Author Topic: Symmetric power supply from 0-15V Power supply  (Read 23967 times)

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Offline MegaWatt

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Re: Symmetric power supply from 0-15V Power supply
« Reply #25 on: January 01, 2011, 08:37:33 pm »
the chip is use is the MAX038 function generator.

The MAX038 is no longer produced. If you don't have access to old stock it is not a good choice for a new development.

Yes, I have noticed that!
Is there any good replacement chip with some same functionallity?
 

Offline Chasm

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Re: Symmetric power supply from 0-15V Power supply
« Reply #26 on: January 01, 2011, 08:53:57 pm »
I would not call it a problem. I already know how to generate magic smoke in large quantities.  8)
MegaWatt wants to power a +/-5V device out of his 0-15V supply. The virtual ground circuit will enable him do that.
Now I would like to make it a bit more operator proof. (In a different way than a pair of zener diodes.)

Maybe I have a brain fart, but I still think that a pair of 7805/7905 will work on the output side of the virtual ground generator.
The real problem is the combined dropout voltage which will increase the minimal input voltage to 15V, and I've ignored it.

Using one 10V regulator on the input side, and then the opamp to generate the virtual ground will definitely work. But so will zener diodes as long as you are not using low power types.


"Just" using a pair of 78xx/79xx regulators without a (virtual) ground in between them will not work.
That is actually a nice thing to try and proof if you have a pair of them.
 

Online Zero999

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Re: Symmetric power supply from 0-15V Power supply
« Reply #27 on: January 01, 2011, 09:37:49 pm »
Is there any good replacement chip with some same functionallity?
There's no pin out compatible replacement.

Function generator ICs don't seem to be very popular nowadays. If you want to make a square/triangle and sine wave generator, your best bet is to build a triangle wave generator and use a wave shaper circuit to get the sine.
 

Offline Bored@Work

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Re: Symmetric power supply from 0-15V Power supply
« Reply #28 on: January 01, 2011, 09:51:43 pm »
Is there any good replacement chip with some same functionallity?

I am not aware of any. The MAX038 was really special, and I don't know any chip with similar performance. The old XR2206 is still available, but generates rather mediocre signals. The ICL8038 (notice the similar number?) is also obsolete.

The state of the art for some time now are DDS chips, e.g. from Analog Devices. To use them in a functiong enerator you usually need to have an MCU to control and configure them.
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Offline MegaWatt

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Re: Symmetric power supply from 0-15V Power supply
« Reply #29 on: January 01, 2011, 09:55:19 pm »
Hmm, too bad! For me it doesn't really matter, I'm making a function generator for home use, and I got a MAX038 from ebay, but too bad they don't make it anymore, it's a nice chip!
 

Offline Bored@Work

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Re: Symmetric power supply from 0-15V Power supply
« Reply #30 on: January 01, 2011, 10:13:10 pm »
If I remember the story (don't quote me on this), they had to stop production after the fab burned down, they couldn't find another fab with the same process and didn't invest in rebuilding the fab or building a new one.
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Offline Simon

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Re: Symmetric power supply from 0-15V Power supply
« Reply #31 on: January 01, 2011, 10:21:20 pm »
I must say it seemed odd that they are no longer available, last i looked they were not too cheap (10 years ago) but I've never seen such a versatile chip. maybe they lost the blueprints ? or it's cost meant it was not popular enough
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alm

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Re: Symmetric power supply from 0-15V Power supply
« Reply #32 on: January 02, 2011, 03:35:00 pm »
I believe they outsourced the production of the MAX038 to TSMC, and TSMC discontinued the process it used due to lack of demand. Selling a few MAX038s for building function generators isn't enough to keep a semiconductor process profitable I guess ;).

Quote
This product was manufactured for Maxim by an outside wafer foundry using a process that is no longer available.
 

Online Zero999

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Re: Symmetric power supply from 0-15V Power supply
« Reply #33 on: January 02, 2011, 04:02:16 pm »
What amazes me is that there are still lots of ICs which are obsolete for commercial devices and only useful to hobbyists still in production such as the NE555 timer and the µA741 op-amp.
 

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Re: Symmetric power supply from 0-15V Power supply
« Reply #34 on: January 02, 2011, 04:04:33 pm »
there must be a lot of us out there using them  ;D
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Offline Chasm

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Re: Symmetric power supply from 0-15V Power supply
« Reply #35 on: January 02, 2011, 04:57:47 pm »
There are a zillion designs that use the parts. It is easier to keep the part alive than to redesign them.

As for the µA741...
I'm currently ramping up to build a linear power supply that uses two of them (or the various replacement types).
The design is from 1973 - and it was and is still the underlying principle in many brand name linear power supplies from Gossen, HP and Agilent and the like.

The design may be old design but it is still very applicable, still easily changed for different voltages and currend. Especially with a couple of mods that fix bugs and make life easier. Not that I came up with any of the modifications, but I'm not going to reject them just because of that. ;)

And on that note, back to the layout.
 

Offline Wim_L

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Re: Symmetric power supply from 0-15V Power supply
« Reply #36 on: January 02, 2011, 10:38:38 pm »
I think the real question was why you'd put a 741 in there when you can get another opamp that's better and cheaper these days. The only reason I can think of is that they're the typical textbook opamp and people just naturally reach for one when they want a noncritical opamp and don't want to bother searching for devices.
 

Online Zero999

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Re: Symmetric power supply from 0-15V Power supply
« Reply #37 on: January 02, 2011, 10:51:31 pm »
Most modern op-amps follow the same pin out as the old 741 so it can be replaced without changing the PCB most of the time.

The NE555 can be directly replaced with the ICM7555 or the TS555 in old designs or anything from CMOS logic gates to a comparator or microcontroller for new designs.
 

Offline Chasm

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Re: Symmetric power supply from 0-15V Power supply
« Reply #38 on: January 03, 2011, 12:34:32 am »
One part of the answer is that the circuit does does not really need a that much faster opamp.
Here is a link to the lastest, modified version. The author has recently measured his power supply. Switching a (FET) load from 0.3 to 1 Amp at 300Hz takes 20 micro seconds to regulate. Others have confirmed these values.
Ok...  admittedly using a TL071 but still not to shabby for a not much newer old op-amp. ;)


A less obvious part of the answer are the involved voltages.
This is old(er than me) analog tech. Most of todays op-amps do not survive Voltages in the 30V region.
(This modified version has been a bit defanged, it it uses different voltages in the voltage regulation than the original.)


Back to topic:
The  important "trick" and reason for the fast response time in this form of circuit is that the ground of the op-amp rails and the and reference voltage float on the positive output voltage. This way you always have the full regulation output at the base of the power transistor(s). That is why the op-amps and reference voltage have their own transformer.

Several Gossen Konstanter, Several Anatek power supplies, HP/Agilent E361xA, E3630A, E6236, Leader LPS 151&152 - all use the same principle.
There is also a HP Appnote, No.90 from 1967 on this topic, it was base for the development and publishing of the circuit in 1973.

There are some later clones that use the negative output as reference. They are much worse because you now need a much higher voltage difference - even worse, depended on the output voltage- to regulate the system. In comparison a quite obviously stupid idea.

The mentioned dual power supplies the ground for the op-amps floats on the (middle?) ground of the output. A contradiction? No, look where the power transistors are in that case. ;)


If I'll manage to get this thing build I'll post a bit more about it.
« Last Edit: January 03, 2011, 12:37:01 am by Chasm »
 

Offline MegaWatt

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Re: Symmetric power supply from 0-15V Power supply
« Reply #39 on: January 14, 2011, 08:46:22 pm »
It's actually pretty easy to solve.

Set the voltage to 10V and use a virtual earth circuit consisting of an op-amp and a couple of transistors.



Could you tell me what the influence is of the 10k resistor at the op amp output?
I'm struggling a bit with understanding what is happening with the resistor and the two transistors :P
 

Offline Zad

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Re: Symmetric power supply from 0-15V Power supply
« Reply #40 on: January 14, 2011, 10:03:21 pm »
Texas Instruments TLE2426 Precision Rail Splitter / Virtual Ground

http://focus.ti.com/docs/prod/folders/print/tle2426.html

Used in quite a lot of low power audio circuits (<20mA) for simulating a dual power supply.

Offline Psi

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Re: Symmetric power supply from 0-15V Power supply
« Reply #41 on: January 16, 2011, 09:45:44 am »
When building just one of them yourself the component prices add up quickly.

Sometimes its just easier (and often cheaper for hobby stuff) to buy a little module to do it for you.

USD $4.22

10-13V input / +5V and -5V output at up to 100mA each

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=102-1342-ND

Of course it depends if you have enough other stuff you want to buy so you get free shipping :)
« Last Edit: January 16, 2011, 09:56:12 am by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Online Zero999

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Re: Symmetric power supply from 0-15V Power supply
« Reply #42 on: January 16, 2011, 03:09:00 pm »
It's actually pretty easy to solve.

Set the voltage to 10V and use a virtual earth circuit consisting of an op-amp and a couple of transistors.

Could you tell me what the influence is of the 10k resistor at the op amp output?
I'm struggling a bit with understanding what is happening with the resistor and the two transistors :P

That resistor is not essential. It will work without it. The resistor just improves the transient response. I hope you can see that the transistor buffer the output of the op-amp and that negative feedback overcomes the 0.7V voltage loss caused by the base-emitter junctions. The 10k resistor bypasses the base emitter junctions so the op-amp's doesn't have to jump around quite so much when the load changes.
 

Offline MegaWatt

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Re: Symmetric power supply from 0-15V Power supply
« Reply #43 on: January 17, 2011, 04:37:59 pm »
So if I understand correctly, the current my circuit can draw to/from ground depends on the collector currents of my transistors?
 


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