Oposite current on one wire?

[bd13]

Can you have oposite current flow, from two different power sources, on one wire? i am trying to get a relay to trigger from a 9v power source but stay latched from a 1.5v battery (drop voltage is only .25v), but when the relay is latched it also needs to short the 9v accross a igniter (rocket type) to get it go off. This leaves both the 1.5v battery and the 9v using the same common to NO pins on the relay. I tried using oposite current flow and diodes to try and keep the 9v current from trying to go accross the smaller 1.5v battery, but it doesn't work.

opperation description:
My main goal is to get the relay to turn on from the "timer" input (which pulses), and only the 9v turns the relay on. the 1.5v is activated when the relay turns on and it used to keep the relay on (creates a latching relay, forms a circuit through the coil, common, and NO pins). When the relay is latched it will also short the 9v over the common and NO pins and then through an igniter (rocket type) to set it off.

 

[Exo]

Once again, you've got the transistor wrong, like i said before this will just short out the speaker output over the transistor.
Then, a mechanical relay will probably be too slow to react on the short pulses the speaker gives. (it has to be on before the pulse is gone or it won't stay on)...
presumably your relay is 9V? it won't stay on by using a 1.5V battery.
Some of the diodes are also placed wrong...

 

[bd13]

the transistor won't short the speaker, because there is no speaker (took it out). second, i tested it and the transistor arrangment does work for turning the relay on, and the relay is not a 9v, it is a 5v and has a drop voltage of only .25 volts (meaning once it turns on it will turn off when the voltage at the coils is less than .25 volts, so a 1.5v batt can and will hold the latch, tested and proven). i know some of the diodes are wrong, and i have come to realize that this circuit can't work, so i think if i add one more relay it will. 9v turns the relays on, both hold the latch position with the 1.5v activated by one of the relays (forms a circuit through both coils and one relay's common-NO), and the other relay shorts the 9v over the other relay's common-NO, just splitting the load.

 

[Exo]

the transistor won't short the speaker, because there is no speaker (took it out).

Yah, obviously. I mean it shorts out the circuit that drives the speaker. The BE junction can be seen as a diode.

second, i tested it and the transistor arrangment does work for turning the relay on, and the relay is not a 9v, it is a 5v and has a drop voltage of only .25 volts

Explain how a relay's coil only has a 'drop of .25V'. Always eager to learn new things.

 

[bd13]

ok, i see what you mean now...and i have a question: how do you get the maximum CE voltage out of a transistor (in respect to BE connection)?

and to your question: a relay, as i have observed, needs X amount of volts to get it to latch, turn on. that is the hardest part of the prosses, from there on it requires less voltage to "hold" itself on, and the very minimum of that voltage is the "drop out" voltage, or the voltage at which the electro magnet can't hold the latch and thus the relay returns to its original state. this is very similar to you pushing a brick up a hill, it requires more force to push the brick than it does to keep it from moving back down the hill, and if you apply even less it will start to move backwards.

i was trying to take advantage of this in my circuit, by using the 9v to switch on the relay i get it to latch (no crap) then i can use a small 1.5v button battery to hold that latch, 1.5v>.25v, and thus freeing the 9v from relay duty and making it short over the common-NO pins and through the igniter. for this to work i would need a <=5v DPDT relay and a way to make the timer pulse once, or else on every pulse the 9v would try to turn the relay on and take voltage away from the igniter. problems: i don't have a <=5v DPDT relay and don't have the money, or will, to get one (only place i know, 6.25$ shipping, .75$ part...worth it? no...) and radio shack only has a 12v DPDT. every step of the way i run into problems, and most relate to power sources and them being too big to fit in the space or too weak and expensive to do anything for very long... i need to do a lot more testing. and too think, if that 9v could hold the relay on AND ignite the igniter this would be over in a second. and work twice as good.

 

[Exo]

ok, i see what you mean now...and i have a question: how do you get the maximum CE voltage out of a transistor (in respect to BE connection)?

You want minimum CE voltage. So the rest can be used for your relay/igniter. The way you're using the transistor is as a (badly connected) emitter follower. Collector voltage would never get higher then base voltage. I would just use the transistor as a switch.

The timer you've got, is it really a loudspeaker that was in it? or some kind of buzzer with a steady supply voltage?
And your ignitor,... Does it really require so much current that battery voltage goes to almost 0?

 

[bd13]

the timer pulses high-low-high-low, and its power supply is only a small 1.5v button battery so the speaker voltage is low. i want to use the transistor as a switch but because of the voltage drop it sometimes can't trigger the relay (when using different power sources). and my igniter, the nichrome wire has <50ohms of resistance (very little) but, for some reason, after the 9v current goes through the relay coil of even the transistor it looses the power needed to heat up the igniter wire, this just makes no sence to me (and i have tested this). i'm thinking maybe the wire is too thick or the resistance takes more away than just voltage.

 

[Exo]

the timer pulses high-low-high-low, and its power supply is only a small 1.5v button battery so the speaker voltage is low.

As you can see it go high-low-high-... I'm assuming it's not a loudspeaker, but some kind of 1.5V buzzer.

i want to use the transistor as a switch but because of the voltage drop it sometimes can't trigger the relay.

You only need 0.7V to turn a transistor on, you just never connected it right (as seen on your drawings)

and my igniter, the nichrome wire has <50ohms of resistance (very little) but, for some reason, after the 9v current goes through the relay coil of even the transistor it looses the power needed to heat up the igniter wire

Well, it shouldn't go trough the relay's coil, but trough the contacts.

But, it seems you misunderstand 'use a transistor like a switch'. In the other post (with the latch circuit) you just replaced the manual switch with a transistor - It doesn't work that way.

Try this, and see what it does. for the upper PNP you can use your 2N2907.

And try it in small steps, don't immediately connect your ignitor, but try to measure the output with a multimeter first. You can turn it on manually (without the timer) by connecting the right side (right on the scematic) of R5 to the + of your battery. Try this first, before you connect the timer to it.

 

[bd13]

ok, i built the circuit and all i get is about a steady 1v when i turn it on (9v power supply). this appears on start and even when i connection the right wire to positive it doesn't change. i went over the circuit and i think i built it right, but because its late i'll re-built it tomarow just incase i did do something wrong. thanks for your help so far.

 

[bd13]

i re-built it many times and the same result happens each time, i get about 1v over the igniter outputs when i plug in the battery.

 

[audioguru]

Hi Exo,
I think that your circuit will work fine with a reasonable load.
BD's problems are caused because his "<50 ohm" igniter with its heavy nichrome wire is a dead short. That's why he needs another battery to hold his relay latched, because the 9V battery's voltage drops down to less than 0.25V!
I wonder how hot his battery gets, it might be hotter than the igniter!

So now his circuit doesn't work because the little PNP transistor is probably shorted (internal meltdown) after having a couple of amps through it.

I was always wondering why the timer must give "pulses". Now I realise that the time between pulses is to allow that over-drained battery a chance to recover. He must go through a lot of batteries. Years ago, somebody probably found that a paper clip across a little 9V battery gets hot enough to ignite a rocket!