One shot switch

[Scarr]

Hi all,

I have a battery project and want to be able to have it sat not using any main battery power until it's switched on for the first time. The switch is an external action.

The only thing I can think of is having a large power transistor fed from a button battery, the button battery has a plastic tab between it's live side and the PCB contact, then when you pull the plastic tab out, it feeds the transistor and switches the circuit on, the power output from the transistor is fed back to the base of the transistor making it a one time switch.

Being a total newbie, is this possible? is there something better? Just thought you could replace the button cell with a reed switch connected to the main battary, once a magnet is placed near by it turns the circuit on, the circuit now reamins on until the main battery is flat.

What sort of power transistor is a very low cunsumption but can handle 3.9v at 3amp?

Thx

P.S. Hope that all makes sense

 

[zachtheterrible]

Hi scarr, I don't think that your circuit will work, althogh I'm not sure since I'm not looking at a circuit diagram.

Look into SCRs. Silicone controlled rectiiifiers. Once there is a signal on the source (i believe that's what it's called) it remains open even if the signal at the source goes away as long as there is current flowing through it.

 

[Someone Electro]

Yea an SCR will do exsactly what you want.It can also hande prety high curents(especilay pulsed)

You just need to send about 5mA of curent in the gate for a short time:and the SCR will keep conductiong untill the curent flowing trough it falls below its holding curent.

 

[Scarr]

Could I not....

Unfortunatly I need to keep the current consumption down, 5ma is far too much as it's battery powered. Could I use a set/reset latching relay, have a push button when you press it it latches set and simply dont connect the reset side. this would not use any current in operation or before I first switch it on?

The reason I want this is I'n trying to have it so the user can turn it on externally but never turn it off.

 

[mstechca]

Hi all,

I have a battery project and want to be able to have it sat not using any main battery power until it's switched on for the first time. The switch is an external action.

The only thing I can think of is having a large power transistor fed from a button battery, the button battery has a plastic tab between it's live side and the PCB contact, then when you pull the plastic tab out, it feeds the transistor and switches the circuit on, the power output from the transistor is fed back to the base of the transistor making it a one time switch.

Being a total newbie, is this possible? is there something better? Just thought you could replace the button cell with a reed switch connected to the main battary, once a magnet is placed near by it turns the circuit on, the circuit now reamins on until the main battery is flat.

Have you thought of using a switch that can handle 3.9V at 3 amps?

an SPST switch works very nicely in your application. You can also get away with any other switch. If you just want A switch, and don't care about the appearance, go with a simple room light switch. :wink: they cost under $1.

If you want to know why a switch works, take it apart and look at the insides.

What sort of power transistor is a very low cunsumption but can handle 3.9v at 3amp?

I assume you want this for your switch. It is better not to worry about it.

 

[Optikon]

Hi all,

I have a battery project and want to be able to have it sat not using any main battery power until it's switched on for the first time. The switch is an external action.

The only thing I can think of is having a large power transistor fed from a button battery, the button battery has a plastic tab between it's live side and the PCB contact, then when you pull the plastic tab out, it feeds the transistor and switches the circuit on, the power output from the transistor is fed back to the base of the transistor making it a one time switch.

Being a total newbie, is this possible? is there something better? Just thought you could replace the button cell with a reed switch connected to the main battary, once a magnet is placed near by it turns the circuit on, the circuit now reamins on until the main battery is flat.

What sort of power transistor is a very low cunsumption but can handle 3.9v at 3amp?

Thx

P.S. Hope that all makes sense

Why have a transistor or SCR at all? Draw your schematic that shows the battery powering the circuit all of the time. But when you install the battery, put in the isolation tab so the circuit does not run. Once you pull that out, that's your one time switch! And the circuit is running on the battery. That's the cheapest way to do it. No extra parts other than a paper tab.

If this isnt what you want, you'll have to clarify.

 

[Scarr]

My question looks stupid!!!

Sorry, I re-read my post and it look stupid, I'm talking about having a switch to force a relay to switch something on!!!! I know this sounds stupid, but here why I want it.

The critera

IP67 rated box
Switch only works once to turn on but CANNOT turn off
No current consumed by the switch before or after turned on

Project has a sealed box and I have found a IP67 rated push switch, the idea being to have have access to the switch. The user pushes it (project powers up) and now it's on until the battery goes flat, and nothing can turn it off.

Hope this makes it a little clearer

 

[Roff]

This will draw nothing until it is switched on. When on, the N-channel MOSFET will draw 3.9uA.

 

[ljcox]

Re: Could I not....

Unfortunatly I need to keep the current consumption down, 5ma is far too much as it's battery powered. Could I use a set/reset latching relay, have a push button when you press it it latches set and simply dont connect the reset side. this would not use any current in operation or before I first switch it on?

The reason I want this is I'n trying to have it so the user can turn it on externally but never turn it off.

I suggest you read Someone electro's post again. The 5 mA is only drawn for a very short time, ie. long enough to trigger the SCR. Once it is triggered, the SCR will remain on until the circuit is opened, or the SCR is shorted.

 

[Roff]

Re: Could I not....

Unfortunatly I need to keep the current consumption down, 5ma is far too much as it's battery powered. Could I use a set/reset latching relay, have a push button when you press it it latches set and simply dont connect the reset side. this would not use any current in operation or before I first switch it on?

The reason I want this is I'n trying to have it so the user can turn it on externally but never turn it off.

I suggest you read Someone electro's post again. The 5 mA is only drawn for a very short time, ie. long enough to trigger the SCR. Once it is triggered, the SCR will remain on until the circuit is opened, or the SCR is shorted.

But even a big SCR will drop around 0.8V at 3 amps. That's a pretty big percentage of 3.9V. The MOSFET solution will drop around 30 to 40 millivolts.

 

[ljcox]

Yes, a good point. I had missed that he only has 3.9 V to play with

 

[Thunderchild]

i'd go fo the paper/plastic tab :lol:

 

[Roff]

i'd go fo the paper/plastic tab :lol:

You take all the fun out of it.
I agree with you, though.

 

[Scarr]

How can I get a IP67 rated plastic tab / pull device?

Hi,

The box is to be IP67 rated, is there a plastic / paper tab product that is somehow sealed :wink:

If not and I do like the Mosfets Ron but not as much as a ltaching realy unless there's a reason why a latching relay with a IP67 rated push button switch won't work as a one shot / no power loss when on or off solution?

Thx

Steve

 

[Thunderchild]

e relay will consume either as off or on. the lowest i have seen was 50 mA and that is not much on 3 amps is it ? but is is up to you

 

[Someone Electro]

Mosfets are way better

 

[Scarr]

Are you sure?

I thought a latching relay was held by a magnet and you applied a voltage to overcome the magnets power, it then flips the relay over and it stays in the position until you flip it again, of course I'm talking about a dual coil relay like this from omron "G6AK-234P".
Thx

P.S. Thanks for everyones comments and help so far.

 

[ljcox]

That's right. I think (from the dim past) latching relays are sometimes called bistable relays.

 

[Gorgon]

Re: Are you sure?

I thought a latching relay was held by a magnet and you applied a voltage to overcome the magnets power, it then flips the relay over and it stays in the position until you flip it again, of course I'm talking about a dual coil relay like this from omron "G6AK-234P".
Thx

P.S. Thanks for everyones comments and help so far.

Hi Scarr,
Depending on the mechanical stress you want to use on your 'black box' I would propose to use a reed switch and a magnet. You can use both the normally closed type and the normally open type.

The normally open reed switch is activated by a magnet, when activated the current to your application is running through the coil around the switch and keeping it activated.
When the battery is empty the switch disconnect it from the load, saving the battery from deep discharge. This circuit does not use any extra power for maintaining the switch in on condition.
You could also use 2 magnets, one disabling the other. When you remove one of them momentarily the other activates the switch.



The normally closed reed switch is kept open with a magnet and when removing the magnet the switch is closing the circuit. If you reapply the magnet the switch would open again, breaking the circuit. This is then not 100% after your spec!

TOK

 

[Roff]

If you do decide to use the MOSFET solution, this is an alternate method. You probably should add the cap in either case.