[SOLVED] Getting AC output from a purported DC-DC boost converter. What gives? - Page 1
Forum section: electronics / beginners

author: mindcrime
author_rank: Unknown
timestamp: June 04, 2022, 12:50:19 am
content: Hey EEVBlog crew, got a "thing" here that has me confused. Briefly, I'm working on a project that needs +5VDC to power a load, but also needs -5VDC at one point in the circuit, to send to the Gate of a P-Channel MOSFET to keep it off (the Source will be at 0 potential in this configuration). However, my input voltage is only right around +5VDC, so my thought was to use a boost converter to get 10VDC (approx), create a resistor voltage divider network and a "split rail" and get my +5 and -5 VDC that way.Sooo... I ordered some adjustable DC-DC boost converters from Amazon, and connected one up. With approx 5V input and the pot trimmed just so, the multimeter indeed shows a 10 VDC output, exactly as expected. And with a voltage divider network, I was able to see +5VDC and -5VDC as expected. So far so good.But... when I connected a capacitor across the output rail to help clean up any ripple, I noticed that the boost converter started getting super hot. I realized that it must be seeing the capacitor as a short, which AFAIK means it is outputting AC! WTF?Anyway, I measured it on the scope, and sure enough, it shows that I'm getting an AC output at 60hz, with a max voltage of +27 or so and a minimum voltage of -32 or so. The average voltage is, indeed, the expected voltage, (5V here, because I probed from the +5V side of the voltage divider) but this AC is useless to me.This has me scratching my head, because I totally expected some ripple/noise from a converter like this, but I did *not* expect to get literal AC from a part that's advertised as a DC to DC converter.So, here's the question: Is this normal, and I need to add a rectifier and capacitor(s) to clean up the output, or did I get a bogus part, or "other"? (Where "other" translates to something along the lines of "Phil, you're a dumbass and totally have no idea what's going on").EDIT:At the recommendation of a user on the Electronics StackExchange site, I tried with another device, but made sure to add a resistive load and a small capacitor before powering it up. After doing so, I now find that I get a relatively clean 10V DC signal.So my guess is that on the previous attempt, either A. I fried the device by powering it up and trimming the voltage with no load, or B. it was dead on arrival all along. These are fairly cheap / low quality parts, so I could buy either option at this point.The key thing to me now, is knowing at least how one is supposed to behave. I haven't used boost converters in a design before, so I wasn't sure if what I was seeing was actually wrong, or if it was just the case that my understanding of what to expect from the device was off kilter.:https://www.amazon.com/gp/product/B07XG323G8/

author: Terry Bites
author_rank: Super Contributor
timestamp: June 04, 2022, 09:38:14 am
content: Hot cap- reversed, under rated or misconnected or dead from the get go. Tantalum?It sounds like you have hum superimposed on the output. Likely a grounding fault or error.Check your scope is DC coupled a working properly. Probe gnd fail?With no external connections, check the continuity between probe end gnd and input gnd and output gnd on the PCB.What does the module input waveform look like?These are not isolated conveters so if they were configured like this, the output and input of the lower module would be shorted to GND.