1200mA @ 12V

[1793]

I'm looking to control some LED film strips with magnetic contacts for an Armoire.

Any thoughts where I might be able to get something to work for this situation?

 

[gar]

101209-1841 ESR

What is a magnetic contact? Are you referring to a magnetically actuated reed switch, or something else? There are power reed switches. I believe AB had some for use in pushbuttons and maybe even contactors.

If just want an electromagnetic relay, then any with silver contacts and rated at 5 A would be fine.

1200 MA might better be presented as 1.2 A.

.

 

[ELA]

Looks like they use a transformer -from the link?

Are you planning on switching the dc side? Is switching the AC side -not an option in your application?


Like the site says 1200MA ... 1.2A would just not be right.

 

[1793]

Looks like they use a transformer -from the link?

Are you planning on switching the dc side? Is switching the AC side -not an option in your application?


Like the site says 1200MA ... 1.2A would just not be right.

Yes there is a transformer and I would like to switch the DC side. I'm very limited to switch types and locations so I think the DC side has more flexibility.

 

[GeorgeB]

I'm an engineer (and hobbyist) so apply appropriate consideration ...

All it sounds like from the limited description is a transistor, resistor, and your reed switch. To go more conservatively, a DC solid state relay is more appropriate than the transistor.

What part of your question am I missing?

 

[1793]

I'm an engineer (and hobbyist) so apply appropriate consideration ...

All it sounds like from the limited description is a transistor, resistor, and your reed switch. To go more conservatively, a DC solid state relay is more appropriate than the transistor.

What part of your question am I missing?

I'm trying to find a way to light the inside of a custom Armoire. The best I have found are the LED ribbons in the OP. I need to be able to turn the LEDs on/off and the use of a magnetic contact switch is what I have come up with and I'm trying to find a switch system that would work.

 

[weressl]

I'm trying to find a way to light the inside of a custom Armoire. The best I have found are the LED ribbons in the OP. I need to be able to turn the LEDs on/off and the use of a magnetic contact switch is what I have come up with and I'm trying to find a switch system that would work.

So your magnetic switch is activated by the Armoire door OPENING. Use the 12DC power suply for the LED strip to activate a 12VDC relay whose contacts will switch the power ON/OFF to the LED strip. I presume that the magnetic(reed switch?) switch is not rated 12VDC/1.2A. If it is, you should have no problem directly switching the LED's. Like these ugly Betties: http://www.homesecuritystore.com/p-1563-ams-38bw-amseco-surface-mount-switch-no-nc.aspx

 

[gar]

101209-1255 EST

Here is a reed switch rated up to 2 to 3 A depending upon ampere turns sensitivity.

http://www.reed-sensor.com/Reed_Switches/High_Wattage_Reed_Switch.htm

This should be able to directly switch your LED string.

Otherwise use a lower current rated reed switch to control the gate of a low on resistance HEXFET.

.

 

[ELA]

I would favor the idea of a lower "drive current" through your magnetic door switch to a relay coil (either magnetic or Solid State) and let that device handle the higher current of the LED transformer.

Gar,
In the reed switch you pointed to what would you consider the inrush capability of this unit to be?

One thing designers often do not consider is inrush current -welding the contacts of small contact area switches.

If possible find out what the inrush current of the "transformer" is so that you can use that information to assist in your decision. If not listed and you cannot measure it, then lean towards a more conservative design.

 

[gar]

101210-1238 EST

ELA:

I do not know the weld current of the contacts. However, I have used small reed switches in series with 100 ohms to trigger a Triac without a problem. I have done no life testing. Peak current in the Triac application is about 1.7 A. This is a lot of current for a small reed switch.

LEDs produce no appreciable inrush current. And because the LED voltage drops at a given current with an increase in temperature it is likely that the initial turn on current is less than steady state.

But I agree with you that using a small reed switch to pilot a solid state device is probably the most reliable approach. A magnetically sensitive solid state is even better, but I think the goal here is simplicity, and minimum cost and effort.

.

 

[ELA]

With respect to inrush current I was speaking in terms of -if you elected to switch the input to the "transformer" .

Somehow I favor the idea of turning the transformer off when not in use, unless the light will be used often. I do understand a desire to switch the DC side.

 

[gar]

101210-1632 EST

ELA:

I also prefer switching the AC side. If this was the approach, then a small reed switch controlling the DC input, at maybe 5 to 12 V, of a solid state switch, and grounding at least one side of the 5 to 12 V control circuit would work.

This also makes it easy to get the necessary inversion to work with a NO reed switch contact.

.

 

[wireguru]

there are alot of ways to do this, do you want something prepackaged or do you feel like making a small PCB or assembling some components on a breadboard?

 

[1793]

there are alot of ways to do this, do you want something prepackaged or do you feel like making a small PCB or assembling some components on a breadboard?

Prepackaged would be my first choice. As you can see I'm not very savvy in this area.

 

[ELA]

As mentioned there are a lot of options. When looking for prepackaged options, are you expecting to spend much?

Here is one possibility if you can afford the money for the two components required.
http://www.smarthome.com/2421/TriggerLinc-INSTEON-Wireless-Open-Close-Sensor/p.aspx

May seem a little overkill?
This would make a nice clean install but may not be optimum for you?

 

[wireguru]

cheapest, simplest way to do this would be to use a 12v automotive relay ($3 or so) and a magnetic alarm contact (make sure you get the proper arrangement of NC or NO).

 

[gar]

101212-1432 EST

Another suggestion. Switch the AC input to your 120 to 12 V power supply with an OPTO-22 120D3 120 VAC 3 A solid-state switch (most likely a Triac) with isolated low voltage DC control. To power this use a Wall-Wart with 12 V DC output. As a load on this 12 V supply use a 390 ohm 1 W resistor in series with the OPTO-22 input. The OPTO-22 has a 1000 ohm input resistance. Thus, the current flow is I = 12/1390 = 8.6 MA. Under these conditions your LED string lights will be on.

Connect a normally open reed switch in parallel with the OPTO-22 input. When the magnet closes the reed switch, door closed, then the OPTO-22 input is shorted and it turns off. The current thru the 390 ohm resistor under these conditions is I = 12/390 = 31 MA and the power dissipation in the resistor is 12*0.031 = 0.38 W.

Get the normally open reed switch and associated magnetic from a hardware store. When the magnet is close to the reed switch the switch will close.

Connect the negative side of the power supply to the negative input of the OPTO-22 and also to earth.

OPTO-22 also makes a Safety Cover for the SSR.

.

 

[BILLY101]

The KISS method would be to use a Honeywell Microswitch (V7) series. They are very small (1" long) and easily recessed into a wooden door frame.
Electrical capacity is not an issue.
Allied electronics stocks several styles. Pricing is under $5.00 each.

BILLY

 

[wireguru]

i would stay with switching the DC side. If you switch the AC side you are introducing a second wall wart, a receptacle box to plug the original wall wart into, something to put the SSR in, etc.