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Switching power

2009-08-13 17:47:17.809342+00 by Dan Lyke 14 comments

Hey, Larry, how do you switch household circuit level (15-20A 115v) current? I was thinking about the Tweet-A-Watt that I've got parts on order for, and thinking that it'd be cool if it could switch, but that it'd then be drawing current to keep the relay (or whatever device I used to switch) in one state or the other.

So I typed "latching relay" into Digi-Key's search field, and got one device that looked like a reasonable candidate, the coil voltage is 6VDC (control on voltage is 4.5VDC, which makes power supply easier), coil current is over an amp, gulp, but that's what capacitors are for, however the sucker costs over $44 at 25 units...

Automation Direct has a 16A latching relay with a 24V coil rating for ten bucks, that'd require some additional power supply wrangling.

Is this the sort of thing one could do with a pair of SCRs or a transistor?

[ related topics: Hardware Hackery ]

comments in ascending chronological order (reverse):

#Comment Re: made: 2009-08-13 18:00:16.987666+00 by: Larry Burton

See if this terminal relay meets your needs. I don't have a full understanding of why you would need a latching relay unless your output is only pulsed.

#Comment Re: made: 2009-08-13 18:07:24.890963+00 by: Larry Burton

Okay, I reread that. Is 4.5mA @ 115VAC too much of added current draw? Transistors and SCRs are not being used in this sort of application.

Call me if you want to. Email me if you don't have my cell number.

#Comment Re: made: 2009-08-13 19:08:30.260029+00 by: meuon [edit history]

JHJ aka Xiamen jinhejie investment & holding http://www.jinhejie.com makes many variatons of what you want. I'm holding an JE11-5 which is a small 90a 250 volt version. http://www.jinhejie.com/ENGLISH/cpzs/index.html has some variations..

I don't know here you can get low quantities though. In quantiity they are very inexpensive.

#Comment Re: made: 2009-08-13 21:53:24.872903+00 by: Dan Lyke

Hmmm... First one I found on Larry's first link is 38mA at 4.5v holding current, but that still works out to 4 bucks a year at 100% duty cycle at $.12/kW/hr.

So, yeah, I think the idea of doing this in every plug needs something latching.

#Comment Re: made: 2009-08-13 22:25:33.723756+00 by: meuon

Latching also holds state if power fails, changes, etc.. It's mechanical 1 bit ram.

#Comment Re: made: 2009-08-13 22:46:14.67642+00 by: Larry Burton

The problem I have with a using a pulse for a signal to latch a relay is the same problem we've discussed with X10. Fire and forget doesn't always register so you fire once to turn something off and it misses and stays on. You fire it again to turn it on and it is now off. Your control scheme is now reversed until you notice to correct it. If it happens once a year that may be acceptable unless it happens that once a year on the one time it isn't.

If you go with a latching relay using a pulse from your controller make sure it is at least a 500ms pulse.

#Comment Re: made: 2009-08-14 01:42:43.482953+00 by: meuon

I haven't looked at the exact circuit specs and how they work, but it seems the controller knows the state. Possibly because it can measure voltage and current as well. I'll look tomorrow.

#Comment Re: made: 2009-08-14 12:40:59.194301+00 by: meuon

Adding: These are simple pulse latching relays, and yes, the controller verifies state with voltage taps on each side. It also measures current, but does not use that for verifying relay state.

#Comment Re: made: 2009-08-17 10:31:51.965555+00 by: andylyke [edit history]

What logic do you plan to use to control the relay?

Will you use it to turn something off? If so, according to what criteria? The vast majority of receptacles and fixtures in my house are drawing no power 90 to 100% of the time. Some, like refrigeration and water heating, are automatically controlled, others like lights are manually controlled but could be put on presence sensors, but the majority just sit there waiting for something to be plugged in. I don't see a great advantage in putting a contactor on every circuit if you aren't peak billed.

Peak shaving is very productive if you're billed for peak demand, or if the serving utility has a peaking problem. What the utilities usually do in that case is put a control on your A/C and water heater, to allow the utility to shut them down at peak demand, usually for 15 or 20 minutes at a time. They then roll these shutdowns around the community to lower the demand when they hit capacity limits, or are using extra expensive generation to meet demand. The customer is rarely aware that his a/c is off for 15 minutes, and the utility realizes great economic efficiency, either from reducing marginal cost of production or deferring investment in new capacity. From the utility's point of view, their peaking equipment may have a marginal cost per kWh vastly exceeding the retail price of energy (e.g. one utility I'm familiar with sells at retail for about $0.12 per kWh, but pays around $0.30 to produce power from its least efficient gas turbine unit), or they may have hard capacity limits , so have a strong incentive to shutdown coincident loads whenever possible. Large commercial and industrial consumers will do the same thing internally, in some cases turning on standby generation, if it can be paralleled, to shave their own peaks if they're penalized for peak coincident demand. I once consulted on a multi million dollar large motor and generator automated test facility, which was economically justified in part by moving the tests to midnight, when they would run under automatic control, incurring no additional labor costs and moving substantial power demand off the peak time.

Any residential consumer, as far as I know is billed for consumption only, so has no direct interest in artificially creating demand diversity.

What kind of loads would you want to control? The only ones that come to my mind at the moment are lights if you're not conscious about turning them off when not in use, and "wall warts". Many , if not most, of today's wall warts are switching supplies that consume damned near nothing when not loaded, but you could put them on some device that you have to manually turn on when you plug in the cell phone, and which turns off when the demand drops below some level. In that case, the relay (contactor) would draw holding current only when the device was charging, but if the power supply is well designed, probably yield only small marginal saving.

Incidentally, be wary of using "relays" as power switching devices. They're rarely designed for repetitive switching, particularly of inductive loads like motors, transformers and fluorescent or HID lamp ballasts.

#Comment Re: made: 2009-08-17 16:16:03.160847+00 by: Dan Lyke

My original thinking was that if I was going to be building a little current sensor on an individually addressable microcontroller for every socket, and a relay added five or ten bucks per socket, then why not throw it in? At the least I'd then have the most flexible light switch system ever. And since we have a number of timers in regular use in the house, something where the override and interface to those timers could be the iPhones or the laptop appealed a lot.

And some of this was a little thought experiment based on some business ideas I've talked to some people about, with a market that wasn't residential.

Larry and I talked a bit by phone a bit ago and came to the conclusion that it really wasn't a practical thing to do. His experience with automatic peak shaving in industrial environments wasn't positive, he said (and feel free to correct me, Larry) that keeping the human in the UI loop was important, that if devices started shutting down without human decisions the reaction was "the stupid machine is flakey".

I haven't wired up a more sensitive meter yet, but the Kill-A-Watt shows zero with any of my more modern wall warts plugged into it when they're not attached to any devices.

So I'll build a few Tweet-A-Watts or similar and see what that tells me, but I'm guess it'll tell me that I should use those parts to build something else.

#Comment Re: made: 2009-08-17 16:31:57.404632+00 by: andylyke

I certainly agree with Larry that punch presses shouldn't just stop operating for the hell of it. In a large plant or office building/complex, there is some opportunity to peak shave with HVAC, as there's a lot of thermal inertia lying around almost anywhere.

For flexible light switching, there are X10 modules (http://en.wikipedia.org/wiki/X10(industry_standard), both plug in and integral to the receptacle that could be integrated into your system. there are probably other standards of which I'm unaware.

#Comment Re: made: 2009-08-17 16:55:53.246345+00 by: Larry Burton

HVAC systems were what gave me the worse problem in shedding on approaching peak. You think you have a good strategy going but stop the fans feeding an occupied area and people complain fast. People are very sensitive to moving air and they will complain with a change either way of just a couple of CFM in air flow. Of course most of my experience with that was in the early 80s and VFDs were just starting to be used on ventilation motors at that time.

Basically what I've found to be the most effective way to control demand costs are to design the control loops for process efficiency and then use process scheduling to avoid peak demand periods when it is feasible. In batch processes that strategy works pretty good. In continuous processes it isn't always possible.

#Comment Re: made: 2009-08-17 17:12:51.663317+00 by: Dan Lyke

What I've heard about the X10 switches is that the lifespan of them is pretty limited, and because of that and the open loop commands they lead to the "frustration that the switching technology isn't working when the problem is that the light bulb is burned out" sorts of user experiences.

There are some newer technologies that have a (slightly) better reputation (if only because X10 burned so much of their reputation by pioneering annoying web ads, although lack of actual deployment for the newer technologies may help), like Insteon and Z-Wave (the latter being a standards consortium with multiple vendors). Sure seems like the right solution for now is to solve problems as they come up (ie: ceiling fan smart enough to turn on when the temperature at the ceiling gets warm enough) rather than trying to wire for every contingency.

#Comment Re: made: 2009-08-17 19:18:57.056721+00 by: andylyke

Larry, I agree - and to the emergence of VFDs changing HVAC. I've got a "geothermal" (ground source) heat pump with variable speed fans, and it's way more comfortable than anything I've lived with before.

Dan, I agree. I've worked with two large companies that both "broke their picks" trying to come up with some kind of market acceptable "smart house". I'd say get CFLs, a programmable thermostat, some kind of smart fan as you described and turn the lights off when you leave the room is about enough "smart house" for me.

**who'd want to live in a house that was smarter than he??**