Can some one give me a schematic on a current source, adjustable around 3 A. - diyAudio
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Old 13th November 2005, 08:26 PM   #1
Progg70 is offline Progg70  Sweden
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Cool Can some one give me a schematic on a current source, adjustable around 3 A.

Can some one give me a schematic on a current source, adjustable around 3 A. Iím going to have it in the output stage, current stage in a power amp, to drive the speaker output.
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Old 13th November 2005, 08:30 PM   #2
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Look at some NP amp, there are many these circuits...
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Old 13th November 2005, 10:32 PM   #3
AKSA is offline AKSA  Australia
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A good current sink which can dip to within a volt of the rail can be made with two green LEDs, a 0R22 5W resistor, and an IRFP150.

This is a current sink, located at ground. For one supported from the rail, invert devices to an IRFP9140.

The two LEDs are run in series at around 2V each from the positive rail via a dropper resistor, dimensioned to give 4mA.

An electrolytic around 47uF is placed across the two LEDs to kill noise.

A 220R stopper resistor is passed from the anode of the upper LED to the gate of the mosfet. A 12V zener is placed (kathod uppermost) from gate to ground to protect against gate spikes.

The source is connected to ground via the 0R22 resistor.

The CCS output is the drain of the mosfet. Current can be adjusted by altering the value of the dropper resistor on the LEDs. For a 24V rail, start around 4K7.

The advantage of a mosfet here is thermal reliability and positive tempco over about 300mA. These devices are far superior from the POV of thermal abuse, since as they get hotter the current reduces, rather than increases as it does with bipolar devices.

Cheers,

Hugh
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Old 13th November 2005, 11:17 PM   #4
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What plays we are game now, Hugh ? Exist many primitive circuits,which we can take " as standard ". I'm so tired, to explain it to beginers again and again....
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Old 13th November 2005, 11:32 PM   #5
AKSA is offline AKSA  Australia
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No game, Epops,

This is a simple circuit which works well. That was what the guy asked; I explained it as best I can. What exactly is the problem? Are you criticising, or participating - it is not clear.

Hugh
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Old 14th November 2005, 03:21 AM   #6
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Hi Progg70,

I guess you are going to build a SE output stage?

Attached I will cheer my idea of a CCS that is far more superior regarding current and thermal stability.

I checked the suggested FET IRFP150 by Hugh and think you should not just fix the gate to a voltage refference, this is because the FET change the Vgs so dramatically by temperature.
The datasheet I looked up is Fairchilds counterpart and for a given Vgs at 25 C degrees corresponding to 3 A you will have 8A at 175 C degrees which can lead to thermal runaway.

Now you wont probably experience such a high temperature change but this shows how bad it is to use FET's as CCS with a constant gate voltage.
The positive tempco of Rdson will never compensate for that as it is not working fully saturated state but rather in it's linear region, so that's why you need adding a device supervising the current, this is done as in my drawing by Q1.
Moreover it will also compensate for all kind of deviations like changing current by Vds changes (eg. your music signal).
However I deeply agree with Hugh to use FET's because of their robustnes and easy driving.

You will actually have a slight negative temperature coefficient when Q1 becomes warmer of surrounding components when the output power grows which might be welcome.

With R1 0,22 Ohm you will have around 3A.
Q2 is your own choice, but Hugh's suggestion is fine.

Cheers Michael

EDIT: btw, Hugh, are anyone of us misunderstanding what Progg70 want's?
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Old 14th November 2005, 06:10 AM   #7
AKSA is offline AKSA  Australia
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Michael,

It's not clear precisely what Progg70 wants; the voltage at the rail, the swing required, etc.

I agree your bipolar sensing network is superior from a current stability POV, no question. However, I've used a couple of the IRFP150s in parallel with 2 x 0R47 source resistors referenced from a couple of LEDs for about ten years on one of my amps and it is absolutely stable and settles very nicely at 2.8A total.

I earlier used your bipolar drive approach, but found the LED solution was no less consistent after warming up and there are visual indicators of operation!

Of course, you could combine both systems easily......

Cheers,

Hugh
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Old 14th November 2005, 09:38 AM   #8
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Quote:
Originally posted by AKSA
Of course, you could combine both systems easily......

Cheers, Hugh
Yes!

And if we would make a combination with LED's, I would put it "above" the gate instead of "below"!

I just want to clarify and add some further comments to my previous post, I mentioned:
Quote:
The datasheet I looked up is Fairchilds counterpart and for a given Vgs at 25 C degrees corresponding to 3 A you will have 8A at 175 C degrees which can lead to thermal runaway.
I should say that above statement is valid when there's no source resistor, when we have a source resistor the problem is of course not that severe, just for Progg70's information.

Cheers Michael
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Old 14th November 2005, 10:22 AM   #9
Progg70 is offline Progg70  Sweden
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Yes itís corrected that I want to build a Single-end power amplifier, (I guess you are going to build a SE output stage?).

Iím really glad people answer my question. Iím going to look on your recommendations.

What happens if you donít have the series resistor and have a bias voltage on the current transistor that then opens up and then you have a current floating true it?

See the picture
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Old 14th November 2005, 10:32 AM   #10
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You will get thermal runaway in that case. When the transistor gets hotter you will get a higher current for the same Vbe voltage, which means it will get even hotter and the current will be even higher ...
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