Adjustable Voltage Regulator Problems - Page 1
Forum section: electronics / beginners

author: Mprentice1
author_rank: Newbie
timestamp: November 07, 2012, 05:56:43 pm
content: Hi guys, I'm trying to build my own homebrew workbench power supply, and so i've been prototyping a adjustable output regulator, the MIC2941, However as you'll see in my blog post, i'm having some problems. can anyone help?http://theelectronicbyte.wordpress.com/2012/11/07/adjustable-voltage-regulator-problems/

author: david77
author_rank: Frequent Contributor
timestamp: November 07, 2012, 06:35:25 pm
content: You calculation is correct.But why are you feeding 1,3V into the circuit and where?

author: 6502nop
author_rank: Regular Contributor
timestamp: November 08, 2012, 01:03:15 pm
content: Two things come to mind:1) As mentioned on your linked page, breadboards are notorious for corrosion problems, broken leads, bent leads not gripping smaller diameter wires, and promoting wire breakage inside the insulation. I recently had this issue with an LM386 I was designing (with the help of a 5-year old). I could not get the damn thing to work. Later, without the "help", I found that the power input, through a 9V battery snap, wasn't making contact with the positive rail of the BB. Why? I always used the top left hole for this, and over the years that hole has seen mostly 22ga wire, and the contacts were used to it. When that battery lead went in, it was only 28-30ga, and didn't fit well. Lesson learned.So, to test this, just move everything on the BB over an inch (3cm) or so. Hopefully, not every tie point is bad. Also, swap all the leads out.2) See that pin on the left labelled "Shutdown"? Well, that shuts down the Vreg. It's active-high, in that when high, the Vreg shuts down. So, you want this to be low at all times. I'd just shove a 47-100K resistor from that pin to GND to keep the Vreg from shutting down.Never leave an input floating!nop

author: jeroen74
author_rank: Frequent Contributor
timestamp: November 09, 2012, 03:56:38 pm
content: QuoteUnless the datasheet says it's fine to do so; for example when it has an internal pull-up/down. Or one of those pins that see an floating input as a valid state.

author: mariush
author_rank: Super Contributor
timestamp: November 09, 2012, 04:06:40 pm
content: Check the ESR of the output capacitor, datasheet says it's unstable at low loads if the ESR is higher than 1 ohm. If you used a crappy capacitor, chances are it is higher than 1 ohm. If you can't determine the esr (see datasheets, if it's not a no-name or some chinese brand), just try a 100uF-330uF 16v or better capacitor in its place and see if that changes anything, those should be low enough in esr.Page 6:http://www.farnell.com/datasheets/36861.pdfPage 7 also says: Note: Using 1k for R1 will ensure that the input bias current error of the adjust pin will be negligible.Do not bypass R1 or R2. This will lead to instabilities.later edit: the above is forLM2941 , should be about the same as MIC2941 but I notice the Micrel datasheet also has those notes about ESR but does have instructions about using a bypass capacitor:http://www.led-treiber.de/MIC2941A.pdf

author: Unknown
author_rank: Unknown
timestamp: November 09, 2012, 10:52:21 pm
content: This is a bit misleading, the figure on page 5 of the National datasheet shows that it should be between 0.1 and 1 ohm for low currents (i.e. ceramic will probably have a too low ESR), and a slightly wider range for currents above 200 mA. The Micrel datasheet talks about 'about 5 ohms or less', though I doubt it's stable with a very low ESR. I would just use the National data and assume they did a better job writing the datasheet and testing stability, as opposed to Micrel designing a more stable regulator.