datasheet & op-amp question

[kavkav]

I have a 6v 1.3A battery and I want an opamp that works with it. I want the opamp tp draw as much current as it can from the battery.

My question is, which part of an opamp datasheet should i be looking at in order to meet my needs. I think i understand the the voltage part but i don't understand how you can tell how much current will be drawn or the amount of current your battery must have in order for the opamp to work.

Also, if anyone already knows an opamp that fits my needs feel free to blurt out the part number/type.

 

[Takeshi-san]

Hi, not sure, what exactly you looking for, but try to "google" low voltage operational amlifier. There is hundreds of them

 

[amitjagtap]

Well, why you want to draw a current as much as possible. As per my knowledge current drawn form battery depends on its load. Actually what exactly you want is not clear from your post. Do you want an opamp with maximum power dissipation?

 

[kavkav]

I'm new at circuits but I am making a sinewave generator from this website (the second diagram) **broken link removed**

I want to use an opamp that works with 6 volts and when I connect something to the output, I want as much current as I can get. I don't know if i'm asking the right question but if my battery gives 1.3Amps what opamp feature from an opamp datasheet am I suppose to look at so that my output (from the diagram) has as much current as i can get.

 

[Audioguru]

The sine-wave generator circuit can drive an audio power amplifier IC. Or you can make your own 1500W (or more) amplifier.
But most circuits do not work from a power supply as low as 6V.

 

[Takeshi-san]

This is not good idea to load oscillator too much - bad stability, shape of the signal. But you can also use some sort of amplifiers or drivers to get high power output. Nothing wrong to power up circuit with 6V, most of modern IC's can work from 3V

 

[kavkav]

I want to connect an electromagnet to it so i want my magnet to be able to actually lift at least a paperclip or a steel pin. If I go with a low current, how low exactly are we talking about. and also I was planning to use a transistor so the strength of the magnetic field would intensify. I do Want my sine waves to be stable and clean but how much current should my op amp draw in order to do the above.

 

[Takeshi-san]

Just use standart opamp and driver

 

[Audioguru]

Just use standard opamp and driver

But the 6V supply is too low for an opamp plus AC drivers.

An MC34071 opamp works from a supply as low as 6V and has a typical max output of 2V p-p when its output current is 20mA.
Using output complementary emitter followers the output voltage is reduced to almost nothing when the output current peaks at 300mA.

 

[FvM]

I don't understand the purpose of driving a sine wave or generally AC current to a solenoid to lift paper clips. It mainly reduces the available current from a low voltage battery. The electro magnet perfectly works with DC. You may want to drive a small opposite current to overcome the core remanence and free the paper clips.

Apart from this doubt, a H-bridge amplifier topology would be best. It's also the usual solution for portable audio amplifiers. Most of them are using class D (switched mode with PWM) amplifiers at these days. But unfortunately all low voltage portable audio amplifiers are designed for battery voltages below 5.5 V. To drive "as much current as I can get" from a 6 V battery, you have to design your own cicruit with a discrete low voltage MOSFET output stage. It can effectively switch the full battery voltage to the output, with just a few 10 mV voltage drop at the transistors.

 

[kavkav]

I don't understand the purpose of driving a sine wave or generally AC current to a solenoid to lift paper clips. It mainly reduces the available current from a low voltage battery.

To be honest it's for an experiment i am doing with a specific frequency generated by the combination of the resistors and capacitors. I just want the amplitude of the wave (electromagnetic) to be strong so that even if i were to attach a solenoid it would at least be able to attract a paperclip. I plan to do this with a transistor at the output of my device but i wanted to start off with a good amount of current rather than something really low like 50mamps. It is the reason I bought a battery capable of 1.3A.

I'm just curious, what kind of op-amp i would have to use. Takeshi-san mentioned using a standard op-amp and driver but by standard do you mean like an LM741 opamp? and by driver do you mean like a normal transistor?

Audioguru explained something i didn't quite understand only because i am new to circuits. What does "2v p-p" mean (mainly the "p-p" part)? and if Audioguru is correct, how can i fix this using the same circuit design i had in the first post. My goal is to have a high amplitude output that is capable to attract at least a paperclip if i choose to connect it to a solenoid.

Lastly if all ends in me just deciding which op-amp to use, just leading back to my original question, where in a datasheet should i be looking to determine how much output current the op-amp would give off in correlation to my supply current and voltage.

Thanks.

 

[FvM]

I bought a battery capable of 1.3A

I guess, you mean a 1.3 Ah sealed lead-acid accu? It's capable of much more than 1.3A for short duration.

Audioguru explained something i didn't quite understand only because i am new to circuits.

It's a funny way to plan experiments. You have no clue of a working high current AC amplifier circuit, but decide that it's good to use a 6 V supply. You have no actual idea about the required current for the solenoid but want "as much current as I can get". And the next question is, how do I calculate the solenoid? Did you understand by the way, that current can be exchanged with voltage by using a different number of turns?

For an experiment, I would use a more standard supply, e.g. 12 V, or even more simple for AC generation a +/- 12V or +/- 15V bipolar supply. It allows to make simple current boosters by just adding two transistors to an OP, or use standard power OPs.

 

[kavkav]

yes you are correct on the battery it is labelled "battery lead acid 6v 1.3ah Q" i dont know what accu stands for. The reason i am doing this experiment is because I came across an idea but i have no one to help me. I wouldn't be attempting building this circuit if I had someone to do it for me but i have no choice so please bare with me. i know i sound kind of dumb asking these questions but i don't think it is very difficult to do what i am trying to do i just dont have enough knowledge to do it myself.
The technician at the store said I could use the 6v battery for what i was doing so that's the reason I decided to buy it.
I made the solenoid and i have a few others so i am not too concerned about that because i can always adjust the turns myself. I was hoping one of the solenoids i have would become magnetic either way. If it really does play abig role in the experiment, which formula should i be using?

I am kind of on a budget so unless its absolutely necessary, then i would buy a 12v. but there has to be a way to do the same with 6v maybe not as much power but to attract a paperclip i'm sure theres a way using the circuit design in my first post.

 

[Audioguru]

I just want the amplitude of the wave (electromagnetic) to be strong

The amplitude is limited to almost nothing because the 6V supply voltage is too low.
The opamp has a 2V loss for each peak and the output transistors that boost the current have an additional 1V for each peak voltage loss. Then 6V is lost so there is almost no output amplitude.

I'm just curious, what kind of op-amp i would have to use. Takeshi-san mentioned using a standard op-amp and driver but by standard do you mean like an LM741 opamp?[/quote]
A 741 opamp is 42 years old and was designed to use only a 30V supply. Few work from a supply as low as 10V and maybe none work from only 6V.

by driver do you mean like a normal transistor?

What is a normal transistor? If you want power then you need ordinary power transistors.

What does "2v p-p" mean (mainly the "p-p" part)?

A sine-wave with an amplitude of 2.122V RMS is 6V pp. pp means peak to peak.

how can i fix this using the same circuit design i had in the first post. My goal is to have a high amplitude output that is capable to attract at least a paperclip if i choose to connect it to a solenoid.

You never posted a circuit.
A high amplitude requires a high supply voltage that you do not have (6V is not high).
But maybe you mean a high power which requires enough voltage for a high enough current.

where in a datasheet should i be looking to determine how much output current the op-amp would give off in correlation to my supply current and voltage.[/auote]
All opamps that work from a supply as low as only 6V have a max 20mA output current. Some little audio amp ICs like an LM386 have a max output current of 500mA when the supply is only 6V. But the output swing s only 4V p-p. The load is 8 ohms.

 

[kavkav]

My mistake, here is the circuit diagram, it is the one on the bottom: **broken link removed**

I'm just making some clarifications

so decision 1:
should i work with a 12v battery? You say a few work with 10v so if not 12v what is a common voltage to work with (on the lower voltage end). Then I should connect output to a power transistor correct?

decision 2:
Using the Lm386 in the circuit diagram as a low-voltage opamp then combining that that with a power transistor. According to what you said the Lm386 brings it down to 4v and the transistor would bring it down another 2 which leaves me with a final 2volts. In the circuit diagram there are two opamps so I'm assuming it would drop the voltage even more? Either way I think 2v is pretty low.

I think decision 1 makes more sense thanks a lot for clearing up things Audioguru. I am just curious if the two options above make sense.

 

[Audioguru]

The LM386 is a power amplifier designed to drive a speaker, it is not an opamp. If its supply is only 6V then its output swing is only 4V p-p. If its supply is 12V then its output swing is 10V p-p. It is too weak to drive a solenoid.
Adding a power transistor will increase the output current but will cause a voltage loss. One power transistor will apply a varying DC current in the solenoid and cause it to become magnetized. Two transistors plus an output coupling capacitor will allow AC in the solenoid with some additional voltage loss.