How do I tell which a device is?
I want to connect the following to a 12V source. Which need resistor consideration and which can be connected directly?
12V Fan
12V heater
12V solenoid valve
None of the above
Hello there,
All of those devices listed can be run from 12 volts.
Many devices are voltage operated in that you apply a fixed voltage and they run fine. That's usually because at that voltage they draw a certain amount of current and if the voltage changes by a small amount the current changes by a small amount. If the device under consideration is run by a voltage like that but when the voltage is changed by a small amount the current changes by a large amount (looking at the ratio of voltage to current) then the device is usually considered current operated. In other words, we want good control over what is happening to that device so we use whatever method works best.
This comes about because it is normal for the supply voltage to change by a small amount in many applications, and if that causes too much of a current change then something might burn out like the device itself, so rather than try to regulate the voltage to some extreme accuracy we drive the device with a current instead and let it seek its own comfortable voltage operating point (knowing the current will always be close to perfect).
The good example of voltage operated is the light bulb. Applying the right voltage causes the right current to flow and everything works fine even if the voltage changes by a small amount.
The good example of a current operated device is an LED, where the current must be set to some specific value and held relatively constant but there may be a range of voltages that different physical pieces of that part might operated at. In other words, when we buy an LED rated for 20ma at 3.5v, the most important spec there is the 20ma not the 3.5v because the 3.5v is just an approximation and may be as low as 3.2 or somewhat higher. This means we can not use a voltage supply of 3.5v because that may not cause 20ma to flow but rather either too little (like 5ma) or too high (like 50ma). If, on the other hand, we drive it with a current of 20ma then we dont have to worry about what the voltage is as it will be whatever that particular LED happens to be at that time and temperature. Note temperature changes the voltage characteristic too, so we can not even use a highly accurate voltage supply because it would only work at one temperature.
For LED's, often a current limiting resistor is used in series with it to make sure the current doesnt go too high when it is run from a voltage source. That means that an LED can be run from a voltage source when it has a resistor in series with it of the right value, but should be run from a current source if it does not have a resistor.
Note that it is the non linear devices that have to be looked at more carefully.
Another example of a current operated device is a bipolar transistor.
Voltage operated devices are characterized as "Ohmic Devices" (their current is more or less proportional to the applied voltage, I = E*R)
Current operated devices are characterized as "Non-linear Devices or NON-Ohmic devices" (the current through them is NOT proportional to the applied voltage).
Voltage operated devices are characterized as "Ohmic Devices" (their current is more or less proportional to the applied voltage, I = E*R)
Current operated devices are characterized as "Non-linear Devices or NON-Ohmic devices" (the current through them is NOT proportional to the applied voltage).
There is no such thing as 'waste' current.
This has already been proven beyond the shadow of a doubt by several senior members on this site.
You hope correctly. I was speaking functionally, as do most circuit design engineers. I don't give a rat's posterior about what the theoretical equations for the BJT may or may not imply about the internal behavior of the electrons and holes, if they are counter to how the device appears to behave and is normally used in the macro world. This subject has been thoroughly beat to death by you and others in another thread. You quixotically support an interesting theoretical view of how a BJT operates and you are welcome to it, but I imagine you will continue to espouse your view whenever anyone mentions your perceived blasphemy that the BJT is a current operated device.When you say "current operated", I hope you mean functionally speaking, and not operationally caused by current, or being current activated. Taking the causal point of view, the voltage applied to the base-emitter junction lowers the barrier voltage between the junctions, and allows the charge carriers to diffuse into the base region and on to the collector. The BJT needs the voltage across the junction to operate. The inevitable base current that occurs is a waste current that is not needed to operate a BJT. It is useful for design and calulation because it is somewhat linear with respect to collector current when the BJT is operated in the active region. So a BJT mimicks a current controlled device, but it is really a voltage controlled device. The only true current controlled device I know of is a magnetic amplifier.
No there isn't. The current is essential to the operation of the BJT. No such thing as waste current. No base current; no BJT operation.
Beta relates collector current to the controlling base current, and does not make the base resistor less relevant -- it's just as relevant as if Beta was lower.
Just try to bias a transistor without it and you'll quickly see how relevant it is.
This has all been proven.
You didn't have a thesis, just some made up stuff about waste current, of which no such things exists.
Senior members have proven current control of BJT with terminal voltage and current equations, device physics, real charts, all of which were in perfect agreement. You can’t just make up stuff and pretend it’s real.
You are wrong, because it is not the current that lowers the barrier voltage caused by the junction diffusion. Without that counter voltage, the BJT ceases to provide diffusion current. You are confused by the current involvement. The base current exists, but it is the base voltage that is needed.
The collector current is exponentially related to the base-emitter voltage. So is the base current. Dividing the collector current by the base current cancels out the expontial terms and give a more or less linear relationship, which is useful. That does not prove that Ib is essential for transistor operation. Only that if it exists, it can be useful.
Beta can never be made so high that base current is irrelevant or nonexistant. Base current will always be essential to BJT operation. Using a simple voltage divider doesn't eliminate base current. In fact, transistors are already biased that way. The voltage divider resistors not only divide the voltage, but also limit the base current, just as a resistor in series with the base input resistor would do. You cannot bias a transistor without limiting current, and a series resistor is always required, no matter how high the beta is. Even a voltage divider has a resistor in series with the base bias voltage.Actually, it would be easier. If the beta were so high as to make the base current irrelevant or essentially nonexistent, then the bias could be accomplished by a simple voltage divider.
You said that before. So I will say again that my thesis has been argued against, but not disproven. If you say that again, I will say the same thing in return.
Of course I had a thesis. You might not have agreed with it, but it was a thesis. The stuff I "made up" was given careful consideration and analysis. In fact, I am not the only one who thinks this way.
No they did not. Many of the arguments were irrelevant, put downs, or misunderstandings. They did not by any means "prove" it. As I said, I did not just make up my theses willy-nilly.
Well, Rachit will insist on voltage control until kingdom come. Don't believe it, no matter how many cartoons he posts. Just stick with the definitions given before his posts, and you'll be fine.
You hope correctly. I was speaking functionally, as do most circuit design engineers. I don't give a rat's posterior about what the theoretical equations for the BJT may or may not imply about the internal behavior of the electrons and holes, if they are counter to how the device appears to behave and is normally used in the macro world.
This subject has been thoroughly beat to death by you and others in another thread.
You quixotically support an interesting theoretical view of how a BJT operates and you are welcome to it, but I imagine you will continue to espouse your view whenever anyone mentions your perceived blasphemy that the BJT is a current operated device
From an external point of view, the base voltage you mention can be just as easily (and more intuitively) be interpreted as the base emitter forward diode voltage necessary to generate a specific base current which is how it appears on a curve tracer, not the other way around.
In practical applications (other than perhaps at RF) the BJT looks like a current operated device, acts like a current operated device, and is spec'd like a current operated device in the data sheets. To state otherwise just confuses the issue for the neophyte circuit designer. But you appear to enjoy doing that so I suppose we're sort of at a Mexican Standoff.
If the base current is just waste current, then is it theoretically possible to build a BJT that takes no base current? And if it is, why can't it be done?
P.S. I totally agree with your sign-off phrase.
You are wrong. The base-emitter junction is current controlled. No voltage can lower another voltage. As we've already proved, the junction voltage is lowered by charge which neutrailzes depleation layer ions. No other physical means exists to lower barrier voltage. No voltage can lower another voltage.
It's not meant to prove Ib is essential, but Ib is essential. If it stops, all transistor action stops. Show me any example where base current stops and a BJT continues to operate. Unless you show a real example, then base current is proved to be essential.
Yeah, any 9 year old argues that way.
Hardly. Nobody espouses the fantasy of waste current. You can't just make up stuff and pretend it's real. Let's keep the discussion about real physical quanties, and not make stuff up.
Our senior members do not misunderstand. They have the experience and background to know what they are talking about. Those who have not the same experience would do well to learn from those who do.
Well, Rachit will insist on voltage control until kingdom come. Don't believe it, no matter how many cartoons he posts. Just stick with the definitions given before his posts, and you'll be fine.
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