| Electronics > Projects, Designs, and Technical Stuff |
| Replacing Zener with vreg? |
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| TERRA Operative:
Yeah, I'm not sure why they went with such a precision part. Maybe it was just a jellybean part that was in their internal catalogue. The circuit is designed to set a DC offset across a capacitor under test. The inputs go through the AD7530LN DAC that set the reference voltage then the transistors amplify and output to the front panel terminals. There is two ranges, 0v - +/-9.99V (10mV resolution) and +/- 10.0V - +/- 40.0V (100mV resolution). I've included the two pages from the manual that explain how it works. I have TL431's here so they are a viable option. |
| SiliconWizard:
--- Quote from: ogden on November 09, 2018, 05:23:50 pm --- --- Quote from: SiliconWizard on November 09, 2018, 05:16:17 pm ---Yeah, that doesn't seem to make sense to have such specs for the zener diode and then buffering it with an opamp --- End quote --- It makes a lot of sense to BUFFER voltage reference/zener using opamp. --- End quote --- Of course you have to buffer it. What it does to absolute precision (which is not what they seem to be after here) and temperature stability is another matter. We can suppose they selected the opamps carefully and mounted them appropriately. Still, the "inverting position" poses even more questions here IMO. |
| iMo:
--- Quote from: ogden on November 09, 2018, 05:19:01 pm ---..Thou I would try mentioned (trimmed to 6.3V) TL431 shunt reference first and see how it does, maybe do some cold-spray test (using "canned air") as well. TL431 is in your junk bin for sure, it's very popular in isolated switching supplies. --- End quote --- LM431 claims 50ppm/C - that fits well.. |
| SiliconWizard:
--- Quote from: TERRA Operative on November 09, 2018, 05:29:40 pm ---Yeah, I'm not sure why they went with such a precision part. Maybe it was just a jellybean part that was in their internal catalogue. The circuit is designed to set a DC offset across a capacitor under test. The inputs go through the AD7530LN DAC that set the reference voltage then the transistors amplify and output to the front panel terminals. --- End quote --- Maybe they just use it as a reference in other parts of the design? Again when you look at the whole chain, I kind of doubt you get such a temp. stability in the end anyway? |
| exe:
--- Quote from: SiliconWizard on November 09, 2018, 05:13:29 pm ---you will have to use a negative regulator --- End quote --- Or a shunt regulator. Final voltage can be trimmed, if needed. So, it doesn't need to be precisely 6.3V. But replacing resistor and adjusting for zero tempco can be a challenge. For me it's unclear how precise and stable the circuit should be. If it allows some deviation, I'd probably just put lm329 or even a simple zener. May be trimming even not needed if it can be re-calibrated to the new range. |
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