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  #1  
Old 10-09-2009, 09:53 PM
WoodDuck WoodDuck is offline
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Default center off DPDT relay?

Hi all, does anyone know if this is available? I'm trying to control a small pump (1a windshield washer) and I need to reverse the flow back and forth as needed. Right now I have a DPDT switch cross-wired to reverse it but I need to automate the process. I've searched and found a power window reversing relay but it seems specialized for it's application. Any ideas?

Thanks..
WoodDuck
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Old 10-09-2009, 11:32 PM
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Bernard Bernard is offline
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Just one bad idea. Use 2 ea. DPDT relays and a SPDT- center off SW. Cross strap NO contacts between the two relays, motor to one set of NO contacts. Control relays with DP-center off SW so that one or the other relay is on, but never both at same time, conter both off.
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Old 10-10-2009, 04:56 AM
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SgtWookie SgtWookie is offline
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Here's one way to approach it:



M1 represents your pump's motor.
The relays can be commonly available SPDT 12v automotive relays.
S1 and S2 are normally-open pushbutton switches. They could be replaced by a single-pole double-throw center-off switch or switching transistors if you wished.

D1 and D2 are common silicon rectifier diodes. They take care of the high reverse-EMF pulse that is generated when current through the relay's coil is interrupted.

The advantages of using this design is that the parts are readily available, it's pretty simple, and nothing bad will happen if both switches are pressed at once.

Not shown is a fuse between the 12v supply and the circuit, which is a must.

There are diagrams of automotive-type SPDT relays on this page:
http://www.the12volt.com/relays/relays.asp
The left image is the bottom view of a SPDT relay. The middle image shows the connections when the coil is not energized, and the right shows the connections with the coil energized.
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File Type: png MotorReversePushbuttonRelay.PNG (9.5 KB, 137 views)
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Last edited by SgtWookie; 10-10-2009 at 05:04 AM.
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Old 10-10-2009, 05:57 PM
WoodDuck WoodDuck is offline
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Thanks for the replies, I think I'm on the right track now. I think I'll go with your idea Sgt Wookie since I already have all the parts.

This project is actually more involved but due to my noobness I'm taking it one step at a time. The next step is to put a delay circuit on the relays so the pump shuts off after a few seconds. I found this design on Aaron Cakes site but I have a few questions:

1) Can I use the same delay circuit for both relays or do I need one each?

2) Are the diodes in the delay circuit duplicating the ones in your design? Maybe they could be eliminated?


Thanks...
WoodDuck
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Old 10-10-2009, 06:21 PM
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SgtWookie SgtWookie is offline
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Quote:
Originally Posted by WoodDuck View Post
Thanks for the replies, I think I'm on the right track now. I think I'll go with your idea Sgt Wookie since I already have all the parts.
OK.

Quote:
This project is actually more involved but due to my noobness I'm taking it one step at a time.
Good plan.
Quote:
The next step is to put a delay circuit on the relays so the pump shuts off after a few seconds.
What is the range of seconds that you are looking for? Say, 1 to 20, adjustable? A flat 5 seconds? Times will be approximate, of course.
Quote:
I found this design on Aaron Cakes site but I have a few questions:

1) Can I use the same delay circuit for both relays or do I need one each?
One each.
Quote:

2) Are the diodes in the delay circuit duplicating the ones in your design? Maybe they could be eliminated?

Due to the way 555 timers work, it would be better to use 2N2222 transistors in place of S1 & S2 in my schematic, with a resistor on the bases of around 1k Ohms connecting directly to the pin 3's of the 555 timers. This would eliminate D1, D2 and the relay coil from Aarron Cake's schematic.

Aarron Cake's schematic does not show the bypass capacitors across the Vcc and ground terminals of the 555 timers. You will need a 220uF and a 0.1uF to take care of transient voltage spikes/dips, caused by the timers themselves.

A single 556 timer could be used in place of two 555 timers.
Do you have a 556 timer, or multiple 555 timers?
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Old 10-10-2009, 07:35 PM
WoodDuck WoodDuck is offline
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I need an approximate 1 - 10 sec delay - adjustable.

Re: the timers, I have neither but space is a premium so I'd like to use a 556.

I'll draw up your suggestions and re post as soon as I can.

WoodDuck
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Old 10-11-2009, 03:18 AM
tkng211 tkng211 is offline
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Quote:
trying to control a small pump (1a windshield washer) and I need to reverse the flow back and forth as needed. Right now I have a DPDT switch cross-wired to reverse it but I need to automate the process.
If the pump used is a DC type bidirectional one, the supply voltage polarity is reversed when you switch the DPDT (cross-wired), and the flow direction will be changed.
To simulate the original circuit, you can buy any DPDT relay with suitable rating in contact current and pull in voltage and connect it as the mechanical DPDT switch. The circuit uses only one relay and one timer IC 555. If you can't get the DPDT relay, you will need two SPDT relays, one 555 and one transistor to drive both relays.
If you need to turn off the pump, another SPDT power swith or relay will be required.

Last edited by tkng211; 10-11-2009 at 03:26 AM.
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Old 10-11-2009, 06:11 PM
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SgtWookie SgtWookie is offline
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Quote:
Originally Posted by WoodDuck View Post
I need an approximate 1 - 10 sec delay - adjustable.

Re: the timers, I have neither but space is a premium so I'd like to use a 556.

I'll draw up your suggestions and re post as soon as I can.
OK. For a 1 to 10 second delay:
Use a 10uF capacitor for C1.
For R1, use a 47k to 75k resistor in series with a 1 Megohm potentiometer.
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Old 10-12-2009, 05:17 AM
WoodDuck WoodDuck is offline
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Well, here is my drawing based on our discussion. I decided to go with 2 - 555's for now to make it easier. I don't think it will take up too much more room anyway. I'm not sure I got all the connections right though- particularly around the switches, the power connections and the new 75k resistors...

tkng211 , the only relay I could find was a dpdt w/o the center off. I wanted the center off because the pump needs to be on only for a few seconds at a time. When it goes on again it's for a few seconds in the other direction.

Thanks again for everyone's help - it's very much appreciated.

Cheers..
Woodduck


Pump circuit.jpg
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  #10  
Old 10-12-2009, 06:02 AM
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SgtWookie SgtWookie is offline
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Looks good, but there are just a couple of little issues:
1) Both of the 555's have reference designators of "U1". One should be U1, the other U2.
2) +12v is not directly connected to either of the 555's pin 8's & pin 4's.
3) There is a wire connecting the left 555 pin 4, 8, and 10k resistor to the junction of the right 555's 75k and 1M pot, which is not correct.

I've marked up your schematic and attached a revised version. The marked up areas are circled in pink.

By the way, .PNG format files are preferred; .JPG files are "lossy" and lose detail every time they are modified.
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File Type: png Pump circuit.PNG (158.4 KB, 19 views)
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