Transistor/capacitor confusion

[Vincent]

Hi all,

I'm trying to build a cheap one shot relay driver. I had some electronics during my uni years but that's been a while and I'm having fun digging it all up again but I have a few questions I hope someone can answer for me.

I would like a bi-stable relay to be switched by a pulse, even if the input (switch) stays closed a bit longer (let's say 10 seconds). Lots of people tell me to use a 555 but I'd rather use 'simple' components. Not that I can't understand a 555 but I just like this better and it makes me understand the components better. So, let's forget about the 555 OK?

My first circuit looked like this:

The idea being that after closing the switch, C1 has no charge and T1 would be saturated, powering the switch until C1 has so much charge that Vbe would be too small and T1 would close. Once the switch is opened, C1 would recharge through R2.

I had this in a test setup and it worked. A few people had some recommendations which lead me to this changed schematic:

D2 is added to allow C1 to properly discharge. People told me that without it it would only 'leak'. Anybody have any comments on this?

Also for T1 (=bc457) there is a max Vbe of 7V so I added a voltage divider R3 to apply less than 7V to R1/C1/T1.

Another question I have is this: Ic for T1 is Ib*Thfe. But A capacitor does not let DC current through, correct? If the latter is true, then howcome the transistor conducts at all? It's confusing because some people say all it takes is Vbe around 0.6V for T1 to start conducting but they don't mention Ib at all. So is a base current really needed or is a Vbe of around 0.6V all it takes? Or does C1 conduct?

Any comments on my circuit and/or answers to my questions would be much appreciated.

Thanks,

Vincent.

 

[ccurtis]

But A capacitor does not let DC current through, correct?

There is charging current through the capacitor that occurs from switching of the voltage from one level to another level. A change in magnitude is not strictly a DC voltage (or current) -- therefore current flows.

So is a base current really needed or is a Vbe of around 0.6V all it takes? Or does C1 conduct?

A base current is what is needed to operate the transistor. Vbe is reached when the current reaches a certain level. Base current will flow before Vbe is reached, but it will be very small compared to when Vbe is reached. This means that a voltage of at least Vbe must be applied for a significant base current to flow, unless a current source is used.

BTW, you say your first circuit works, but the relay does not stay latched. No?

 

[kchriste]

D2 is added to allow C1 to properly discharge. People told me that without it it would only 'leak'. Anybody have any comments on this?

That is correct. D2 allows C1 to discharge through R1 and R2 when SW1 is opened.

Also for T1 (=bc457) there is a max Vbe of 7V so I added a voltage divider R3 to apply less than 7V to R1/C1/T1.

They really should have said Veb instead of Vbe. The Veb of 7V is for when the base is more negative than the emitter (reverse biased). D2 prevents this from happening when C1 is discharging by clamping the negative voltage on the base to within -0.7V of ground. So you don't need R3 at all.

 

[colin55]

Here's the circuit you need:
**broken link removed**

The circuit produces a strong pulse to latch the relay ON and the input voltage must remain HIGH. The 220u gradually charges and the current falls to a very low level. When the input voltage is removed, the circuit produces a pulse in the opposite direction to unlatch the relay.

 

[Vincent]

@ all: Thanks for your replies

@ ccurtis: It actually IS a latching relay. I use a copy of the circuit to switch the relay back.

So are you saying that while the capacitor is charging, a current flows through? I thought since a capacitor kind of acts as a container, current could only flow 'in' but not 'through'.

@kchriste: The thing about Veb sounds good and it is indeed according to the datasheet information so i will remove R3. Thanks for pointing that out!

What i don't understand though is this: I was taught that voltage goes the opposite way of current (when 'thinking' about it). If we speak of Ibe = 5mA (current flowing from base to emitter) should the voltage created of 0.6V not be called Veb because it's opposite?

@ collin55: I'm using a bi-stable latching relay so that it does not have to stay powered to be switched in a certain direction.

 

[kchriste]

If we speak of Ibe = 5mA (current flowing from base to emitter) should the voltage created of 0.6V not be called Veb because it's opposite?

No. Then the voltage is called Vbe. An easy way to think of it is this way: Your positive lead of your Voltmeter (Vbe) goes on the 2nd letter of Vbe and the negative lead on the 3rd letter of Vbe. ie: 7Veb = -7Vbe.

 

[colin55]

In the circuit I have provided, the relay does not have to be powered, but the CIRCUIT has to be powered. The circuit draws almost no current once the electrolytic is charged.

 

[ccurtis]

@ ccurtis: It actually IS a latching relay. I use a copy of the circuit to switch the relay back.

Then I have the same question as you do. What is sustaining the base current when S1 is open? Nothing that I can see. Maybe the transistor is in avalanche condition? Is the relay, itself, a latching type?

So are you saying that while the capacitor is charging, a current flows through? I thought since a capacitor kind of acts as a container, current could only flow 'in' but not 'through'.

Through is not the correct term, as it is electrons rushing onto one plate to match the charge on the other plate of the capacitor, until the charge is equal on each plate. As a practical matter, this action manifests itself as if a current is flowing through the capacitor. And so, though not technically correct, it is very often said as such.

 

[duffy]

Vincent, what the heck is your avatar?

 

[Vincent]

@ kchriste: Thanks for clarifying that

@ colin55: I choose a bi-stable latching relay so that I could use a circuit that consumes no power at all. Even if it uses very little power, why use any power at all if it's not required? Thanks for showing me your idea though, it's always interesting seeing alternatives But I prefer to stick with my own design.

@ ccurtis: The relay itself is latching, that is correct. All it takes to bring the switch in a certain position is one short pulse. When the pulse 'dies' the swith stays in that position. The relay actually has two coils so it can be switched in both directions. That's why I use two of the circuits I posted. Regarding the electrons. In uni we were told that electrons actually move in the opposite direction of what we 'think of' as current. So electrons move from - to +. To fully understand how a capacitor works I should probably read up on that stuff now. For now your explanation is enough for me to at least continue the way I was going.

Thanks everybody for your replies, it helped me a lot

Have a good remainder of your weekends!

 

[Vincent]

@ duffy: It's a robotic chicken from the computer game World of Warcraft.

 

[ccurtis]

Regarding the electrons. In uni we were told that electrons actually move in the opposite direction of what we 'think of' as current. So electrons move from - to +.

What we "think of" as current is usually wrong, but it is of little consequence in practice. At the atomic level, an individual electron moves only a very little distance at once and drifts very slowly in one direction (for DC) and does not flow smoothly as water in a pipe. An electron moves to fill in a "hole" in a nearby atom that does not have a full set of electrons (an atom with a net positive charge). This is more equivalent (though not fully accurate either) to that famous toy with a set of balls in sequence, in a straight line, hanging next to each other from strings attached to a bar above them. When one of the end balls is swung to hit the sequence of balls, the ball on the other end flies away. It is the action at a distance that occurs very quickly. A "hole" is often visualized (incorrectly) as a positive charge moving in an opposite direction to that of the electron (conventional current flow). The hole is actually a lack of an electron in an immobile atom, and not an object having mass.

 

[duffy]

that famous toy with a set of balls in sequence, in a straight line, hanging next to each other from strings attached to a bar above them.

It's called a "Newton's cradle".