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Re: RESET circuit tweaks

Posted: Sat Nov 10, 2018 8:45 pm
by PhilC
I noticed that the 7705 is also designed to perform a reset on voltage loss scenarios (probably obvious to you all) is it worth checking the VCC on pin 8 and 7 to see if its dropping in time with the pulses?

only other thing I can see is the cap on REF is too small, should be 0.1uf not 0.01uf. so that might be worth a try I guess?

Re: RESET circuit tweaks

Posted: Sat Nov 10, 2018 8:50 pm
by Icky
I got it working. :?

Have been testing using bench PSU at 5v. Cranked it up to 5.5v and the circuit is steady.

Screen Shot 2018-11-10 at 20.46.50.png
Screen Shot 2018-11-10 at 20.46.50.png (22.01 KiB) Viewed 933 times

Re: RESET circuit tweaks

Posted: Sat Nov 10, 2018 8:53 pm
by PhilC
:D well we got there in the end then

Now you've fixed it I have to get distracted by X Factor...... *sigh*

Re: RESET circuit tweaks

Posted: Sat Nov 10, 2018 8:57 pm
by Icky
Forgottenmyname wrote:
Sat Nov 10, 2018 8:53 pm
:D well we got there in the end then

Now you've fixed it I have to get distracted by X Factor...... *sigh*
Yes - Thats the prototype board sorted. Now need to translate it to the reset board and STF as I think there are a few value changes to be done.

Enjoy Xfactor :D

Re: RESET circuit tweaks

Posted: Sat Nov 10, 2018 9:01 pm
by PhilC
Icky wrote:
Sat Nov 10, 2018 8:57 pm

Enjoy Xfactor :D
Do I have to? :geek: :roll:

Re: RESET circuit tweaks

Posted: Sat Nov 10, 2018 11:35 pm
by exxos
Just catching up here... Doesn't seem right that it needs higher voltage to work.. Maybe some resistor is to high in value and its not charging the cap up ?

Re: RESET circuit tweaks

Posted: Sat Nov 10, 2018 11:41 pm
by Icky
exxos wrote:
Sat Nov 10, 2018 11:35 pm
Just catching up here... Doesn't seem right that it needs higher voltage to work.. Maybe some resistor is to high in value and its not charging the cap up ?
On the prototype board I am using 1K as per the Mega ST schematic as the pull up resistor connected to the reset switch. Apart from the pull-up on the ~Reset (Mega ST schematic and my test board) there are no other resistors.

Re: RESET circuit tweaks

Posted: Sat Nov 10, 2018 11:59 pm
by exxos
What about 2 1k in series on the sense on pin 7 to bias it to 2.5v ?

Re: RESET circuit tweaks

Posted: Sun Nov 11, 2018 12:02 am
by exxos
Also pin 5 needs 10k pull up and pin 6 pull down according to the data sheet...

Re: RESET circuit tweaks

Posted: Sun Nov 11, 2018 12:14 am
by Icky
exxos wrote:
Sat Nov 10, 2018 11:59 pm
What about 2 1k in series on the sense on pin 7 to bias it to 2.5v ?
Gave that ago on 5V with no response. output just sat at 0V

Section 10 of the data sheet found by Forgottenmyname is interesting around having to terminate both outputs and also putting a resistor in series with the Charge Threshold capacitor http://www.ti.com/lit/ds/symlink/tl7705b.pdf