Analyzing GreatScott's USB Power Stick - Page 1
Forum section: electronics / projects

author: LoveLaika
author_rank: Frequent Contributor
timestamp: March 30, 2022, 03:26:25 pm
content: I recently sawon hisFire TV USB Power Cable. I thought the concept was very interesting as well as his reasoning as to why it can be necessary, so I wanted to simulate it in LTspice, if only for my own understanding. I've attached his schematic along with my own LTspice schematic and picture of said schematic (for people who rather just look at a picture). For simplicity's sake, I used pre-included parts that are packaged with LTspice, so there should be no need for third-party models. It more or less functions the same as his, though I did try to make some changes based on what the comments on his video suggested.Basically, he broke it down into three parts: charging the supercaps (limiting it to 500 mA), measure the voltage across the supercaps (which will be the Fire Stick's source), and turning on a 'switch' to connect the Fire Stick to the supercaps when properly charged. Running it through LTspice helped me make sense of how it functions, but I have some questions about certain functions of it. I thought to share this and see what you guys think.The choice to have his caps in series: it seems to be because of the cap's voltage limitation? The supercap was rated for 2.7 volts. Assuming that at steady-state it is seen as a voltage source, putting 2 in series would "add" up the voltage to the needed voltage of 5 volts safely, though at the cost of reducing the capacitance. Had the voltage ratings for the supercaps been higher, you could put them in parallel, though I think the whole configuration would have to be reworked. Speaking of which, what other circumstances would you put caps in parallel?In GreatScott's schematic, he took the voltage across the supercaps and passed it through a difference amplifier, reducing it down to 3.3 volts (assuming a 5-volt difference). Now, initially, I was confused by why it was necessary when one could just do a 1-to-1 ratio? I mean, the voltage could have been cut to not have it go to the rails for sure, but is that the only reason?This may tie in with the second item, but there's no real way to stop the current from the main voltage source. When the Fire Stick is connected, I see it jump to 1-amp. This seems like a big concern, unless it wasn't properly addressed in his original schematic?So, the current limiting part of the circuit was interesting, but I was wondering how it could be applied to a different configuration? He used the principle that the op-amp will output a voltage in an attempt to make the voltage the same at the two inputs. His feedback loop had the output going to the NMOS which controlled the current going to the shunt resistor. That voltage across the shunt resistor was fed back into the op-amp completing the feedback loop. I can see how it works, but in his case, he was directly controlling the NMOS switch while tying it to the shunt voltage, whereas in my example circuit to control the current, I'm controlling an NPN which controls a PMOS which limits the current. Right now (CC_TEST), it looks like a comparator, but how do I incorporate a feedback loop to achieve the same purpose?

author: thm_w
author_rank: Super Contributor
timestamp: March 30, 2022, 09:17:37 pm
content: Quote from: LoveLaika on March 30, 2022, 03:26:25 pmYeah 2.7V limit, you'll even often see "5V" rated supercapacitors that are just two 2.7V caps heatshrinked and connected together.https://www.digikey.ca/en/products/detail/illinois-capacitor/DGH105Q5R5/7387509

author: LoveLaika
author_rank: Frequent Contributor
timestamp: March 30, 2022, 10:04:59 pm
content: Thanks for your reply. That's interesting, though in the case of failure, you would be stuck with that design. So putting caps in series would allow it to overcome the voltage rating of one cap by itself, huh? I never thought of using caps like that before, seeing as how I don't have an application that needed it. But you would be reducing the capacitance all the same, so less energy storage?Speaking of, any thoughts on my other questions?

author: thm_w
author_rank: Super Contributor
timestamp: March 30, 2022, 10:57:16 pm
content: Yeah capacitance is reduced in half, so its usually only used when absolutely necessary.Energy:https://www.omnicalculator.com/physics/capacitor-energyQuote from: LoveLaika on March 30, 2022, 03:26:25 pmhttps://daycounter.com/Calculators/Op-Amp/Op-Amp-Voltage-Calculator.phtmlhttps://cxem.net/calc/tl431_calc.php?social_iframe=56012He's comparing voltage with a TL431 reference, with an adjustable pot. That will give you 2.5 to 5V output.So its best to have your expected value in the middle of that range (3.5V) to allow good +/- adjustment with the pot.Otherwise, in terms of values selected, etc. don't expect great scott to have optimized the circuit extensively. He's generally using parts he has on hand.

author: mikerj
author_rank: Super Contributor
timestamp: March 31, 2022, 11:07:00 am
content: A significant flaw in his circuit is that he is switching the ground of the Firestick via an N channel MOSFET. Unless the HDMI ground is isolated from USB ground (either within the firestick or the TV) then this is doomed to failure. Also no current limiting on the output, those supercaps can supply a hefty amount of current in the event of a short.

author: cdev
author_rank: Super Contributor
timestamp: March 31, 2022, 01:46:46 pm
content: I'll need to read up on this. Is it made by Michael Ossmann or the other fellow in the video?In contrast a USB voltage and current monitor I got by Kweieisi partially broke pretty soon after I bought it. Now it shows voltage on the USB but doesn't measure anything else properly. Its supposed to measure voltage plus current and power supplied.Think rather than buy another el-cheapo I may check out some other devices.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: March 31, 2022, 02:00:18 pm
content: Quote from: thm_w on March 30, 2022, 10:57:16 pmTrue, I don't expect this to be totally optimized. Still, I learned some interesting things while analyzing it.I see what you mean now. Having it in the middle of the range allows for leeway for comparison values. However, in that case, if 5 volts equates to something like 3.3 volts, then he would have to set his reference voltage at that level or lower in order for the stick to be connected, unless the Fire Stick has a minimum voltage that can be operated at. You can't set it for something higher than 3.3 volts.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: March 31, 2022, 02:22:02 pm
content: Thanks for replying. One comment mentioned that he should have done high-side switching instead of at the ground-side. I tried to incorporate that in my "Improvements" through the use of an NMOS and a PMOS. That seems pretty typical. In his video, it seems to work, so from what you mentioned, there has to be some point where the HDMI ground is not isolated from the USB ground. I think I can see why one would keep it separate, but with the fire stick (and maybe the TV), isn't it easy to check with a multimeter? Just check for continuity/shorting between the HDMI plug shell and the USB ground pin/shell?Wait, so if you said high currents in the event of a short....what if a crowbar circuit was included at the output as well? I recently read about that as a method to prevent overvoltage, or maybe a simple circuit of BJTs to protect against shorts. Perhaps that would be enough.

author: mikerj
author_rank: Super Contributor
timestamp: March 31, 2022, 08:43:47 pm
content: A crowbar circuit is to clamp voltage, for overcurrent you could use a polyfuse (self resetting semiconductor fuse).

author: aheid
author_rank: Regular Contributor
timestamp: April 01, 2022, 12:28:12 am
content: Quote from: mikerj on March 31, 2022, 11:07:00 amActually his low-side switch design is doubly flawed. If the USB and HDMI grounds are not isolated, the it has the problems you describe, no switching and no output current limiting on the supercapsIf, unlikely as it seems, the USB and HDMIareisolated, then the circuit is still flawed, because then the Firestick ground is routed through the constant-current portion, raising the ground level of the Firestick, likely causing brownouts of the Firestick when current draw peaks.Sad thing is you can make a high-side version (both inrush limiting and output switching) using almost exactly the same setup just with PMOS instead of NMOS. Though I'm not experienced enough to tell if the roll-your-own differential amplifier used for the inrush current sense would work well in practice.