MOSFET - AVR output isolation from drain/source volts ???

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Does a mosfet offer any isolation to the output pin of a microcontroller.

 

I'm going to make a simple lead/acid battery float charger (sick of them dying when in storage). So we are talking a voltage of up to say 15v at the drain of the mosfet while the gate can only have a max of 5v (connected to the AVR).

 

I'm having a hard time finding info on drain gate breakdown voltage. With an N type mosfet I'll have the source connected to ground so when the mosfet is off, the full 15v will be at the drain terminal.

 

I was thinking of protecting the AVR output by using a 10K "dump" resistor to ground then a diode to the gate of the mosfet to prevent any higher gate voltage getting back to the output pin. The 10K dump resistor would be to dump any reverse leakage current from the diode. The mosfet I'm using can turn on with quite a low gate voltage so the diode voltage drop won't be a problem.

 

Silly ideas ?? Unnecessary ??

 

I'll be looking into other things like gate inrush current etc but my main concern is the higher voltage getting back to the AVR output pin.

 

Keith

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The gate is isolated, so it is effectively a capacitor. if you get breakdown, then it is usually accompanied by smoke. This is based on experience with switchedmode supplies - when the mosfet explodes, it normally takes the drive circuit with it.

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Well, there is isolation, of a sort. DC wise, the output (eg, drain to source) voltage or current does not feed back to the gate. AC wise, it certainly does (capacitance). HOWEVER, the control is gate to source voltage. So, there is a strong dependence. You cannot just float the source-drain circuit with respect to the gate.

 

There are a number of single cell and multi-cell battery monitoring ICs out there. But, for a few currency units, you can get "battery maintainers" that do the job, well. Why bother building something?

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Cheers Russell,

 

I like the explanation for laypersons.

 

Keith.

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Cheers Jim,

 

I first realised about the gate to source control when I had the drain connected to positive and the load connected to the source terminal. The subsequent voltage at the source terminal when switched on, meant the gate/source voltage was reduced. I fixed it by putting the load on the positive (drain) side and connecting the source to ground. Then I got the full gate/source voltage drop and the FET turned hard on.

 

Why bother building something when I could just buy something ??

Started a new job that steals a great deal of my time. In the pockets of time I have I'm trying to learn an ARM microcontroller. As a result I find myself having to think about the sequence of events to write a program for, and program AVR again.

I.E. I'm getting a bit rusty and don't like it so this is just a little refresher for me. Plus nothing beats using a tool which you designed and built. Same with my little excavator, it never gets old using the grab, rake, ripper hook, narrow bucket, etc that I designed and built. It just wouldn't be the same if I bought them LOL.

 

I've also got parts lying around. Old ATX power supplies (hack em to increase volts), old laptop power adaptors (hack em to decrease volts), or just use one with $2 adjustable power supplies from Ebay (with current limiting), plus a $3 Arduino Nano and a logic MOSFET.

 

So pricewise I'll get out of it cheaper than if I bought a float charger.

 

Keith.

Last Edited: Sat. May 13, 2017 - 11:36 PM
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ka7ehk wrote:
But, for a few currency units, you can get "battery maintainers" that do the job, well.
What I have on a diesel one-ton truck :

Site Logo

Battery Tender® Plus Selectable 6V/12V @ 1.25A

http://products.batterytender.com/Automotive/Battery-Tender-Selectable-6V-12V-1-25-Amp-Plus.html

Did make a mistake of exposing the AC side to light rain causing the circuit's GFI to trip; recommend an extension lead :

http://products.batterytender.com/Accessories/

Winters are mild here relative to northern states but does keep the most near battery warm enough for a squirrel to sleep on top of the battery (stash of acorns and acorn remains)

I brake for squirrels ... truck's a squirrel motel wink

ka7ehk wrote:
Why bother building something?
One can fix it when it expires though a COTS battery tender has some essential safety features like no arc (IIRC it senses +3Vdc before applying current)

Would recommend either a complete battery box (box and lid) or the vehicle's battery box with hood/bonnet/trunk shut and battery connected via the ring terminal harness accessory.

Why : https://www.avrfreaks.net/forum/ballistic-batteries-red-wire-red-wire by DocJC

For if or when a brute force battery charger is misapplied to the battery (severe operator headroom problem)

IIRC it's common for a deep cycle battery to be in a marine battery box.

 

"Dare to be naïve." - Buckminster Fuller

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Gchapman,

 

you know what, I'll probably buy something like that down the track, AFTER my home made float charger has been up and running for a while LOL. I have a terrible habit of building a gadget to a working state and that is "my fix". Later on I'll just pay for something similar.

 

Keith.

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One possible solution.  Crude, but workable as a starting point.

 

If you want full isolation, stick an Opt-Coupler in the gate circuit, as shown.

 

EDIT TO ADD:

Updated schematic to show Opto-Coupler.

 

EDIT TO ADD:

Changed 12V_IN to Vx_IN_P

Added Vx_IN_N, to show the GND return reference between the supply and the battery.

 

https://www.avrfreaks.net/sites/default/files/forum_attachments/P-Channel_Driver_schem_1.jpg

Attachment(s): 

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

Last Edited: Sun. May 14, 2017 - 03:22 AM
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The pass FET will short when the load shorts unless the source is current limited.

Alternatives :

  • protected load switch per AofE
  • AVR's comparator detects the overcurrent then pass FET's gate drive is removed with reverse current control

12V automotive protected load switches are somewhat common with 24V automotive on-coming (bus, medium truck, semi tractor, RV)

What's interesting are the new 48V (IIRC) automotive products (hybrid engine start-stop, weird to hear such when walking near a crosswalk)

 


https://www.fairchildsemi.com/products/power-management/advanced-load-switches/advanced-current-limited-load-switches/FPF2700.html

http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyFPF2700MX

https://www.fairchildsemi.com/products/power-management/advanced-load-switches/advanced-current-limited-load-switches/FPF2895.html

http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyFPF2895UCX

http://www.cambridge.org/us/academic/subjects/physics/electronics-physicists/art-electronics-3rd-edition

 

"Dare to be naïve." - Buckminster Fuller

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gchapman wrote:

The pass FET will short when the load shorts unless the source is current limited.

 

 

I didn't see where the OP asked for current limiting.

 

My intent was to provide something that would be a starting point, as far as isolation goes - nothing more.

 

Besides, what fun would the OP have if we simply design the whole thing for him/her?

 

A little demolition-derby is in order, you think???  How else will he/she learn, unless he/she lets a bit of smoke out???

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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What!

 

Carl, did you  really mean to switch your 12 volt rail to a 5 volt battery? Smokies time!

 

Ross McKenzie ValuSoft Melbourne Australia

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Ha ha, yeah demolition can be fun too. 

 

No, I'm not asking anyone to design the whole thing. As my opening post asked, I was mainly wondering about the isolation / DG breakdown voltage for isolating the AVR output from the higher voltage. 

 

Everything else I can pretty much find on the net or in my books, and that is where I usually haunt. Just couldn't find much info on the gate isolation topic.

 

I did mention current limiting when I mentioned a $2 power supply (I said it was built in). If I was using a bare bones supply I'd probably use a decent sized transistor in the output line to do the job - plenty info on the net for that one.

 

Regarding the isolation, it's not really a case of do I >> want << it, it's more a case if do I >> need << it. Based on Russells' first reply I'd say I don't.

 

This is going to be a pretty simply float charger, just to keep a battery from discharging and sulphating when not used. Supply maybe an amp and cut off when upper voltage is reached, switch back on when lower voltage is reached. I've got a "real" battery charger which I can use for the main charging of a discharged battery, but for float charging I may want to have 3 or 4 simple float chargers all going at the same time. Take them all off a common supply source and it's a cheap homemade solution.

 

Thanks for all the input.

 

Keith.

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Actually, no.  But I'm relatively new to Fritzing and was not able to simply set 5V to Vx, or something.

 

Fritzing has limited voltage supply selection 0V, 1.7V, 3.3V, 5V, & 12V.  I suppose I could have put a "Node-TAG" and simply labeled it Vx.

 

But the point being, the circuit demonstrated a method of galvanic isolation between the PWM source and the battery.

 

 

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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You're forgiven... cheeky

Ross McKenzie ValuSoft Melbourne Australia