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#69221 - 09/02/06 12:44 PM Very low duty cycle timer?  
NJwirenut  Offline
Joined: Sep 2001
Posts: 806
Bergen County, NJ
Does anyone know of an inexpensive method of automatically turning on a small 120VAC load for a couple minutes once a week or so?

The application is an automatic water drain for an air compressor. I want to install a solenoid valve at the bottom of the tank, and turn it on once a week or so to drain out the accumulated water. The exact time of day that it actuates isn't important, and neither is the exact frequency of operation. A standard intermatic type time switch gives a minimum "ON" cycle time of 15 minutes, which is much longer than needed to blow out the water, and would waste a lot of air. I have seen digital programmable devices, but they seem like overkill for this application, with mulitple outputs and setpoints which I would never need. Time switches would also need to be reset after a power interruption, which is annoying. I am thinking of something like a time delay relay with a repeat cycle mode, but I don't know of any with such a long cycle time.

Yes, I know that there are automatic float type valves that are made just for this purpose, but we have tried several models, and they all failed shortly after installation. A solenoid valve/timer should be a lot more robust, since it isn't relying on floats which tend to bind up over time.

[This message has been edited by NJwirenut (edited 09-02-2006).]

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#69222 - 09/02/06 12:58 PM Re: Very low duty cycle timer?  
Tiger  Offline
Joined: May 2005
Posts: 706
Crystal Lake, IL USA
Isn't there a small tank & pump used in HVAC to pump whatever drips out of furnace/a.c. units?


#69223 - 09/02/06 01:17 PM Re: Very low duty cycle timer?  
JCooper  Offline
Joined: Dec 2002
Posts: 228
Kendall Park, NJ USA
I think I know what you are looking for NJWirenut, my firehouse has the same thing on our compressor but it fires once every half hour or so for about a second. I will take a look at what is running it later today and post back.

#69224 - 09/02/06 01:34 PM Re: Very low duty cycle timer?  
circuit man  Offline
Joined: Nov 2001
Posts: 273
harbor freight sells an automatic tank drainer that is hooked up through the unloader line on the pressure empties when it comes on & when it shuts down. works pretty good. has every thing ya need in the box.some of the supply houses should have these.

#69225 - 09/02/06 02:45 PM Re: Very low duty cycle timer?  
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
I have used these electric auto drains with good success.

[Linked Image]

They did not clog up in a very damp and dirty environment.

If you must build your own I know timers that will work but they are not inexpensive. $100 - $200.

[This message has been edited by iwire (edited 09-02-2006).]

Bob Badger
Construction & Maintenance Electrician

#69226 - 09/02/06 04:57 PM Re: Very low duty cycle timer?  
CharlieK  Offline
Joined: Mar 2003
Posts: 21
Baltimore. Md. U.S.A.
NJ. try a Intermatic C8855. It has an on renge of 21/2 seconds to 5 minutes every 24 hours and sells for about 100.00


#69227 - 09/02/06 06:46 PM Re: Very low duty cycle timer?  
JoeTestingEngr  Offline
Joined: Nov 2005
Posts: 790
Chicago, Il.
NJ, If you follow your 24 Hr />=15 min timer with any of a number of "interval on" timers, your final result would be a shorter purge every day. You could fine tune the combination any way you wanted.

#69228 - 09/03/06 10:26 AM Re: Very low duty cycle timer?  
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Joe brings up a good point about the timing.

A few seconds a day is IMO a better idea than once a week.

The electric one I posted above we might even set for 1 or 2 seconds per hour.

A quick PSSHHHHT every hour and the tank stays dry. [Linked Image]

Bob Badger
Construction & Maintenance Electrician

#69229 - 09/03/06 12:11 PM Re: Very low duty cycle timer?  
Helectric  Offline
Joined: Jan 2005
Posts: 90
Brandon, SD
They make automatic drain valves for just this pupose. If you contact an air compressor supplier or visit their web site you should find one. Here's a link to one that is similar to those that I've used in the past. They're available in all voltages. The ones that I've used typically come equiped with a 110v cord.

#69230 - 09/03/06 12:28 PM Re: Very low duty cycle timer?  
NJwirenut  Offline
Joined: Sep 2001
Posts: 806
Bergen County, NJ
Looks like McMaster-Carr carries the one Iwire posted. Will probably order one of those.

Hopefully the electric setup will hold up better than the float type.

Thanks, all!

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