I am looking for help in selecting a proper MOSFET for our project. We will be utilizing a MOSFET to switch higher current up to 5A at 24V to a coil. We will be utilizing a microcontroller to control the gate voltage to determine the current across the coil. So the VGS(th) will need to be quite low to handle the small voltage range (0-5V) of a microcontroller. I can provide more detail if needed. Thanks
Hello, welcome to the TechForum! Would something like 497-13975-5-ND work for you?
I would suggest looking at this forum post.
Thank you @Steven_14444,
An alternative for @Apex4446 is to use a Darlington. They are a bit easier to drive directly from a microcontroller. Also, it is possible to place devices in parallel for even more current provided a small emitter resistor is used.
Ref:
Best wishes,
Aaron
Sorry for the late response, notifications were going to junk mail.
But thanks guys, ill read through all of this stuff and come back with any questions.
Really appreciate it. This project has been a struggle and even more so finding the help along the way so really appreciate it.
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Read through all that stuff and continued to do some more research into Darlington pairs. Definitely feel this is the way to go. But I’ll be honest trying to understand some of the graphs on the datasheets gets intimidating.
Would something like the BDW93C work. My worry is the output current from the Teensy 4.1 would not be adequate for this transistor (as in too low). Our goal is to eventually be able to vary the current to the base to adjust the current to the coil. I’m pretty sure the output on a Teensy 4.1 is only 4mA but some pins can output 20mA if I understand.
Hello @Apex4446,
To summarize, you are looking to switch a 5 A inductive load (coil) operating at 24 VDC. You desire a simple to use interface between the microcontroller and the coil.
One more question, is turn on and off speed important to you?
Assuming no, I recommend using an interposing relay. This technique uses a small control relay as in intermediary between the microcontroller and larger load. Another possibility is a solid-state relay.
Best wishes,
APDahlen
While we can
Yea we have a coil where our max current is 5A at 24V. We are generating a magnetic field and by controlling the current across the coil we can change the magnetic fields.
Switching speeds is very important. The current will be changing rapidly (ms range) to varying conditions based on telemetry sent to the microcontroller. How we generate that signal is still to be determinded (pwm, dac…). But i think right now our primary goal is proof of concept so we will manually create a signal to be sent to the transistor, even if it means using a potentiometer to change the current to the base of transistor.
If linearly variable current control with any degree of precision is the objective, something slightly more involved than a plain 'ol transistor is likely going to be needed. One approach may be a circuit of the sort below:
The concept here would be to fake an analog signal by filtering a PWM output from the aforementioned Teensy, and feed that to an op amp which gets its feedback signal from a current sense resistor (R2) monitoring current flow through the coil and driving some BJTs to cause said current flow. By magic of op amp, coil current then becomes a very close approximation off the filtered input signal. Above is drawn as such because the slate of BJTs good for upwards of 5A/24V is shrinking rather quickly.
Important conditions to evaluate for component selection would be at maximum coil current, so that things can happen, and at the point where potential across coil and Q1 are equal, where stresses on Q1 are likely to be at maximum.
Very interesting, thanks rick. I’ll run this one though multisim and play around with it.
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Hello @Apex4446,
The circuit described by @rick_1976 is a classic and reliable solution. It works very well, but I once forgot the heatsink and burned my finger on the BJT.
Let me add two more thoughts to this conversation:
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The speed of turn on an off is dominated by the coil inductance. This article presents the situation for a coil. Another option is to focus on the tau = L/R aspects by adding a series resistor and increasing the drive voltage as described here.
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You could shift back to MOSFETS and PWM the coil directly as described in this note.
Welcome to open ended circuit design. May I recommend you refine your requirements to help guide you to a solution. Be sure to ask the meaning of a “wicked problem.”
While you are there, ask it why PWM drive of a 24 VDC via a series connected 240 VDC supply has merit. Know that it’s hardly ever done as it’s not worth the safety risk. Yet, you are on a quest for speed and should know di/dt.
Sincerely,
Aaron
Thanks guys. Man I wish I had just half the understanding on microelectronics like you guys do. This stuff quickly reminds me why my interest and career path leads to power engineering. But this has been a fun little rabbit hole of a project that’s led me down some interesting study paths. Just gonna keep reading and trying to understand more and widdle away at this. I’m finding just tweaking things in Multisim and observing results has helped. Unfortunately play time is coming to and end and I just got to get parts ordered and get a working proof of concept completed.
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Hello @Apex4446,
Play time never ends if you do it correctly!
I wouldn’t compare yourself to @rick_1976 and I. We both have a lifetime of experience. Keep the fires alive and you will one day be in a position to help others.
Sincerely,
Aaron
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