Emergency Power out release

[rmcingle]

Maybe I'm too wordy or boring for people for people to carefully read my posts.

They use to make dimmer switches with Rheostats.

The light dimmers that have a rotary knob to control the light level had a rheostat (variable resistor) but it was in the control circuit to the "SCR" and didn't directly control the light.

As someone else pointed out, a light dimmer switches on, full for a period and then shuts off for a period. This works well on a resistive load, such as an old fashion light bulb. It does NOT work well on a reactive load, like a coil. In fact, a pure reactive load would probably kill a light dimmer very quickly.

I don't recommend making a DIY release mechanism based on electricity if you don't have a good understanding of it. Using a variable resistor (rheostat) to control the voltage/power to an electromagnet is a very bad idea. Any circuit that gets hot in operation is a bad idea.

As someone else stated, you can reduce the current that an electromagnet requires by simply adding more turns of wire. The magnetic strength is a factor of "ampere turns" and the magnetic qualities of the core. If you currently have a magnet that has, say, 100 turns of wire and requires 1 amp to hold your key you can get the same result by having 200 turns of wire and only 1/2 an amp. When you double the turns of wire you more than double the length of the wire (because the outer windings take more wire per turn) so the required voltage might increase a slight amount.

if you desire an increase in voltage (so you don't have to reduce it from your source) then the easy solution is to use more turns of a smaller diameter wire. A smaller wire has more ohms per foot, plus it fits better on a coil.

I noticed in your pictures that you were using a full insulated wire for your windings. This is unnecessary and, actually, not advised. If the coil gets warm the plastic insulation will melt (and at a relatively low temperature) and once the plastic gets soft all bets are off. "Enamel" wire is what you wind a coil with. The insulation if very thin so it doesn't take up much space, which allows you to get more winds of wire in a given space. The enamel will also withstand higher temperatures than the typical plastic insulation.

The biggest "bang for the buck" is in the choice of the core for your magnet. Hard steel typically has very poor magnetic qualities. Good old soft iron is a whole lot better! You can get two or three times the magnetic strength from the core, even with the same ampere turns. There are more exotic materials that can be even better, but probably not what a DIY person would have access to.

Why don't you just buy an electro-magnet? Get something like this:
http://www.ebay.com/itm/DC-12V-Open-Fra ... 4d18af3ab2

You may need to make a slight modification so that the plunger will fall all the way out, but it should be easy. The solenoid should easily hold a ring of keys.

Then get a 12 volt power supply, rated at something more than 1 amp. Easy to come buy, and not expensive. You can control the power supply at the AC mains side (lamp timer) or on the DC side (relays, switches, etc.) Cheap, easy, reliable.

Ron

[BornThisWay]

Yes, a simple 12V relay can also be useful but most everything else has been covered or completely inaccurate.

Here are some problems with your arguments.

In order for the wire to get hot there has to be a substantial amount of current going through it or it would have to have some resistance.

900mA isn't going to do anything to the insolated copper wire. The adapter might get hot, but the wire never will.

The resistance of a copper wire of this size is so low its virtually not relevant.

And even if it was, the steel core is one hell of a heat sink.

Steel is a very good material to make a magnet out of, it may not be as good as soft iron, but last I checked they didn't have a soft iron section at the hardware store.

I wanted this to be something that can be easily reproduced, so I used commonly available parts.

This can also be used with a simple D Cell battery as a time release.

And lastly, you don't have to be extremely knowledgeable with electricity to build something like this.

It is a very good DIY project as long as someone was willing to do a little research.

[tiemeupalso]

i built one of these in elementry school as a science project.that wasnt too long after they descovered electricity(if you listen to some young kids)

[Sir Cumference]

?...
Here are some problems with your arguments.

......

900mA isn't going to do anything to the insolated copper wire. The adapter might get hot, but the wire never will.

The resistance of a copper wire of this size is so low its virtually not relevant.

And even if it was, the steel core is one hell of a heat sink.
........
And lastly, you don't have to be extremely knowledgeable with electricity to build something like this.

The arguments are not problematic, they are very valid.

You can't deny Ohm's law, and basic electronic knowledge is paramount.

When your supply says "900 ma", that means, that it can be expected to safely deliver that as a maximum.

Have you measured the resistance in the coil?
The resistance is probably the most relevant part of this.
Let us assume it is 6 ohms.
12V/6ohm=2A
And there is nothing stopping it from drawing that from the supply.
There may be a fuse.... It blows and your supply is dead. If not, you are overloading it, it heats up and either just dies if it is a good design, or catches fire if it is a bad design.

If you have an expensive lab power supply, it probably has current limiting, in which case you can actually limit the current through the coil. But if you do not, the only thing controlling the current, is the resistance of the system (which when battery powered, includes the internal resistance of the battery).

If you draw too much current and has negligible resistance in your coil, the core's value as a heat sink is an illusion.
(It is anyway, but for other reasons. Please re-read rmcingle's post).


I agree that you do not need to be extremely knowledgeable to do this, but to be totally ignorant is bad!

[BornThisWay]

I'm not trying to be hard on rmcingle, I'm sorry if it came out like that.

the 900mA directly connected to the coil was never my intent.
I tried that during my first test and it failed.

Without a resistor its like shorting the wires of the adapter. It will run at full capacity and fail.

The Transformer would overheat and die long before the wires on the core could get hot.

The resistance is 0.5 Ohms.

12V/6ohm=2A
And there is nothing stopping it from drawing that from the supply.

The inline resistor is stopping it from drawing that from the supply.
And if there was no resistor, then failure of the transformer would stop it from drawing that much current long before a 24 AWG wire got hot.

If the adapter is too complex for a DIY, then I would encourage people to only connect it to one D Cell battery as a time release.

[rmcingle]

First off, a standard multi-meter is a very poor tool to measure a half an ohm. Fluke makes meters with a "low ohm" range that is good for measuring under two ohms, but it doesn't appear to me that the model you show there has that mode.

24 awg should have a resistance of about 27 ohms per 1000 feet. Measure the length of your wire and figure out what the resistance should be, it would be a more accurate number than what the multi-meter lies to you. Measuring the current and voltage would also be a much more accurate way of establishing the resistance.

24 AWG wire is rated at 3.5 amps in free air. But this is NOT a free air situation. It degrades a lot when you put a thermal insulator around the wire (the plastic insulation). It will degrade MUCH more when you wrap several layers of the wire over itself. The limiting factor becomes the insulator. Note that it doesn't have to get hot enough to "melt" the insulator, only hot enough to make it pliable enough for the hard wire to migrate through the softened plastic.

If you assume your 1/2 ohm is correct, and you are putting the 900 ma from your supply through it (regulated by the external resistor) then the power dropped in the coil will be .9 x.9 x .5 = .125 watts. Okay, not very much, probably not a heating issue. The resistor, on the other hand, will drop the rest of the power. 900 ma x 12 volts is 10.8 watts. all but .125 of that will be dissipated in the resistor. That is a big resistor. If you get a resistor rated at 10 watts it will survive, but it will get rather hot. I would recommend getting one rated at 50 watts or more to ensure that you didn't create a problem based on the heat.

Or you can add more wire to the coil. As I said above, use enamel wire and go to 30 awg wire. If you went to a 30 awg enamel wire instead of your 24 awg PVC insulated wire you could easily get four times as many wraps in the same overall coil diameter. The 30 AWG wire has about 4 times the resistance so overall you would get about 16 times the resistance for the coil, or about 4 ohms.

Now, here is the best part: the strength of the magnet is a ratio of the number of amps x the number of turns. If you put four times as many wraps of wire, you will need 1/4 the current to get the same holding power. So now, instead of giving it .9 amps, give it .225 amps, and your magnet is just as strong. Ditch that 12 volt supply and get yourself a much smaller one, you only need 0.9 volts. Similar supplies for 6 volts, 5 volts and even 3 volts are common. If you use the same resistor approach to limit a 3 volt supply the resistor will only dissipate 1/2 a watt.

Or go to an even smaller wire, add even more turns, get an even higher resistance and get rid of the resistor altogether.

Or just buy the solenoid I referenced earlier, it would be cheaper than buying the spool of wire.

If you look around this forum you will find other threads where I have posted information (including pictures) of the setup I use, which is a door lock magnet. It was made to operate on 24 volts but I use 12 volts and it has more holding power than I can break loose. I paid about $20 for the magnet off Ebay. 12 volt power supplies are cheap. I connect the power supply to two (2) timers. One timer is a standard lamp timer set for a fairly wide window of time, and is the back up. The second timer is a spring wound timer that can be set for up to 1 hour. I use two timers in case one fails (which one did one time....) I have been considering adding additional switches for turning the power off, such as a smoke detector.

Ron

[jjim]

There are probably a lot of ways of doing this. It might be a good idea to latch the power off if there's a brief outage, or if a humungous power spike causes the electronic timer to reset or otherwise malfunction without releasing the keys. This could happen during a lightning storm. You could latch the power off to your release mechanism with a 120VAC relay that was normally open. Instead of connecting the relay coil to the home power, only connect one side of the coil. The other side of the coil is then connected to one terminal of the normally-open switch in the relay, and the other terminal of the switch is then tied to the other wire of the home power. The only way to energize the relay when it is connected this way, is to put a wall switch across the normally open relay terminals. Turn that switch on for a moment only, and the relay stays energized.

Or, instead of winding your own solenoid so that its magnetic field can hold your keys, try carefully removing the plastic shell of a 120 VAC power supply, like the one in your picture. If the power supply is powering something, the transformer inside might have enough of a field to hold the keys, which would drop if the power went out.

[Sir Cumference]

Except that the plastic shell has the crucial role of keeping all the nasty electrons isolated from the rest of the room.

The moment the shell is gone, you could have bare high voltage leads available to touch.

[bound_jenny]

you could have bare high voltage leads available to touch.

Now what could possibly go wrong with that?

Jenny.

[Sir Cumference]

you could have bare high voltage leads available to touch.

Now what could possibly go wrong with that?

Jenny.

I know.
Just me being an over-cautious old fart.

[lj]

OK Guys,

it is clear that some here have a good knowledge of electrics, and some do not.

The OP came up with an idea that is fundamentally flawed. We, who DO know what electrics are about, have tried to explain why it is flawed.

You can buy electromagnets designed to operate on a range of supplies for very little money, or as has been suggested, a low-voltage relay could easily be modified by stripping the moving parts and just using the coil.

Unless you have the knowledge, don't risk your life with electrics, either by messing around with it directly, or indirectly by using it as a safety release.

PS a D cell can supply anything up to 10 amps for a short while, certainly enough to melt a thin wire and start a fire, and a rechargeable D cell can melt a 4"nail

[BornThisWay]

There has been a lot of good feedback and safety considerations posted.
And I appreciate it. Whether I got frustrated with them or not, they forced me to research, test, and ultimately improve my product.

Safety is extremely important to me and it would destroy me if someone got hurt trying anything I'm doing here.

*****Here is my challenge to anyone who want's to take it up:
1. Develop a release that will only be triggered when the power is cut.
2. It should be low cost.
3. Made of easily acquired parts.
4. Safe
5. Not excessively difficult to make.

Once it's made it can be connected to a timer as well.

I will continue working on my own ideas, but I wont post them until I believe it meets the five points listed above.