Electronic Switches General

[segaloco]

So I've enjoyed chatting here and learning more about transistors on the fundamental level, and just got my first batch of tubes from the antique store to play with "the old way". In general I am fascinated with the applications for binary switching, be it relays, tubes, transistors, or whatever is next. Rather than endless piecemeal topics swirling the bowl, figured I'd make this one as its a topic I just like learning and talking about. Anyway.

So I find myself with a question of tube function. My understanding is you heat a filament, like a light bulb, but the goal is electron rather than photon emission. A plate is placed a distance away such that without other influence, even with a potential between the filament and plate, the current cannot flow through the air between. The grid, placed in between and given a potential, creates favorable conditions to then draw the electrons and flow of current in a controlled direction towards the plate. This of course is then what transistor action is analogous to, what with changing the electrical conditions of a space between two potentials.

However, is there anything like a vacuum tube but that operates in a "depletion mode"? In other words, you apply a potential to turn it off, rather than on? I picked up a random array of vacuum tubes at a shop today and am trying to draw various parallels to transistor circuit design.

 

[hmb]

So I've enjoyed chatting here and learning more about transistors on the fundamental level, and just got my first batch of tubes from the antique store to play with "the old way". In general I am fascinated with the applications for binary switching, be it relays, tubes, transistors, or whatever is next. Rather than endless piecemeal topics swirling the bowl, figured I'd make this one as its a topic I just like learning and talking about. Anyway.

So I find myself with a question of tube function. My understanding is you heat a filament, like a light bulb, but the goal is electron rather than photon emission. A plate is placed a distance away such that without other influence, even with a potential between the filament and plate, the current cannot flow through the air between. The grid, placed in between and given a potential, creates favorable conditions to then draw the electrons and flow of current in a controlled direction towards the plate. This of course is then what transistor action is analogous to, what with changing the electrical conditions of a space between two potentials.

However, is there anything like a vacuum tube but that operates in a "depletion mode"? In other words, you apply a potential to turn it off, rather than on? I picked up a random array of vacuum tubes at a shop today and am trying to draw various parallels to transistor circuit design.

The operation of a vacuum tube is not quite as you described.
1. there is no air inside the tube, hence the reason they are referred to as vacuum tubes
2. the filament heats the tube's cathode. This facilitates emission of electrons from the surface of the cathode.
3. when a positive charge is applied to the anode (with respect to the cathode) electrons emitted by the cathode are attracted towards the anode.
3. in a vacuum tube with a cathode and an anode, with a positive charge applied on the anode with respect to the cathode, current flow is in one direction with electrons travelling from cathode to anode. If the anode is not positively charged with respect to the cathode, no current will flow. A vacuum tube with just a cathode and an anode functions as a basic diode.
4. in a vacuum tube with a cathode, a grid located between anode and cathode, and an anode, electron flow from cathode to anode can be controlled by changing the voltage applied to the grid. This type of vacuum tube is a basic triode.
5. additional grids can be added (creating a tetrode or a pentode) to further enhance the tube's performance

 

[segaloco]

1. I'll make sure I pay my editor better to appease people who can't appreciate that I'm not a moron and "the air" meant the space between, not literal air.

2. I said you heat a filament. That filament emits electrons. Once again, I'll pay my richly paid editor a bit more to ensure I can ask questions and not have to question whether I can describe something casually *adjacent* to a question without that being picked apart like a carcass in the desert. Maybe that'll help focus discussion on the things to discuss.

3. Okay, so its already "depletion" and I had it backwards in that the grid is charged to interfere, not enhance?

4/5. Indeed.

Sorry to be bristly but I really am tired of the way every piece of internet content has to be absolutely perfect. I'm just a person, if you want perfect content go pay for a subscription to a magazine geeze, I can only be so paranoid that every single thing that is not the subject is going to be overanalyzed. I tell myself to get over it, it won't happen, just hit the post button...and it happens.

Be nice, sometimes you're literally making someone's avoidance come true by doing the exact thing they're anxious about.

 

[hmb]

1. I'll make sure I pay my editor better to appease people who can't appreciate that I'm not a moron and "the air" meant the space between, not literal air.

2. I said you heat a filament. That filament emits electrons. Once again, I'll pay my richly paid editor a bit more to ensure I can ask questions and not have to question whether I can describe something casually *adjacent* to a question without that being picked apart like a carcass in the desert. Maybe that'll help focus discussion on the things to discuss.

3. Okay, so its already "depletion" and I had it backwards in that the grid is charged to interfere, not enhance?

4/5. Indeed.

Sorry to be bristly but I really am tired of the way every piece of internet content has to be absolutely perfect. I'm just a person, if you want perfect content go pay for a subscription to a magazine geeze, I can only be so paranoid that every single thing that is not the subject is going to be overanalyzed. I tell myself to get over it, it won't happen, just hit the post button...and it happens.

Be nice, sometimes you're literally making someone's avoidance come true by doing the exact thing they're anxious about.

My apologies... I really hadn't intended to be bristly or condescending. Just clear and precise.

 

[Bentendo64]

In an oversimplified sense, a vacuum tube (triod, tetrode, etc...) acts as a type of transconductance amplifier. When the grid voltage is varied, the plate current is controlled.

This is a little unlike a BJT, which in a similar oversimplified sense is a current controlled current amplifier. You vary the base current to control the emitter current.

I think a tube is more similar to a MOSFET, which in its oversimplified explanation is a voltage controlled resistor. When the gate voltage is varied, the source current is controlled.