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Old 19th May 2004, 12:38 PM   #21
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Default Its like this lattice of coincidence

Quote:
Originally posted by mfr
Git a reply from China: (relevant text)

----start---
The polarity of K246 please insert like the white silk screen components outline show on the pcb.

The voltage output only -3.74 or other is too low, change the K246 to K246GR, if the voltage is stil un-normal you can change the fet to zener diode.

The pcb of the DAC38 is fully tested and working good.
Hope it may solve your problem.

----end---


As I mentioned before the K246 is marked on PCB as reversed from schematics - Drain/Gate connected together instead of Source/Gate.
The jfet is symmetrical and it should not matter......... at least according to John Curl! I guess maybe I was a good troubleshooter on the rest.
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Old 19th May 2004, 01:17 PM   #22
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Default For future reference..........

Quote:
Originally posted by janneman
The easiest way to get exactly (more or less) 5V is to replace the zeners with floating integrated 5V references. Search the sites of Analog Devices, LTI, Texas etc, there's tons of them. They are more accurate and have better (lower) dynamic impedance than zeners which means better regulator performance.

For example, the ADR550 will beat the hell out of any zener regulated supply.

Jan Didden
And lower noise. I would put a cap in the range of 470 uF to 1000 uF across the voltage reference, very close to it. For a very fast op amp you may want to add a 0.1uF cap in series with a 10 ohm resistor network across the reference and electrolytic. Short leads and small 0.1uF cap (50 V MKP should work well). A voltage reference uses feedback and will most likely be unstable inside a feedback loop with no cap or too small a value cap across it. Use a low voltage cap just a little over the voltage of the reference for low Q and low inductance. Something like a 50 V high Q ( low ESR cap) will be both inductive and likely the resonate with the inductive output impedance of the reference. Think of it as a noise filter AND a zobel network. LM336 impedance vs. frequency below:


Pretty much no charge with all the dead beats here.........
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Old 19th May 2004, 01:22 PM   #23
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Hey Fred,

What do you charge as a ghost writer?
Did you identify my avatar yet?

Jan Didden
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Old 19th May 2004, 01:44 PM   #24
mfr is offline mfr  Canada
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Interresting, but forgive my lack of certains basics, such as how to connect a zener instead if the FET? I'm not at the level of understanding transistor bias, FETs and the maths behind these circuits.

I have learned a lot throught these postings + reasearch on regulators etc, and I must thank you all for the input.

Here is an attempt at putting zeners instead of the FETs (attached). Is it correct?

As for the integrated 5V references, i'll have to order some, but I would like to try the zeners today.
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Old 19th May 2004, 02:18 PM   #25
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Default Did I mess up.........

The zeners replace the 2K4 ohm resistors to the output of the op amps. The direction shown is correct. Leave the 2SK246 jfets where they were. Sorry if I confused anyone.

For some good regulator background:

http://tangentsoft.net/elec/opamp-linreg.html

http://home.comcast.net/~walt-jung/w...ed_PN_Regs.pdf

http://home.comcast.net/~walt-jung/w...g_Circuits.pdf
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Old 19th May 2004, 04:01 PM   #26
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Default Interesting circuit.....

Too bad I missed it first time around.

"You did, Oscar, you did."

Why didn't you come up with it, Phred?

"It was one of Wilde's!"

"I wish that I had come up with it."

"You will Oscar, you will."

Etc.
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Old 21st May 2004, 02:25 PM   #27
mfr is offline mfr  Canada
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Default Working well!

Here are the results:

Bias resistors 668 Ohms

Zeners 5.1v (measured at about 4.7v)

Opamps outputs (using 10v zener instead of 2.4k)

+ side 4.31v
- side -4.33v

Regulators are working at zener voltage !

After further measurments it looks like the zeners I was using were way off: 4.6v instead of 5.1 and they required more current. I have some other zeners (same wattage) and these are closer to specified value and do not require as much current (tried them with 1k, 2.4k 4.5k bias resistors), almost the same output. I settled for 5.6v zeners (measured at 5.25v) with 2.4k bias resistors: Regulator output at + and - 5.25v

Thanks for help !
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Old 21st May 2004, 02:41 PM   #28
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Looks good!
Just out of curiosity, when you say 'opamps output' you mean the output pins or the pass transistor base? In the final circuit, what is the actual voltage on the opamp output pins? I guess you put the fets back in?

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Old 21st May 2004, 06:37 PM   #29
mfr is offline mfr  Canada
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Just re-measured another PS:

Output voltage is +- 4.97v

Opamp output pins 4.13 and 4.17

On the FET side 5.54 and 5.57
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Old 21st May 2004, 09:21 PM   #30
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So, that's about 1.4v across the 2.4k or .6mA, that's the reserve for driving the pass xsistor if the supply is being loaded. Was this measurement under load? What load?
Assuming an Hfe of 100 of the pass xsistor gives you about 60mA load current spare. Is that enough? What's the raw input voltage at the reservoir cap?

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