Current Amplifier

[yasir_ali]

hello every one.
I have designed an Oscillator using 741 op-amp. it can supply 20mA maximum But i have to have 3A for my circuit purpose. so please suggest me the current Amplifier that i can design to have 3A at the output.

thanks

 

[bychon]

Have the 741 control the gate of a MOSFET. The MOSFET will handle 3 amps if you choose the right one.

 

[ericgibbs]

hello every one.
I have designed an Oscillator using 741 op-amp. it can supply 20mA maximum But i have to have 3A for my circuit purpose. so please suggest me the current Amplifier that i can design to have 3A at the output.

thanks

hi,
What is the supply voltage to the 741 opa.?

Also what is the 3Amp load, a motor etc..?

 

[yasir_ali]

i am adding my Oscillator circuit.
And yes Eric, you are right.The load is a motor.which require 2.4A. i am thinking that i must amplifier to 3A.

thanks

 

[bychon]

Does this drawing mean you're using an AC motor?

 

[ericgibbs]

i am adding my Oscillator circuit.
And yes Eric, you are right.The load is a motor.which require 2.4A. i am thinking that i must amplifier to 3A.

thanks

hi,
I would ask the same as Bychon, looks like an 'ac' motor drive.??

If the answer is yes, then you will most likely need a push/pull FET drive.
The resistive split 12V supply will give you problems at 2.4A if you try to reference the FETs to the virtual 0V.

 

[yasir_ali]

yes the motor in A.c and requires 165v,2.4A and 300Hz. i'll design the oscillator with 300Hz but the main problem is how can i boost the current?

also Eric tell the way i can connect the this Oscillator with single supply.

Thanks

 

[yasir_ali]

Isn't there anyone who can help me?

 

[ericgibbs]

Isn't there anyone who can help me?

hi,
At that voltage and current any transistor being driven by a sine wave input and producing a sinewave output will have to be highly rated and mounted on a heatsink.

Look at this complementary transistor pair pdf for an example.

What you are trying to do is not easy, is it essential that the motor is powered by a sine wave rather than a 300Hz square wave.??

 

[yasir_ali]

@ Eric
yes it is written on the motor that i requires these things to run in a fine mood.
and what about this Transistor? can i just simply bias it like we bias the normal common emitter Amplifier to give me 3A?

 

[ericgibbs]

@ Eric
yes it is written on the motor that i requires these things to run in a fine mood.
and what about this Transistor? can i just simply bias it like we bias the normal common emitter Amplifier to give me 3A?

hi,
With these transistors you will require another transistor to drive them.
If you look at the datasheet you will see the HFE is about 10 at the collector current you require.
So for a collector current of 2.4A, a base drive current of at least 2.4/10 [ 0.24A] is required.
A driver transistor for each power transistor would require a 0.5A collector rating.

You could consider using N and P MOSFET's in place of the the power transistors.

Most AC motors will work well with a square wave drive, a square wave power driver would be easier to design and also be more efficient.

 

[yasir_ali]

hi
OK. i got the idea that how drive this power Amplifier. and i think it is very easy. look at the attachment what i am think is correct?

 

[Hero999]

What you're doing will work but it won't be very efficient.

Is 165V the RMS voltage or is it the peak?

The typical class AB amplifier required will be typically only 50% efficient meaning half the power is wasted as heat.

A more efficient method is to use a class D amplifier which is >80% efficient but much more complicated.

Are you aware that you can buy ready-built variable frequency AC motor drives to do this or you you want to make it for educational purposes?

 

[yasir_ali]

@hero
yeah! i am not aware of the drives if available in market. and i am designing it for educational purpose. i wanna learn and also want to know about available drives.

165v is RMS value

 

[ericgibbs]

hi
OK. i got the idea that how drive this power Amplifier. and i think it is very easy. look at the attachment what i am think is correct?

hi yasir_ali,
That sketch of yours shows the basic block diagram.

What plans do you have for the HT supply to the output transistors/FET's.?

 

[yasir_ali]

@ Eric
Sorry! Eric i didn't understand.what do you want to ask me.

 

[ericgibbs]

@ Eric
Sorry! Eric i didn't understand.what do you want to ask me.

OK,
If you plan to drive a sine wave signal of say 165Vrms thats approx 235Vpeak. [~400Watts]

That means a power supply of +/-250Vdc rated at say 3A to 5A

 

[yasir_ali]

yeah! you are absolutely right. i only have a truck battery of full size (i don't know what we would name it ).

what i was think is that, that first i'll convert D.c into A.c with 300Hz and then amplify the current and then after having enough current i'll step-up the voltages.
What do you suggest? is it possible?

 

[ericgibbs]

yeah! you are absolutely right. i only have a truck battery of full size (i don't know what we would name it ).

what i was think is that, that first i'll convert D.c into A.c with 300Hz and then amplify the current and then after having enough current i'll step-up the voltages.
What do you suggest? is it possible?

hi,
If I follow you, its a truck battery 24V.????

If yes, this would mean a step up to about 165Vrms [240Vpk] using a 300Hz switching inverter, with a centre tapped winding in order to give 240Vppk.

Even at 100% efficiency for a 2.4Arms motor current thats at least 24Vamp draw from the battery.
If the 'η' of the inverter was say, 70% thats approx 35A.

I would suggest you post a more detailed drawing of how you see the project connected to the motor.

 

[yasir_ali]

No. it's a 12v battery.
I am attaching the block diagram, what i was thinking that i would design my circuit in this manner. but you people are saying that this is not very efficient so i'll change my idea according to yours.

Thanks