MOSFET circuit with low value resistor.

Thread Starter

markdil

Joined Mar 31, 2009
11
I see this in a lot of MOSFET circuits with inductive loads. There sometimes is a low value resistor, usually less than one ohm from the source to ground. What is the purpose?
 

ronsimpson

Joined Oct 7, 2019
2,063
In this circuit there is a low value resistor on the S of the MOSFET. The "ISENSE" pin on the IC measures the FOSFET current this way.
1651330349740.png
p.s. Boost power supply.
 

Thread Starter

markdil

Joined Mar 31, 2009
11
One possible explanation is that it can be used to measure the current through the device.
I had considered that, but saw no reason sensing the current would be needed on the circuits I have looked at. I attached a picture of a circuit board. The resistor on this example is just a circuit trace that measures 0.265 ohms. It goes from source to ground. The inductive load is between V+ and drain. Nothing else is on the load circuit except a 10K resistor from the gate to ground which I imagine is for making sure it shuts off.
 

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Papabravo

Joined Feb 24, 2006
18,409
I had considered that, but saw no reason sensing the current would be needed on the circuits I have looked at. I attached a picture of a circuit board. The resistor on this example is just a circuit trace that measures 0.265 ohms. It goes from source to ground. The inductive load is between V+ and drain. Nothing else is on the load circuit except a 10K resistor from the gate to ground which I imagine is for making sure it shuts off.
If it is just a circuit trace, how do you imagine that it is a resistor?
The picture is nearly useless, a schematic would be more helpful.
 

DickCappels

Joined Aug 21, 2008
8,707
Thank you for the schematic.This looks something like and RF heating circuit. That would be degeneration for one reason or another.
 

Thread Starter

markdil

Joined Mar 31, 2009
11
Thank you for the schematic.This looks something like and RF heating circuit. That would be degeneration for one reason or another.
It is a Schumann Resonator circuit. The gate of the MOSFET is controlled by a 555 timer. The frequency is set to ~7.83Hz (50% duty cycle). Other Schumann Resonator circuits I have looked also have the same low resistance (all others have an actual resistor instead of a circuit trace) tied to ground. That is why I assumed the circuit trace was a resistance. Either way, I just don’t understand why it is there.
 

DickCappels

Joined Aug 21, 2008
8,707
At that frequency the little squizzled trace on the source can't do much more than be a resistor, and not much of one at that. It may be more for visual effect than for functionality.

1651409548261.png
 

Thread Starter

markdil

Joined Mar 31, 2009
11
I have purchased close to a dozen different units and have been studying them. The reason they got my attention is I saw a Tik Tok video (yes, I can’t help myself when it comes to TikTok) of someone claiming it helped them sleep. Since I have a nightmare (no pun intended) time trying to sleep, I was to a point I would try anything and figure what do I have to loose? So, I purchased one and tried it. It actually has helped me sleep. I have been using it for about six months.

Most of the units I purchased had blatant design flaws, some didn’t even work. So, I decided to design my own. That is why I was questioning the low resistance I saw on most of the units. Maybe they were copying each other, or maybe there is a reason it is there. That is why I posted the question seeking help from people a lot smarter than me!
 

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MrChips

Joined Oct 2, 2009
26,089
The transistor is configured in common-drain mode (aka source follower).

1651411973167.png

This is equivalent to common-collector mode (aka emitter follower) for BJT (bipolar junction transistor).
1651412010921.png

In this case the load is Rs or RE and this transistor configuration exhibits the following characteristics:
  1. current amplification
  2. unity voltage gain
  3. very low output impedance

The squiggly line is the antenna and is the load of the circuit. It is an inherent part of the circuit and should not be removed.
 

DickCappels

Joined Aug 21, 2008
8,707
@MrChips I was not referring to the spiral. Perhaps I should have called it the meandering trace in series source.

1651414422686.png

It is difficult to think the meandering trace does much of anything at 7.83 Hz (not even kHz).
 
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MrChips

Joined Oct 2, 2009
26,089
Sorry, I did not look closely enough nor can I observe where is the squiggly line and the PCB spiral with respect to the MOSFET.
 

DickCappels

Joined Aug 21, 2008
8,707
1651418483526.png
The three in-line rectoval pads are probably those of the MOSFET. @markdil@ix.netcom.com wrote that the Source is connected to the meandering trace.

They just don't look like 8 Hz components, though granted, the sprial can be used to make a magnetic field, probably a small one.

By the way, I am not questioning whether markdil's sleep has improved. His experience is his experience. My vote is that the meandering trace is a decoration.
 

Danko

Joined Nov 22, 2017
1,302
BS and cheating.
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
https://www.indiatimes.com/technolo...schumann-resonance-human-behaviour-536813.html
Schumann Resonances could affect humans
Studies in the past (highlighted by BigThink) have suggested that these waves actually have an impact on humans. A study from 2006 has discovered that the frequencies could be linked to different kinds of brain waves. They called this as ‘real-time coherence’ in variations in the Schumann and brain activity spectra within the 6 to 16 Hz band.

In another study from 2016, conducted by the Behavioural Neuroscience Laboratory of Canada’s Laurentian University saw 238 measurements from 184 individuals over a period of three and a half years showed strange and unpredictable similarities in the spectral patterns and strengths of electromagnetic fields that were generated by human brain activity and the Earth-ionospheric resonance.

Some have even linked the Schumann Resonance of 7.83 Hz to hypnosis, meditation as well as growth hormones in humans, although there isn’t any concrete scientific evidence supporting these claims.

In the area of ‘new age science,’ many believe that the Schumann Resonance can also be affected by human consciousness. So a global rise in anxiety, tension or stress (much like what occurred during the initial months of the pandemic) could affect the Schumann Resonance. Alternatively, even an unusual spike in these frequencies could have an impact on humans and animals.

While all these sound rather scary, one can ignore them due to the lack of scientific backing behind these theories. But if it were to be proven true in the future, it’s a fascinating concept nonetheless.
1651426569636.png
The first photo, a modern printed desktop circuit, Schumann resonator. In the second photo, the floor of Chartres cathedral in France (built in 1220).
 
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