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Old 14th Mar 2016, 5:08 pm   #90
jimmc101
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Join Date: Mar 2009
Location: Portsmouth, Hampshire, UK.
Posts: 585
Default Re: A precision voltage calibrator

I've just added a LM78L15 regulator to my module and it displayed exactly the same behaviour as Bill's; previously I had been using a lab power supply without this problem..
Quote:
Originally Posted by AC/HL View Post
... At first I thought mine was faulty too. Set to 10v it read about 2, gradually creeping up. After a bit of experimentation I found that initially only the 5v setting works, but that once kick started at 5, they all work, and seem accurate (I only have 2.5 digit meters, so can't check to 5 decimal places) Odd. Further checks revealed that left to it's own devices it initially creeps to about 4 than jumps to 10, whereupon all ranges work. But... after setting to 2.5 the slow rise from 2 to 4 starts again. It's reproducible over and over, so must be inherent in the chip (mine at least) They do say to leave it to "warm up" in the instructions.
Once started it would restart normally after power cycling even after waiting a few minutes, I then tried shorting out the capacitor between pins 6 and 7 (With the power off!) and the slow start behaviour returned every time.
Looking at the datasheet the suggested value for this capacitor is 10 to 100nF, the value in the module was 330nF. I removed this capacitor and replaced it with 22nF and have not had any further problems since.

It would be interesting to know what values the other modules use, it looks as if too large a value gives problems with slow rising power supplies.

Jim

Please excuse the bodge, I should have access to an HP33401A tomorrow and needed something in a hurry.
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