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Old 1st December 2005, 02:56 AM   #171
fab is offline fab  Canada
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Default Front End more

I forgot to clarify that the input of regulator must be DC voltage.
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Old 1st December 2005, 12:27 PM   #172
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1) the resistors in series with the regulator output would seem to conflict with the purpose of the regulator.

more to say on the TL431 regulator later -- the standard Texas Instruments model breaks down unless it is used as a shunt regulator.
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Old 2nd December 2005, 12:29 AM   #173
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Quote:
Originally posted by jackinnj
1) the resistors in series with the regulator output would seem to conflict with the purpose of the regulator.

more to say on the TL431 regulator later -- the standard Texas Instruments model breaks down unless it is used as a shunt regulator.
1) The resistor indicated is 10 ohms and is optional. It is used with the 470uF x2 as a low pass filter for residual ripple. I use 2.7 ohms with excellent results.

2) The TL431 is used as a SHUNT regulator in this circuit. The series regulator is the 7818/7918. The TL431 is used as a precision voltage reference to allow more than 37V at input of 78xx/79xx IC.
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Old 2nd December 2005, 01:37 PM   #174
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The TL431 has reactive components which have to be compensated (not just the junction capacitance of a diode) -- TI's model isn't correct (and they are aware of the problem.)
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Old 3rd December 2005, 05:22 AM   #175
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Quote:
Originally posted by jackinnj
The TL431 has reactive components which have to be compensated (not just the junction capacitance of a diode) -- TI's model isn't correct (and they are aware of the problem.)
Do you mean that the "spice" model is not correct or the TL431 design? TL431 has been used for years in many design circuits.
Do you mean that T.I. has a design issue?
Other manufacturers like On Semiconductor produce also the TL431.

What does your simulation shows?
Have you added compensation parts in the simulated circuit?

Thanks
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Old 3rd December 2005, 07:48 PM   #176
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Yes, Texas Instruments SPICE model on their site is not entirely correct, but there isn't a "design issue" with the device.

The TI SPICE models predicts the thermal performance wonderfully but doesn't model the error amplifier correctly. There was an article in the Design Ideas section of the Sept 15th EDN -- written by two TI engineers -- describing the compensation method. The TL431 is "ubiquitous" and it is a great, inexpensive chip to work with, it's just not the right device for this application.

I would just use the correct combination of zener diodes to determine the output voltage. 5 watt diodes are cheap and won't go pop. The bias to the diodes is set with a resistor to the "Ground" pin from the output terminal of the LM7818.

R = V(out)/(Ibias). figure 5 to 10 milliamps for the bias current. I show 10k in the attachment below -- too high a value -- but it's just for purposes of illustration.

We can make it a little more elegant...
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Old 3rd December 2005, 10:10 PM   #177
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Default Voltage regulator

I still not see the relation between a wrong spice model and the real circuit performance. The main goal of using the TL431 is to replace the Zener diode and provide voltage adjustment. I have used a circuit as the one you have shown for 2 years but I wanted to do a more versatile circuit. Remember that "cheap" zener diode are 5% tolerance and 5W diode are not cheaper than a TL431 (70 cent). Since a positive and negative regulator are needed than the 2 zener diodes (one for each polarity) must be matched. The TL431 is a more precise voltage reference than a simple zener diode. From what I remember the performance (ripple) with the TL431 was in the same range as the one with only a zener diode. I believe that the performance for either circuit is good enough for the purpose of the application. Anyway, the pcb layout I have provided can use only a single Zener diode and no TL431 if required.
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Old 4th December 2005, 12:35 AM   #178
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Default Re: Voltage regulator

Quote:
Originally posted by fab
I still not see the relation between a wrong spice model and the real circuit performance.
You have inserted a "reactive component", the TL431 -- into the ground return of the linear regulator made worse with the 56nF bypass. Take out the 56nF for it is really problematic --
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Old 4th December 2005, 02:45 AM   #179
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Default Re: Re: Voltage regulator

Quote:
Originally posted by jackinnj


You have inserted a "reactive component", the TL431 -- into the ground return of the linear regulator made worse with the 56nF bypass. Take out the 56nF for it is really problematic --
OK, I have checked more closely the datasheet on the Stability boundary conditions of the TL431. From my circuit with 7818/7918 it is more difficult to evaluate the problematic cathode current based on the curves provided in the datasheet. So far, I have built 8 circuits successfully but I agree that it would be more safe to remove the 56 nF cap across the TL431.

thanks for the observation.
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Old 3rd February 2006, 02:30 PM   #180
gcohen is offline gcohen  Israel
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Default DH220

Hello all,

Did somebody try to short capacitors C8 (and hence also C7)
in the Hafler DH-220 ?

I know that this capacitor increase the feedback in very low frequencies, below 3Hz, but I do not think that the amplifer
will get into oscllations while there will be sonic improvements.

Who need 470uF electrolytic capacitors which reduce the
sound quality! It is located really on the inverted input signal.

So, what do you think ?

Thanks, Guy
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