In the scenario that you describe, I would file a provisional patent application. If something changes with your invention, then at least you tried to capture as much as you could now with the provisional filing. I don't see any con but for the money and time lost if the invention changes a lot.
1. A provisional application will never be declared "void." But, if not written well it might not provide you with the real benefit of the early filing date.
2. If a provisional is written well, it might not be "added cost." It will just be "included cost" that can be figured in to the total cost of preparing a non-provisional. Basically splitting one big step/cost into two smaller steps/costs.
The best way to ensure your provisional is worth anything is to hire a patent attorney.
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Q: "What are the potential cons of a provisional patent?"
R: Well, the biggest "con" of all: The Patent Office or a judge could find that your provisional application does NOT adequately disclose the invention you claim in your non-provisional. The consequence being that your non-provisional will be afforded its own filing date and not the filing date of your provisional. Which means that if someone publishes a disclosure of your invention anywhere in the world before your non-provisional is filed then that publication would be "prior art" which can foreclose you being granted a patent for your invention. Or even worse --- if someone files an application claiming the same invention before you file your non-provisional then, as long as that person independantly invented whatever's claimed, then that persion could be granted a patent on the invention.
The ONLY time an inventor without long experience in filing patent applications should file his or her patent application [provisional or non] is when the inventor has NO ability to pay a patent attorney or to raise the money to pay a patent attorney.
The above response is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
It is often counterproductive to file a provisional that not drafted by experienced patent counsel. A poorly drafted provisional that an inventor files with the patent office can do substantial harm when the inventor finally retains counsel to file a non-provisional. If the disclosures in the provisional are inadequate to cover the scope of the invention, the inventor may lose the filing date for the provisional. That can have enormous adverse consequences if public disclosures of your invention have been made either before or after filing the provisional.
Most importantly, however, the main disadvantage of filing a provisional is that it can sometimes lead to loss of foreign patent rights. The main reason many inventors file provisionals is that they hope to shop their invention around to investors within the one year period before a non-provisional application must be filed. Disclosures of your invention to the public or to third parties after filing your provisional can totally destroy your ability to obtain foreign patent rights. And in today's global economy, failure to protect your foreign patent rights can be fatal to the success of your enterprise.
Thus, except in cases where achieving an additional one year of U.S. patent protection is critical (such as in the pharmaceutical industry), I advise my clients to file only non-provisionals---and only if they are prepared by experienced patent counsel. Otherwise, any patent that results often will turn out to be worthless. (I know many patent prosecutors feel that filing properly drafted provisionals is a good thing, but in my experience, it is better to suck up the cost and get your non-provisional on file---then work on getting your foreign filings made on time within a year after filing your non-provisional).
Further, it can be dangerous (and financially disastrous) to file a poorly drafted provisional, with unprofessional drawings, and claims that may not be drafted artfully. If you eventually file a non-provisional prepared properly by counsel, and if your provisional was poorly drafted, a litigator like me will use the provisional in court to attempt to prove that your patent is invalid, that you cannot claim the benefit of the filing date of the provisional, that your non-provisional added new matter not supported in the specification of the provisional, and therefore that your attempt to shop around the invention after you filed the provisional but before you filed the non-provisional should bar you from achieving patent protection. Frankly, in my experience, unless your provisional is drafted by patent counsel, you may be better off without filing it. It does buy you time, but in the real world, your patent will often get destroyed in court if your provisional is poorly drafted by a lay person. Since you need patent counsel to file your provisional, you are better off just filing a non-provisional (why pay patent counsel to do the work twice).
Q" I have not finalized the specifics of my invention so I want to file a provisional patent as a "placeholder" so I can establish first to file. "
A: That is not likely to work for you, as YOU cannot likely file a proper PPA without having those specifics. It will allow you to claim "patent pending", but is likely all you will accomplish unless you use a patent attorney.
Q:"I am aware that if the invention changes in the future the provision patent can be declared void"
A: No it can't. And it is not a PATENT it's a temporary form of patent APPLICATION.
Q:I am willing to invest the $65 in the event the invention does not change as long as filing a provisional patent wouldn't hinder a future non-provisional patent."
A: Aren't you the big spender! If YOU write it, you can pretty much count on it hindering. If a patent attorney does it, you should be able to rely on it to protect your priority.
Q:"I already have a rudimentary prototype and would write the provisional patent off that one. "
A: You keep calling it a patent. It isn't. Functionally, it is basically just a detailed priority claim.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
Short Answer: Your understanding is right, but ...
A provisional patent application provides the inventor with temporary protection for what is written in it. In other words, the protection is there but limited to what is disclosed. You have a year from the date of the original provisional filing to file additional patent applications (whether regular or provisional) that claim priority back to the original provisional. The pitfall inventors regularly face in relying on provisional patents is lack of sufficient disclosure to support the later claims in the regular patent application. You would be well advised to consult a patent attorney to make sure your rights are not prejudiced as a result. Good Luck.
You should not file a patent of ANY kind without the experienced assistance from a Patent Claims expert who knows how to differentiate your invention from the prior art upon which it rests. I have seen patent claims for a simple HANDLE that needed to be more than 60 pages. Most patent attorneys are willing to do a free consultation and will furnish a price list, as well as discuss these issues in greater detail. If you are concerned that someone else will scoop your idea, then you need to see someone fast, because doing it WRONG the first time is fatal.
This does not constitute legal advice or the engagement of my services as an attorney.