Positron BASIC Community

I wanted to ask you if using a 0.6 ohm 0.6mH coil at frequencies lower than 6 HZ with duty+ 14% square wave with ZK-PP2K generator turns off completely. Furthermore, at frequencies such as 14Hz 14%duty+ always square wave the power supply always goes into protection when the coil is activated but the PP2K remains on. How to solve these problems?
I power everything with a 12V 3A power supply protected against over current
Thank you

Hi,
At such low frequencies the inductive reactance of your 600uH inductor is much less than its DC resistance so you will go in to current limit of your supply.
Why would you want to use such low frequency PWM?

Cheers,
David

To limit this, can I put a low-value resistor in series with the coil?

Quote from: Giuseppe on Feb 16, 2024, 09:53 PMTo limit this, can I put a low-value resistor in series with the coil?

Yes you can, but you'll just be burning up energy in the resistor instead of putting it into the coil.
Can you lower the voltage of the supply instead?  Maybe down to 1 or 2 volts.

-Rick

Thanks for the replies, I'll try and let you know

Such low frequency PWM is interesting. Several years ago, when I worked for a local company, we used to hire out one of our 2KW variable frequency power supplies to a company who brought them back with evidence of sea water in/on them. They reluctantly explained to me that they were dragging inductive coils along the seabed while varying the frequency in a PWM pattern. I think they were looking for certain, presumably valuable, metals. Perhaps this was a precursor to materials used in modern batteries?

But their frequencies were from 60 to 400Hz.

Charlie

Charlie, I know of an as yet unlocated wreck 1/2 mile off the Dorset coast which has around £75 million in gold. The problem is trifold.

1) The damn stuff moves around with the tides(a bar of gold ONLY weighs the amount of water it displaces when under water)

2) Gold has a notoriously short magnetic response time

3) By now, the gold will be spread over a huge area (it was payment bound for the US for war materiel but the ship was torpedoed just off the Dorset coast and went down). Salvage was attempted at the time, but the area was repeatedly mined by U-Boats at night and as fast as the RN swept them, they were replaced. In the end the risk was deemed too great nad after the war, it was noticed, no one could remember where the wreck was!

Has anybody got a boat Sean and I could borrow?

A lot of money £75 million

That happens because at very low frequencies, the coil stays ON long enough for the inductive reactance to drop, meaning the circuit sees mostly the coil's raw 0.6 Om resistance.

At 6 Hz and 14% duty cycle, the pulse stays high for around 23 ms. Since the L/R time constant for a 0.6 mH / 0.6 Om coil is only 1\text{ ms}, the current ramps up to maximum almost immediately and acts like a near short-circuit (I = V / 0.6 Om). That huge current spike is tripping your power supply's protection circuit or dropping the line voltage enough to shut down the PP2K generator.

A few quick things to check/try-
Add a series resistor- Putting a small power resistor (2Om–4Om) in series with the coil will cap the peak current.
Flyback diode- Make sure you have a fast recovery diode placed across the coil to catch back-EMF spikes when the signal drops.
Use a separate driver- Drive the coil via a MOSFET with its own high-current PSU so the PP2K logic side stays isolated from voltage sags.

If you ever need to quickly cross-check pulse durations and period times for different duty cycles, frequencylama.com has a simple frequency/period calculator that saves doing the math by hand.
Hope that helps point you in the right direction!

Quote from: SeanG_65 on Feb 18, 2024, 12:21 AM(a bar of gold ONLY weighs the amount of water it displaces when under water)

No, the gold gets lighter,but only as much as the water weighs.The weight of the water is subtracted from the weight of the gold.