how does a rear windshield motor reverse polarity?

Thread Starter

eire

Joined Apr 21, 2014
2
Anyone know how a rear windshield motor runs for a few seconds and at the end of its 180' rotation, changes dc polarity to go back? Is it a common switch or what? I am working on a yard ornament and would like it to do that.. for holloween! can you tell I'm bored..
 

alfacliff

Joined Dec 13, 2013
2,458
the ones I've seen use a reversing switch or mechanical linkage. gthe ones with the switch dont reverse current, they usually use an spdt switch to use one of two windings.
 

shortbus

Joined Sep 30, 2009
10,045
They still work that way. Though mostly now days the linkage is under the cowl panel and can't be seen. The only ones I've ever seen that reversed direction in the "motor" were the really old vacuum wipers.
 

BillB3857

Joined Feb 28, 2009
2,570
We had a 1973 Ford Country Squire that reversed the motor to park the wipers below the cowl. Normal operation kept them above the cowl, but when turned off, they reversed and a special linkage took them lower.
 

ErnieHorning

Joined Apr 17, 2014
65
I believe all of the ‘Hidden wiper” designs in the 70’s would reverse the motor. A cam inside the gearbox would lift the electrical contacts, which would stop the motor at the park position.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
The advantage of continuous run in one direction is the elimination of the high inrush as you reverse the motor, especially on immediate reverse where the motor has not yet stopped in the opposite direction, very frequent operations will greatly heat the motor.
Max.
 

ErnieHorning

Joined Apr 17, 2014
65
The advantage of continuous run in one direction is the elimination of the high inrush as you reverse the motor
Now days I would use a FET H-Bridge with PWM and a microcontroller. But…

Using the following circuit with carefully placed switches, the motor would slow down to almost a full stop before it could be reversed. The DPDT switch could again be part of a DC relay that could be controlled by 555 timer circuit.

A stable power supply voltage would be critical for this to work reliably but it would be a very simple circuit.

 

ErnieHorning

Joined Apr 17, 2014
65
Welcome to the forum, Ernie.
Thanks.I normal frequent Christmas Light forums but I’m heavily into electronic design, both in my day job and my hobbies.If you do a Google search using my username, you’ll likely find all of those places and some of the things I’ve done.
 

Thread Starter

eire

Joined Apr 21, 2014
2
So I looked over the one on the back of a salvage yard and it seems to have wires going into the housing, a diode on the surface and wires then going to the motor.. must be magic inside the housing. not sure if I want to buy one just to rip it apart..
front wipers use constant one direction and mechanical arms.. not the back. they figured a smooth working reverse polarity somehow?
 

djsfantasi

Joined Apr 11, 2010
9,156
Do you have the make or model number of the wiper motor? What car and year did it come from?

Are you stuck on using this motor or can you use a front wiper motor? There are many references online for a front motor. Look at Scary Terry's web site.
 

ErnieHorning

Joined Apr 17, 2014
65
I did some searching on the net for rear wiper motors. It looks like they still turn one direction and has the same wig-wag output, it’s just more compact and there’s no need for linkage since there’s only one wiper.

If you still want to change directions of a DC motor and do it totally automatic, here’s a simple way to do it. The Halloween guy’s may already know how to do this but it seems pretty simple. This should be applicable to linear or circular motion. I tried to find something on the net but couldn’t find one that was simple and had a decent schematic, so I did my own.

It uses one DPDT relay as a temporary latch and two momentary limit switches. These could be straight push switches or the edge style that you roll over (micro switch). When it starts out, the relay is off and supplies power to the motor that must make it travel in the direction of Switch 1. When the Switch 1 limit is pressed, it supplies power to the relay which causes the top relay switch to latch the relay in a permanent on condition. This also causes the bottom switch to reverse the power to the motor. When limit Switch 2 is pressed, it removes power to the relay and it returns to turning the motor in the original direction. The will repeat endlessly.

 
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