Battery Powered Inductive Circuit

Thread Starter

hotrodman106

Joined Mar 27, 2021
6
Hello All,
I'm hoping someone smarter than myself can help me out with this one because I am bashing my head clean through a wall.
This is my inductive transmitter circuit. Essentially, I have an ATTINY45 generating a 180khz square wave which drives a MOSFET and subsequently drives a coil to transmit current to the receive antenna. Now, for the most part this circuit works fine on my bench power supply (barring when I drive it a bit to hard) HOWEVER, the minute I attach this to a 12 volt battery, either the polyfuse trips or, if I remove the polyfuse, the MOSFET goes disco inferno. I'm guessing this has something to do with transient current on the power lines feeding the board but I have no idea WTF is going on and how can I fix it?
Thank you all in advance!
 

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Papabravo

Joined Feb 24, 2006
16,814
All that head bashing must have resulted in brain damage. If your antenna is a conductive piece of wire, in a coil or not, you are creating a DC short to ground. That is why the MOSFET is getting hot. Where did you get this concept and why did you think you could get away with it? Did you pay big bucks for a fly-by-night correspondence school course? What are you trying to do exactly?

Antennas don't work with DC and pastrami don't go on white bread with lettuce and mayonnaise.
 

Thread Starter

hotrodman106

Joined Mar 27, 2021
6
While I'll admit I have some brain damage from a combination of inhaling ABS fumes from 3D printing and solder smoke, I do know that a coil of wire is, in fact, a wire. If you look at the design and read my post, I am switching the coil at 180khz and am creating a changing magnetic field. Like I said, it works perfectly fine when I power it on my DC bench power supply but ONLY has a problem when connected to a battery. Also, I disagree which your sandwich analogy, I quite like my pastrami sandwiches on white bread with lettuce AND mayo. Sometimes even cheese and onions too.
 

Papabravo

Joined Feb 24, 2006
16,814
Whatever your duty cycle is it is still making a short and stressing the MOSFET. You gotta find a way to stop doing that. Maybe your power supply has a current limit associated with it. I don't know - I'm not there and can't see what kind of foolishness you are up to. Have you asked anybody else if they think running DC through an antenna is a good idea?
How about AC coupling the output of the MOSFET to the antenna - would that kill you?
 

Thread Starter

hotrodman106

Joined Mar 27, 2021
6
Shockingly the last time I asked someone about running DC through a coil, they put me in one of those jackets that goes up in the back, makes it really hard to get any soldering done. Regardless of that, what would you suggest instead? I'm essentially designing a primative wireless transmission circuit that should be able to provide approximately 12 volts at 1.5a on the receiver side. I'm all ears for a redesign
 

DickCappels

Joined Aug 21, 2008
7,721
1616913857341.png

I usually incorporate some current limiting in series with the coil. Note the 220 ohm resistors. The duty cycle is carefully kept at 50%.

What is connected as your antenna?
 

Thread Starter

hotrodman106

Joined Mar 27, 2021
6
View attachment 233794

I usually incorporate some current limiting in series with the coil. Note the 220 ohm resistors. The duty cycle is carefully kept at 50%.

What is connected as your antenna?
That makes sense. For my implication, the duty cycle is varied for a dimming function but it stays between 50% and 15% however, for testing I disabled this function so it just stays at 50%. For my antenna I am using a wireless charging coil which I salvaged from a different circuit so I don't have super detailed specs on it, however I know its a single coil but dual layer, it's 50mm in diameter, has a sfr of ~180khz, and should have an inductance of somewhere in the 20uH range give or take. What do you think would be the simplest way to add some current limiting to the circuit?
 

Thread Starter

hotrodman106

Joined Mar 27, 2021
6
What is the inductance of the coil?
Alternatively, number of turns and dimensions?
I salvaged the coil from a different circuit so I don't have super detailed specs on it, however I know its a single coil but dual layer, it's 50mm in diameter, has a sfr of ~180khz, and should have an inductance of somewhere in the 20uH range give or take.
 

DickCappels

Joined Aug 21, 2008
7,721
Is there any tuning on the antenna (in the form of a resonating capacitor)?

For lack of any specification I would drop the voltage until current draw becomes acceptably low.

I don't imagine that variyng the dutycycle is a good idea. If you can, how is that supposed to work?
 

Thread Starter

hotrodman106

Joined Mar 27, 2021
6
Is there any tuning on the antenna (in the form of a resonating capacitor)?

For lack of any specification I would drop the voltage until current draw becomes acceptably low.
I don't have any capacitors across the coil, only a 0.1uf film capacitor across the base and emitter of the MOSFET. How would I calculate a resonating capacitor for the coil?
 

AlbertHall

Joined Jun 4, 2014
11,505
I don't have any capacitors across the coil, only a 0.1uf film capacitor across the base and emitter of the MOSFET. How would I calculate a resonating capacitor for the coil?
The circuit in post #1 shows no capacitor on the gate of the MOSFET, but it does have one between drain and source of the MOSFET. That capacitor is effectively in parallel with the coil.
 
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