Hello,

I have read, in the education section, about a current to voltage converter. It is an op-amp circuit with negative feedback. The current signal, generated by a transducer, is converted to a voltage signal in the sense that the circuit acts like an ideal current source: same current no matter what the attached load and its impedance may be. Is that correct?

In general, what do the terms "current signal" and a "voltage signal" mean? I signal is something that carries information, either in the form of a current or of a voltage. But the two seem pretty equivalent to me, since, in virtue of Ohm's law, a current can always be converted to a voltage using a resistance and vice versa.

Thanks,
antennaboy

 

Not everything obeys Ohm's Law.

Consider the power outlets in your house. They are always pretty close to the same voltage, but the current is all over the place.

 

A signal is the output of a transducer, not a generator. A generator outputs both current and voltage, a transducer will output a voltage OR a current. So some amps need to amplify current and other amps need to amplify voltage from the transducer.

Then....it depends on what the amp is driving. Some outputs need current...and some outputs need voltage.

Hence current to voltage and voltage to current amps.

 

Thanks everyone.

I see how sensors can either produce a current or a voltage response.

BR-549, what do you mean that some outputs need current and some output need voltage? Do you have a simple example of each one of these situations? Electrical devices need electric power which comprises both current I and voltage V. For example, when charging a phone, the phone requires a specific voltage and draws a specific current (as long as the source can provide that current).

 

Not everything obeys Ohm's Law.

Heretic!!!! You shall be burned at the stake for your LIEEEEESSSSSSSS!!!!!

 

Thanks everyone.

I see how sensors can either produce a current or a voltage response.

BR-549, what do you mean that some outputs need current and some output need voltage? Do you have a simple example of each one of these situations? Electrical devices need electric power which comprises both current I and voltage V. For example, when charging a phone, the phone requires a specific voltage and draws a specific current (as long as the source can provide that current).

Your problem possibly stems from an imperfect understanding of voltage and current. Using your cellphone example, 5V is not something going into the phone, it is an indicator of the attraction by the positive pole of the power-supply on the electrons in the negative pole of the supply. That is why it's called *potential*. You have up 5 volts of possibility to make electrons move. An electronic circuit is nothing more than a very careful management strategy to control the flow of electrons through it, without upsetting that potential and causing the flow to stop. The power-supply doesn't determine how much current flows into a device-- only the circuit determines that. And it is designed to handle the flow of current through it no faster than the potential it is designed for (in this case, 5V). If you plug a 5V circuit into a 12V power supply, that potential is too great-- it can force current to flow through the circuit faster than components within the circuit can handle- frying them.

 

A good application of both is in process control. You can transmit a value of a transducer long distances using a 4-20 mA current loop. 4 mA is reserved to operate the transducer.

A solar cell can be thought of as a battery, BUT if you maintain the voltage across the solar cell at 0V, it's output current is proportional to light intensity. That's a job for a current to voltage converter.

There are different ways to measure currents (voltage across a resistor, the transimpeadance amplifier and hall effect) to name a few.

How would you measure 1e-9 Amps or 1 Nanoamp?

Measurements introduce errors. The Voltage Burden affects current measurements. The input impeadance of the voltmeter affects voltage measurements.

 

Sorry antennaboy...I wouldn't want to add confusion. I was trying to relate it in a simple way.
Using the questioner's...words. It's just a way of describing an amp.

Perhaps saying a high to low or a low to high, impedance converter amp, would explain it better. I did not say negative impedance converter.

Electric devices do need power. Do you know why? Because signals don't have enough power. That's why we amplify them. We add power to them...with electrical devices.

A transducer and a generator/power supply are two completely different animals.

A current signal will always have a voltage and a voltage signal will always have a current.

Either a resistor or a cap can adjust that ratio. Your purpose, available parts and convenience will determine whether you use voltage or current for your signal. You may use both if you want to.

Did I confuse you more?

 

Thanks again everyone.

BobaMosfet: I see how a power supply (either a voltage supply or a current supply. I don't know of any other type) connects to a load to provide electrical energy. In the case of a voltage supply, the connected load is given the "right", recommended operating voltage. The load then draws the current it needs from the supply (same reasoning goes for a current supply). When charging electrical devices, both the voltage and the current is important. Solar panel almost always provide the right voltage, but depending on the sun level, the current drawn by the charged device may be sufficient or not.

As far as voltage to current converters, the current through the load is the same regardless of the the load's impedance. Simplistically, this happens because there is a large impedance that sets that constant current. The load current has minimal effect in setting that current.

Feel free to point out any heresy I said. Thank you!!

 

I would hate to muck up your understanding, but it's not too bad for now. I wont say it's 100% accurate, but it's a start.

Your "power supply" definition is only a subset.

I would like to introduce the concept of a "load line" See https://en.wikipedia.org/wiki/Load_line_(electronics)

This says the load has a set of operating points. Simplistically take a battery operated radio that might work from 9V to 6V. That I-V curve if straight would be a "resistance" or the load line.

The battery or source also has some sort of characteristics.

The "operating point" is the intersection of what the supply can deliver and what the load can use.

==

We can further classify a constant voltage (CV) power supply as 1) CV, current limit 2) CV, constant current

Current limit can be provided by a fuse or electronic circuitry. Overvoltage protection can be employed.

==

An arc lamp I used may have an entirely different type of power supply. The nominal voltage was about 22 V and the current about 42 Amps. Since the lamp contains high pressure Xeon and two electrodes, that gap is to large for any conduction to take place. You have to superimpose a 40 kV pulse to get the lamp to start.

In this particular source, intensity was controlled which is related to power. The supply is designed to operated in a constant current mode with an open circuit voltage (limit). Based on the light intensity measurement, the set point for the lamp current is constantly changing keeping the light intensity constant.

 

Thanks again everyone.

BobaMosfet: I see how a power supply (either a voltage supply or a current supply. I don't know of any other type) connects to a load to provide electrical energy. In the case of a voltage supply, the connected load is given the "right", recommended operating voltage. The load then draws the current it needs from the supply (same reasoning goes for a current supply). When charging electrical devices, both the voltage and the current is important. Solar panel almost always provide the right voltage, but depending on the sun level, the current drawn by the charged device may be sufficient or not.

As far as voltage to current converters, the current through the load is the same regardless of the the load's impedance. Simplistically, this happens because there is a large impedance that sets that constant current. The load current has minimal effect in setting that current.

Feel free to point out any heresy I said. Thank you!!

A constant current source regulates the current actively, which in reality, with the exception of one value, starves a load of the current it would otherwise allow to pass through it. Ohm's law still applies.