I opened up my ATX Power Supply as I had it for quite a few years but it has been stowed away and used intermittently until I use it a lot more in my office computer in recent years. I just don’t trust any power supplies Made in China, even from a reputable brand as a couple of decades of working with computers tells me that they are bound to break after a few years, and very often it is the capacitor that rotted and the rest are collateral damages. Lo and behold there is one:
After I took the capacitor out, I noticed something odd: the polarity marker on the circuit board is the reverse of how the capacitor was installed! Holy smokes! I just want to verify if the PCB markings is right or the installer was right, so I installed wires to the capacitor to lift it up so I can connect the multimeter leads across it to measure the voltage polarity. This picture also shows the PCB’s capacitor orientation marking:
And the multimeter reads -5V following the original orientation of the capacitor before I took it out. This means the polarity was reversed!! No wonder the capacitor bulged. I was lucky that it didn’t blow up after a few years of use! Probably it was rated 16V yet only -5V was passed to it so the electrolytic capacitor rotted slowly.
To give XFX Force credit, they didn’t slap the power supply together with the cheapest white label components from the gutters. It uses proper Nichicon and Hitachi capacitor, so it might be the reason that reversed capacitor lasted so many years.
It’s the workmanship in China. If you go with a Red Chinese (Yellow-Soviets) brand, they might use junk components, but don’t think you are safe with foreign companies that has a solid process and design. The cheap labor in China who doesn’t give a crap can still manage to fuck it up. So trust nothing
The cutoff frequency of 10Hz on the datasheet is a typo. Better scopes at the time claims 90Hz. 10Hz is just too good to be true.
Found the specs from the service manual:
Don’t be fooled by the -3dB cutoff and ignore how wide the transition band can be (depends on the filter type and the order). Turns out this model has a very primitive filter that AC couple mode still messes square waves below 3kHz up despite the specs says the -3dB is at 90Hz. You better have a 30+ fold guard band for old scopes!
Remember square wave pulse train in time domain is basically a sinc pulse centered at every impulse of the impulse train in frequency domain superimposed. Unless you have a tiny duty cycle (which is not the case for uniform square waves, they are 50%), the left hand side of the sinc function at 1kHz fundamental still have sub-1kHz components that can be truncated by the AC coupling (high pass filter).
I tucked away my PC a little away from my workstation desk and the power switch is located at an inconvenient location. I tried to keep the wiring minimal so I’d rather not wire a dedicated ATX power switch onto my desk.
Unfortunately my motherboard does not support turn on by USB keyboard, and I’m not ready to upgrade because I am using it to test PCI data acquisition cards and it’s the fastest one that has 4 PCI slots and they are hard to find nowadays.
I found a $2.5 wireless module on eBay that claims to switch LED lamps which works on the standard 433Mhz channel and it replicates momentary switch pattern and can operate on 5V (My motherboard is new enough to have 5Vsb from onboard USB header).
Initially I was tempted to get the built-in relay version, but I was worried about the current draw from 5Vsb and those are 12V relays., not to mention the footprint is much bigger (the one above is 22.5mmx 11mm x 8mm).
I thought I can figure out with some sort of BJT switch instead of using a relay that has a much bigger current draw requirement, but I realized it’s a pain in the ass because the output is ‘floating’ differential. The OUT- does not tie to the power ground (it’ll short out the unit when I tried to. That’s why I added quotes to ‘floating’ because it’s only relative to OUT+). I also measured OUT+ which is +5V with respect to power ground.
I tried to power a LED and it only works if current flows from OUT+ to OUT- so it’s really sinking current from source power to do that, and it’s unidirectional.
I’d just take a gamble and hook up with a 5V NO relay that I have around. Turned out it actuates with the 5Vsb from the USB header. I glued the relay to the back of the PCB and hook up a flyback/snubber diode (reverse biased) across the relay coil so the back EMF won’t fry my motherboard.
Seems like the transmitter-receiver pair is on momentary switch mode by default, so no addition configuration is needed other than pressing the learn button and immediately press the transmitter button to pair.
I wired a jumper extension cable (male – female) to the relay output from the middle as a by-pass since I’d like to keep the original power switch’s functionality (so it’s basically OR-ing between hardwired switch and the wireless remote 433Mhz switch)
Here’s an example of taking 5Vsb from USB header and tapping into the power switch jumper in Front-Panel jumper block:
Note that the PWR SW- pin is connected to the ground. Since I’m using a relay, the relay output is floating so the polarity does not matter.
A few years ago I switched my stereo system (I’m using them as computer loudspeakers) to Class D because I was annoyed by the heat the professional (Class AB) amplifiers generate heating up the room (electricity bill aside).
I personally prefer the sound from Tripath amplifiers (started with LP-2020A and LP-2024A), which they called Class-T, but it’s really just Class-D with better PWM feedback mechanism. To drive bigger speakers with stronger bass, I bought a LP-2051 (50W x 2 RMS), which uses TP2150 (200W driver) + TC2001 (Audio Signal Processor) Tripath chips.
However, this amplifier has a very fatal flaw: it has a ridiculous loud turn-off pop! The system has speaker protection relays, so there’s no turn-on pop as expected as speakers are connected 3 seconds after switching on the power.
The turn-off pop bad enough that I was about to toss that in the trash as the amplifier fearing that it might damage by expensive and hard to get ADS vintage speakers. However Class-T chips are no longer produced and the TI Class-D chips for some reason just doesn’t have the clarity in the mid-range and high-frequency range I was looking for in Class-T amps, so I’ve decided to figure it out.
DO NOT TOSS YOUR Lepy LP-2051 Amp because of the loud turn-off pop! There’s a simple way to fix it if you have a soldering gun, some wires AND a relay (normally open, 5V coil)!
Before I get to the solution, here’s a a few things I figured from observing the board which helped:
The power line is 19V (the unit won’t turn on until it reaches 18V, so the acceptable range is 18V~19V)
The unit draws around 1/3 of the turn-on power when the power switch is off. The switch is AFTER the power rail smoothing inductor and tank capacitor so they are charged.
Turns out the speaker protection relays are hooked straight to the 19V plug input BEFORE the switch (two channel’s relay coils are chained in series, then to the collector of a NPN transistor acting as a switch)
The 5V regulated (rightmost pin of 78M05 when viewed from top) is always powered even when the power switch is off.
[Failed] Attempt 1): was to disconnect the left/right speaker wires with 24V external relays switched by the front panel power switch (it’s passing 19V and 19V is enough to activate the relay). The reason is that the power droop faster than the relay disconnect when losing power.
[Mixed success] Attempt 2): I have a big power capacitor 0.12F as external power smoother (initially used to fix huge transient power draw for huge bass transients like drums), which slows down the power droop when the power is switched off enough there’s no power-off pop. However, this solution is very clumsy as reasonably sized 0.022F capacitors won’t slow the power line droop enough to avoid the turn-off pop. This observation gave me the idea that it’s the power-loss detection circuit not reacting fast enough for the system to do the proper ‘shutdown’ procedures (muting the output or disconnecting the speaker wires).
I initially looked into expediting disconnecting speaker protection relay when the power switch goes to off position, but realized it’s already done in the transistor switch logic (cascaded NPN stages) that controls the speaker protection relay as I trace the circuits.
I started looking up the datasheet looking for built-in mechanism in the IC that mutes the amplifier as I switch the unit off (the unit is partially powered once the DC plug is in). Turns out there is: the mute pin is on TC2001’s pin 24 (MUTE):
I tried flying pin 24 to the 5V regulated output (on 78M05) and it muted as expected. I didn’t really bother to check, but when I take out the jumper wire, it’s unmuted as expected (which contradicts with the datasheet description that floating is considered high/mute, so I assumed there’s other logic driving it low by default)
[Failed] Attempt 3): Use a NPN transistor like 2N9304 (and a potential divider to tap the 19V logic to 4.5V) to drive the mute pin (TC2001 pin 24) high (which means muted) when I power the unit off. The turn-off pop is still there because it turns out that there’s a 200ms delay for the mute pin:
So now the goal is to have mute activated (set to high) FIRST a little before the 19V power line gets disconnected. The timing order for turning on does not matter because the speakers aren’t going to be connected until after 2.5 seconds once it’s hooked up to the 19V source, it doesn’t matter what’s the logic transient logic level of the mute pin (it’s going to be NOT connected to the 5V when the SPDT power switch is ON steady).
Realizing that there the power switch acts the fastest, then the mute pin, with relay actuation being the slowest, and the power switch that came with the unit is a SPDT, I don’t even have to implement a timer to delay disconnecting the 19V line before it finishes muting. Here’s the winning solution:
I happen to have a Fujitsu/Takashimaya JY5H-K5VDC reed relay which happens to be a NO (Normally Open switch) which happens to fit the design like a glove. Here’s a picture for the experimental (working) hook up that the turn-off pop is completely gone:
Completed mod after cleanup:
There’s a few cleanup that I did:
I hot glued the relay on the two hard switches as they are not near any heat sources and mechanically stable
I stole the ground (going to the coil) from the nearby C5’s ground pad (C5 is the moderately big 470uF SMD cap nearby).
The old right-angled hard switch’s pins were lifted from the board by bending the pins straight.
The utmost pin/pad (leftmost, closest to the power plug) for the switch is not connected anywhere else on the board. It was a dummy as the SPDT switch was only used as a SPST switch. Good for me so I can solder one of the 3 pins there for mechanical support (otherwise we’ll twist the front two dummy solder joints for mechanical stability when we toggle the lever repeatedly). I chose to bend the middle pin to solder it to the board because the other two are either too far out or too short.
In summary, furthest SPDT switch pin goes to mute pin, the middle is the 5V regulated rail to be switched between the mute pin or the relay coil. The shortest SPDT switch pin goes to the coil. The rest are obvious
Here’s the finished mod with different angle shots for you to see the wiring:
Given the amp itself is cheap (but it’s really an excellent bang for the buck: clear sound, solid bass, class-D efficiency, small size and light weight), those who are not that electrically savvy might not be inclined to do that mod. If you are going to throw it away anyway, please send it to me (I’ll pay for postage).
It’s a very very simple mod that any beginner electronic hobbyists or an electronics student might be willing to do it for you with the instructions here for a small fee. Basically it’s just some wires and a relay and some good solder wick (or if you have a nice desoldering pump, it works fine too).
I typically don’t take petty service orders (under <$500), because of all the hassles shipping back and forth, opening it up, putting it back, and keeping records for the IRS, plus the liability risks. So please find a kiddo, hobbyist or tech to do it.
The two basic laws in circuit analysis, Kirchhoffs Voltage and Current Laws,
[KVL] Voltage across the same pair of points is the same no matter what paths you take
[KCL] Current stay the same along the same path
are often taught in basic circuit analysis, but most of the time, they taught it in the context of nodal analysis, which you have a little more complicated meshes with multiple theoretical power source (voltage or current) that simple series/parallel circuit rules are not enough to solve the puzzle.
However, these two fundamental concepts are useful to develop insights that help you estimate quantities in a circuit quickly like a pro.
Kirchhoffs Voltage Law [KVL] can be applied to a parallel circuit of 2 branches (often the case when measuring additional loading effect). Let say the two branches are applied (loaded) at a voltage output , which might change depending on the branches (loading).
You can exploit the algebra to quickly calculate the current of any branch without first computing the overall resistance or current:
Kirchhoffs Current Law [KCL] is useful in analyzing energy loss over resistance in wires . For example, in high school physics, we discuss why we have high voltage power lines for bulk energy transmission despite it’s more dangerous. The traditional explanation is
so the lower the current is (which can be done through stepping up the voltage, traditionally done with AC signal through transformers, to maintain the same power). But how about other form
Technically, it’s possible, but you have to be very careful that the voltage we are talking about is across the wire with resistive losses , NOT the load voltage .
changes depending on the output load , so you have to derive the assuming an arbitrary , which will happen to cancel itself out and end up the same as if you think of everything in terms of current first:
So the bottom line is that most of the time, it is easier to think in terms of current in most circuit analysis because current won’t change along the same path. This is especially true when your problem has varying impedances/load which will disturb the voltage.
Of course, if the problem screams direct application using KVL, don’t go all the way converting it back to current. You will find the current-first approach useful when we get to semiconductors like diode, voltage references, BJTs,.
I usually think of voltage as a consequence or effect of current flushing into a transducer (e.g. resistor), so it’s subjected to change and therefore messy to use when solving circuit puzzles. Solving circuit analysis problems are often an exercise of identifying invariants and inferring the remaining quantities.
In the time of national emergency against the Chinese Communist Party Virus, or #CCPvirus in short, we are glad to offer FREE basic 100Mhz oscilloscopes (or mixed-signal oscilloscopes) to makers and engineers in the US who are stepping up with innovations to help.
whether you need the logic analyzer function (mixed-signal)
does your project require fancy oscilloscope features like FFT, calculus, phase difference, deep memory, talking to the PC
your name, address and phone number for shipping
and I’ll make the arrangements immediately.
Currently available models (subject to availability)
Agilent 54622D (Mixed-Signal)
HP 6632B Systems Power Supply (20V, 5A, Fast recovery)
These models has a no-brainer learning curve for any motivated maker/engineer who are up to the game innovating something serious. Time is ticking. We want you to use the oscilloscope right away! Higher bandwidth oscilloscopes are available as loaner if your project justifies it.
It’s on an honor system. Please don’t abuse the program so the innovators who genuinely need the oscilloscope will have what they need!
We thank all the innovators who contribute their time and effort in response to the CCP virus outbreak!
Stay safe, wash your hands, and stay home whenever practical.
Save lives by slowing the spread within our medical system’s capacity.
When I was in high school studying electronics on my own as a hobby (it was not taught in the curriculum. No, physics people culturally hates electronics, they consider it a chore.), I followed the logic states of the bistable (two NAND gates) meticulously. However, it was tedious and hard to remember correctly.
There’s a fast way to reconstruct the explanation from scratch. You’ll need these invariants:
‘1’ is ‘let the other input decide’ in AND logic (1 & A = A)
‘0’ is ‘action‘ in AND logic, namely clear (0 & A = 0)
NAND is practically a NOT gate if you tie the inputs together
Two NOT gates chasing each other generates Q’ and Q
NAND gates provides a mean for external inputs to disturb the chasing NOT gates
By leaving external inputs (S and R) at ‘1’, we are letting the state pins decide, behaving like the two chasing NOT gates.
The only way to disturb the state is to create a ‘0’ (clear) action. The circuit is symmetric, so ‘S’ and ‘R’ is arbitrary as long as you are willing to switch the roles of Q and Q’.
Set Q to ‘0’ by sending a ‘0’ (clear action) through ‘S’
Set Q’ to ‘0’ by sending a ‘0’ (clear action) through ‘R’
There are no other valid actions in this configuration.
Side note: persisting the clear action will lead to 0 & 0 = 0 at the applied input and 1 & 1 = 1 at opposite NAND gate, which the achieved state remains. Normally we want to return the external inputs back to 1 to receive future commands (actions) correctly, both external inputs asserting low is invalid.
It’s more natural to have S and R being active high in transistor’s implementation. NAND’s ‘S’ and ‘R’ are active low (so technically, I should use S’ and R’ instead, but I’m following the more common nomenclature for the moment for the NAND gate implementation).
I’d like to charge my phone and use USB devices at the same time, but it seems like it requires a 64.9kOhm resistor from sensor ID pin (micro USB) to ground. Instead of melting a USB-OTG cable, I bought this adapter (schematics here)
so that I can have direct access to the ID pin. This is a USB 3.0 give that I have a Galaxy Note 3. The same principles apply to the USB 2.0 versions for Galaxy Note 4.
According to this website, fsa9480_i2c.h has the table for the resistor ID values. Turns out 64.9kOhm is the one for both charging (slowly) and using USB devices (like mouse, network adapter, etc.).
I just bought a big lot of Data Precision 8200 and some Analogic AN3200 DC Voltage/Current Calibrators with a bunch of hard to find (unobtainium) genuine parts (relays, switches, hardware, regulator and amp ICs, whole modules, transformers) that that I believe it’s the leftovers of a closed down repair shop.
That means I’ll have all the materials needed to service and upgrade Analogic / Data Precision 8200 that you are unlikely to be able to find elsewhere.
Data Precision 8200 is the official unit to field adjust TDS 500~800 series oscilloscopes as the automation software (GPIB) was hard-coded to this model. Nonetheless, I find it a reliable reference for verifying oscilloscope performance and adjusting my multimeters as well.
Call me at 949-682-8145 for a repair quote or if you are interested in buying a unit. GPIB and 1kV option can be ordered for extra, either with the unit or service upgrade.
I knew Duracell is known for leaking when left in equipment for too long (too numerous to count: I had it leaked in wireless mouse, remote control, clocks, etc), but I always thought it’s my fault for leaving them in my electronics for a long time.
Today I got my answer: it’s not my fault that the batteries leaked. I just opened a new box of 4 AAA Duracells, and one of the new unused battery (the marking says it expires in 2023. It’s 2019 at the time of writing). I bought them from Tigerdirect so it’s likely to be genuine (on 10/2015). Here’s the pictures:
Not only I am not going to get Duracell batteries even if they are free, I’m going to toss all Duracell I have. It’s nothing but a menace. It’s worse than white label brands as it’s known to leak. It has to be a design or chemical formula or manufacturing process problem they have. By no means it’s an isolated incident.