminal and just one of the other two terminals, unless it is a momentary switch. 
As Russell says, limit switches would need to be added, maybe operated by cams on your drivehaft.

Andy
 

[John Swift]

Hi Snub ,

you can use two single pole automotive relays


 connect the motor between  the  common relay contacts

the negative supply to the normally closed contacts and the positive to the normally open contacts  (on both relays )

until a relay switches, both motor terminals are connected to the negative supply

when the existing switch  operates  one or other relay, the positive supply is connected to one  of  motor terminals


if the motor torque is not too great you  could just let the motor stall until you release the switch (assuming its normally off)

other wise  wire two limit switches , one in series with each relay coil


   John

[75Plus]

John, could you reduce the size of your schematic? As it is it is hard to see it all.

Joe

[John Swift]

Hi Joe ,

how much smaller do you need it ?  it just fits my screen

   John

attached picture  at 70% of original size

edit
PS  --  settings for my scanner my needs adjusting , had to install the software to scan the  image

reinstalled windows a few days ago   ,, expect it to take  a few more days to get thing back to normal

[Chuck in E. TN]

Try right clicking the picture and choose open in a new window. Works for me.
 
Chuck in E. TN

[snub]

Thanks to all for your help.
The original switch is momentary contact.  There were no limit switches in the original setup, so if one motor was working faster than the other it would still be trying to rotate while waiting for the other one. And this is what chewed up the drive gears.
There was a switch to let you know when the buckets were fully open. It was connected to a light on the dashboard that blinked until they were open.

Since I will only be using one motor ,and based on what worked before, do you think that I would need to use limit switches?

[DMIOM]

hi,

If you do use limit switches and can get two normally-closed ones, and they are of sufficient rating to carry the motor current, you can arrange them as in this sketch, with bypass diodes.  When you motor up to the stop the limit switch interrupts the current BUT the diode will allow the reversed polarity to motor you the other way back away from the stop

Dave

[John Swift]

Hi snub ,

with motors that powerfull , I'd fit limit switches ( one at each end of travel)

or if that's not easy, try adding a wirewound resistor in series with the motor to limit the torque

as a starting point, measure the motor current before you hit the end stops
and divide 3 by your answer to get a resistor value that drops 3 volts 

with the engine running the motor now runs on about 10.5V
both the torque and stall current will now be reduced 

to find the maximum current with the motor stalled
divide 13.5 v by the sum of the motor resistance and your added resistor
the wattage rating for your resistor  is given by  I squared R ( I = stall current, R = added current limiter resistance)


  John


PS    could you use a mechanical torque limiter as used in battery operated drills and screw drivers ?

[andyf]

Here's my contribution, based on John's first drawing and taking into account that your existing dashboard switch is momentary. Giving it a quick flip up or down will energise one or other of the relay coils. When the relay contacts close, +12v from the motor supply is fed back to the coil, holding it on until the limit switch opens, breaking the feedback so the relay switch opens and the motor stops.

If I've made some ghastly error, someone please correct me!



Andy

[Bluechip]

Bottom rail S/B  0V .. 

( Pedantic pillock ... nowt on the telly ... AGAIN    )

BC

[John Swift]

Hi Andy ,

the only problem I can see is if you selected a motor direction and then selected the reverse
before it reached the limit switch and the first relay dropped out

both relays would be latched on and the motor stopped between the two limits

sod's law predicts it will happen sooner or later

 you need to be able to switch off the 12v supply to the relays to reset the circuit
or  use two pole relays to interlock the relays to prevent this

  John

circuit added

[andyf]

Good point, John. Sod's law* would leave my version connecting both motor terminals to +12v, so it would stop, leaving the headlamps stuck halfway up and both relays latched on until the "+12v Motor Supply" was broken momentarily.

More thought required.

Andy

* For the benefit of our US cousins, Murphy has a close relative called Sod who also fouls things up, but whose name isn't mentioned in front of the children.

[snub]

Holy Cr*p....You guys are fantastic. I had a feeling that this would be the best forum to ask this question.

But stupid me, I just spent an hour trying to convert John Swifts excellent schematics into a diagram, I click on here and he's already done it. Ah, what the heck. I'll post it up anyways.

The only thing different ( that I can see), is that I have power going into terminal #30, and out of #87. I'm guessing it doesn't make a difference as it's only a 'switch'.

Thanks again for all your help!

[andyf]

John, great minds think alike  . I drew up exactly the same circuit late last night, after going to the pub and consuming a considerable quantity of beer, so I thought it best to leave it for checking in the morning. I suppose the two new relays RL1B and RL2B, which only handle the small current required by the coils of RL1A and RL2A, could be replaced with transistor switches, with the addition of protective diodes across those coils.

Snub, though it's more complex than your drawing and needs another two relays, John's latest version allows you to work the motor through your existing momentary switch. Good luck, and don't set the car on fire!

Andy 

[John Swift]

Hi snub,

on your diagram just swap terminals 30 and 87 so the motor is connected to the common terminal 30 on each relay
otherwise you will short circuit the supply !

before I posted the wiring diagram ,I had to downloaded the PDF data for an automotive relay to add the relay terminal numbers

if the switch is not allready connected to fuse , you could connect the switch common terminal to the motor supply fuse in the diagram so all the wiring is protected 

   John

[snub]

Thanks, John. After studying the schematic I find you are correct about terminals 87 and 30. I will correct my diagram so no one else fries something.

On another note, I was just thinking about a feature that most cars with power windows has. And that would be the automatic down on the drivers side. Just a quick touch of the switch and the window goes down and stops automatically. I will have to delve into this and see how it's accomplished. Might be something I could incorporate into my design.


In the mean time, I'm searching for a suitable motor. The original ran at about 10 RPM.  The new one would have to have dual shafts like the one pictured below. Finding one with the right RPM has been a struggle, but I guess theres no reason why I couldn't use a PWM controller.

[John Swift]

Hi snub ,

a speed control like UK1102 from Canakit.com could give you the way of controlling the speed
its available as a kit or ready made

using two pole relays as in the circuit in the  earlier post will keep the motor running until the limit switch is tripped

I'll have a look for the info , I know Lucas made them

    John