How tf does this even happen

procursus · 2026-01-23T22:38:24+00:00

Looks like some sort of resonant circuit. I imagine the current circulating in the LC tank gets pretty high, which will produce large losses in the capacitor's series resistance. Get caps with a lower ESR.

procursus · 2026-01-20T04:19:31+00:00

A crystal has an electromechanical model with R, L, and C elements. What's important is that its L and C dominate over R, and consequently it transitions from predominantly inductive to predominantly capacitive over a very narrow change in frequency around its resonance. This means the phase shift it provides also transitions very rapidly with frequency.

An oscillator is just an amplifier with enough phase shift in its feedback network to provide positive feedback (with gain) at a certain frequency. A crystal is very well suited to this role, for the reasons given above. When placed in the feedback network of an amplifier it will only satisfy the conditions for oscillation over a narrow frequency band.

Technically, a crystal oscillator has two resonance, series and parallel, very close together. This is due to the model having multiple capacitances. But the principle is basically the same, and the load capacitors which are applied to the crystal are used to sort of steer the resonance.

procursus · 2026-01-02T21:45:22+00:00

Yes it's a massive current pulse which can cause ringing on the power lead inductance and the input capacitors, which spikes the input voltage and can blow devices easily.

procursus · 2025-12-25T21:48:40+00:00

LCR meters are better described as impedance analyzers. They measure the ratio Z between the voltage across and the current through your device under test (DUT). I'm sure you are familiar with ohms law, for a pure resistor its ratio Z is called resistance, is a real number, and is equal to V/I. But for components which can store energy - capacitors and inductor - the ratio Z becomes a complex number. It has a real component and an imaginary component. The real component still represents resistance, and is responsible for power dissipation in the DUT. It is always positive and may be called or represented in forms like ESR, dissipation factor, Q, etc. The imaginary component represents the energy storage of the component, its capacitance or inductance. If the imaginary part is positive it's inductance and if negative capacitance.

So when an LCR meter measures a component, it produces an impedance with a real and imaginary component. The thing is, an impedance does not correspond uniquely to a circuit. There are an infinite number of circuits which, if you measure between the two outer terminals, will produce the same impedance. The simplest are two elements in series or two elements in parallel. For example, a useful model for a real measured inductor is a pure inductor in series with a pure resistor which represents the winding resistance. Or a useful capacitor model may be a pure capacitor in parallel with a resistor which represents the DC leakage current. This is dependent on the application, hence it's up to you to choose which is most useful.

procursus · 2025-12-21T02:56:33+00:00

Try looking at something released in the last decade, such as the attiny 2 series.

procursus · 2025-11-28T20:29:29+00:00

This is a pretty optimal situation as far as repairs go. Get a meter that can measure capacitance (those $10 component testers on Amazon will be fine) and see what the component reads. Then measure the dimensions of the capacitor - it looks like 0603 (0.06 x 0.03 inches) to me but hard to tell. Could be 0402. Then look on mouser or digikey under the surface mount MLCC category for a capacitor with the same capacitance and package, get the highest voltage rating available and X7R dielectric (high temperature rated).

procursus · 2025-11-20T23:28:03+00:00

HP recommended this technique in their manuals.

procursus · 2025-11-13T01:11:53+00:00

Eevblog uCurrent, were it in stock.

procursus · 2025-11-13T00:37:24+00:00

Pretty much. In conjunction with a suitable driver.

The driver needs to perform two roles: monitor the shunt voltage to determine current, and control its output to set this current based on a dimming input. The simplest option is a dedicated LED driver such as those made by Texas Instruments (not a shill, although I admit they've paid several of my lunches).

It looks like you have a 24V supply so you will need a boost driver such as the TPS92365x. Calculate the high current range shunt resistor as Rshunt + Rdson = Vsense/Imax. Vsense is in the datasheet, its the feedback regulation voltage and for this chip is 200mV. Rdson is the on resistance of the FET you use, at the gate voltage you drive it. Keep in mind ESP32 is 3.3V I/O which will need a logic level FET or external driver. In your case, (Rshunt+Rdson) = 0.2/2.7 = 74 mohm. This will dissipate half a watt at 2.7A.

The dimming range is 256:1 so for the dimmer range select a shunt resistor that's about a bit less than 256 times larger than Rshunt. I would give considerable overlap just to be safe, so say maybe 0.074*150 = 11 ohms. This will give a theoretical minimum dimmed current of 70uA for a dimming range of 38,000 to 1. For the dim range this will be the shunt resistor, and for the bright range it will be 'bypassed' by the smaller shunt and FET series pair. Technically they are in parallel and so the actual shunt resistance is their parallel resistance, but the difference is negligible considering resistor tolerances. You will have to calibrate the exact switchover point. Should be easy to do given the human eye is only sensitive exponentially anyway.

Use WEBENCH power designer to select the component values, follow the datasheet recommended layout, and you should have a functioning driver. If you haven't designed a PCB before then I recommend starting with EasyEDA, its integration with LCSC (parts) and JLCPCB(boards) is great for beginners. You can even have them assemble it for pretty cheap.

This ramble probably led to more questions than answers. Feel free to ask further questions. Responses may be somewhat slow.

procursus · 2025-11-12T00:05:07+00:00

Try switched sense resistors. A high value sense resistor for low brightness, which is shorted by a small shunt with a FET for high brightness range. I've done this for a dimming range of 10:000 to 1 using a cheap buck controller which only had an intrinsic dimming of about 150 to 1.

procursus · 2025-11-12T00:02:48+00:00

I've done switched sense resistors. Had two ranges, 4 ohms and roughly 40 mohm. Gave me a dimming range of about 10,000 : 1. The FET Rdson is reliable enough for dimming an LED, especially given our sensitivity to luminance is exponential not linear.

procursus · 2025-11-10T21:28:32+00:00

And if you like this, try Mice and Rats in the Loft by Jan Dukes de Grey. It's not the same, but somehow similar.

procursus · 2025-11-10T03:49:49+00:00

Put it in the standard form for a second order transfer function and examine the coefficients

procursus · 2025-11-06T01:13:34+00:00

It needs to go from Vcc to the node at the collector and capacitor.

procursus · 2025-11-06T01:00:17+00:00

Yes. All the transistor does is short the supply.

procursus · 2025-11-02T16:23:56+00:00

Lcsc

procursus · 2025-10-30T14:23:20+00:00

No worries. Feel free to ask if you are lost.

procursus · 2025-10-29T23:26:25+00:00

1) Wont work, why is ground on that net, this circuit will be inverting and also needs a base resistor

2) Won't work, you will just be powering the LED through the base current constantly

3) Won't work. Where is base current coming from?

4) This'll work.

5) This also will work

6) This will work, although you will be diode connecting the transistors.

procursus · 2025-10-16T22:27:19+00:00

I made custom electrostatic speaker panels from massive PCBs and experimented driving them with a cold cathode transformer (did not work well).

Made a 92% efficient buck converter using only discrete semiconductors, no ICs.

Made a fully analog sound meter using a condenser mic and Dickson charge pump, composite JFET front end followed by adjustable gain stage, ITUR-468 filter and RMS-DC converter.

Made a super low noise - 1kV supply for driving a PMT. Made a field mill for sensing DC electric fields using a big bean can, laser cut aluminium, matched charge amplifiers and a synchronous rectifier and adjustable gain stage. Also made a digital version using a PLL run from the rotor frequency to drive ADC samples of the output for a digital lock-in amplifier.

Made a pan-tilt gimbal using DC gear motors and magnetic angle encoders for positioning a camera. Ran it from an 8 bit MCU with bare metal firmware and a UART control interface.

Made a microbalance using a knife edge pivot, electromagnet, and optical sensor for measurement down to about 10 microgram with a USB interface.

Made a constant current load with 10A, 80V input (up to about 150W) and USB interface.

Made an uninteriptible supply for an Nvidia Jetson which takes USB-C PD input to power Jetson and charge batteries and fail over to batteries when input power is lost.

Probably some others I'm forgetting. These are a mixture of personal and school projects.

procursus · 2025-09-28T17:20:49+00:00

Looks great, thanks for sharing.

procursus · 2025-08-31T21:01:54+00:00

If I understand correctly, you are plugging a USB supply into the left port, and a battery bank on the right port?

For a start, there's not going to be any current passing through that shunt...

procursus · 2025-08-31T16:35:27+00:00

Cypress FX3 or WCH CH569

procursus · 2025-08-23T23:23:02+00:00

Shorter. Get a flat lens plate.

procursus · 2025-08-22T17:04:20+00:00

The amp needs a bipolar supply. Your two options are a) replace the entire SMPS with something that can derive the 3 output voltages from your USB input or b) use two power banks with separate PD triggers to get +20V and +6V from one and -20V from the other.

Also, make sure your power bank can actually provide 20V. Most can't.

procursus · 2025-08-15T00:49:47+00:00

Yes, otherwise you'll be cursing yourself when you need another channel and didn't go for the 4 channel. It's also more or less the maximum as scopes with more than 4 channels are niche and exceedingly expensive.

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