HELP: Battery Powered Inductive Circuit - Page 1
Forum section: electronics / projects

author: hotrodman106
author_rank: Newbie
timestamp: March 28, 2021, 02:06:38 am
content: 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 WTFis going on and how can I fix it?Thank you all in advance!

author: bob91343
author_rank: Super Contributor
timestamp: March 28, 2021, 03:02:00 am
content: Sorry but that diagram is incomprehensible. I can't read the legends and it's modern style as though created by a computer rather than a person.Redraw it in a sensible manner and maybe I can offer some help.

author: hotrodman106
author_rank: Newbie
timestamp: March 28, 2021, 03:29:30 am
content: Fair enough, I won't be able to redraw it this evening but I'll redraw it for you tomorrow. In essence though, it's simply a n-channel mosfet being switch at 180khz which drives a the 'negative' side of a coil (As in one side of the coil is tied to the source power while the other side is pulled to ground intermittently by the mosfet). Additionally, there is a film capacitor across the source and drain of the mosfet. The rest of the circuit is just a power supply for the micro, a reverse polarity protection mosfet, and some voltage dividers for various voltage testing.

author: Tom45
author_rank: Frequent Contributor
timestamp: March 28, 2021, 03:31:54 am
content: Details of the coil would be important too. In particular, what is its inductance?What is the current limit of your bench supply?

author: hotrodman106
author_rank: Newbie
timestamp: March 28, 2021, 03:41:30 am
content: Current limit of my supply is 5A. As for the coil, it's salvaged from a different board 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.

author: ledtester
author_rank: Super Contributor
timestamp: March 28, 2021, 03:44:59 am
content: A diagram like this is a lot easier to read:You can whip one up over at circuitlab.com(Note I am assuming that your inductor goes between ANT+ and ANT-)It's also important to know the values of the components -- R1, C1, L1 and what device Q1 is.

author: hotrodman106
author_rank: Newbie
timestamp: March 28, 2021, 03:57:06 am
content: That diagram is perfect, thank you! Also btw, your handwriting is gorgeous, I'm jealous. Now for the component breakdown: R1 is 150 ohm, C1 is 0.1uf film, L1 is the coil I described in the last reply, and Q1 is a RFD14N05LSM mosfet. For the mosfet here is the digikey link:https://www.digikey.com/en/products/detail/on-semiconductor/RFD14N05LSM/458751

author: Tom45
author_rank: Frequent Contributor
timestamp: March 28, 2021, 04:10:32 am
content: The reactance at 180 KHz of a 20 uh coil is about 20 ohms. The DC resistance is quite likely well under an ohm. And the on resistance of the mosfet is 0.1 ohms.If you apply the 12 volts before the gate is being switched at 180 KHz the current through the mosfet will be 12 / R. With R being likely << 1 ohm, the current will likely be limited by the 5 amps of your bench supply.Depending on the 12 volt battery, the current would only be limited by the mosfet on resistance and the coil's DC resistance. If the coil is 0.1 ohms, then the current could be as much as 12 / (0.1 + 0.1) = 60 amps. Well over the 14 amp rating for the mosfet.

author: hotrodman106
author_rank: Newbie
timestamp: March 28, 2021, 04:20:08 am
content: Which would explain the disco inferno of the MOSFET, makes sense. Now what's weird to me is that when I run the circuit on the bench power supply, the circuit only draws about 400mA without the receiver being near the coil. If I line up the receiver and transmitter coils together, the max draw is 3 amps which is still plenty under what my power supply can do. But, to your point, shouldn't I expect the circuit to attempt to draw the full 5 amps from my power supply? And, the circuit blows up when the transmitter is unloaded but again, only on the battery? What would be the best way in your opinion to try and limit the current/redesign the circuit?Also I'll say it now, thank you all so far!! I super appreciate it!

author: mikerj
author_rank: Super Contributor
timestamp: March 28, 2021, 08:22:15 am
content: Your PB4 I/O pin needs a pull-down resistor. This pin will be tristated when the micro is initially powered which will allow the gate voltage to float. Which such a low (DC) impedance load this is a recipe for disaster.