Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: John Robertson <jrr@flippers.com>
Newsgroups: sci.electronics.design
Subject: Re: reset circuit
Date: Mon, 9 Jun 2025 22:28:00 -0700
Organization: A noiseless patient Spider
Lines: 81
Message-ID: <1028fp0$11e9o$1@dont-email.me>
References: <7bq84k947t84j9adpbl07lq2ujs695bsr4@4ax.com>
 <10223u2$38m9k$1@dont-email.me> <8p894k17crcqf3hug8gas091d7vgj01idu@4ax.com>
 <1026pqr$iaae$1@dont-email.me> <iard4k5lnned7p9h5jn3ou17quvches2g2@4ax.com>
Reply-To: spam@flippers.com
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Tue, 10 Jun 2025 07:28:01 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="95e90a1af0abf39288466c513d98b9a9";
	logging-data="1095992"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/2OxbeBIB7J7ykmE8GETgpWu+mrvzfYEU="
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:uMYM5Ntcd/3akWfoA213W3sbJgE=
In-Reply-To: <iard4k5lnned7p9h5jn3ou17quvches2g2@4ax.com>
Content-Language: en-US, en-CA

On 2025-06-09 7:36 a.m., john larkin wrote:
> On Mon, 9 Jun 2025 07:07:23 -0700, John Robertson <jrr@flippers.com>
> wrote:
> 
>> On 2025-06-07 1:46 p.m., john larkin wrote:
>>> On Sat, 7 Jun 2025 12:29:06 -0700, John Robertson <jrr@flippers.com>
>>> wrote:
>>>
>>>> On 2025-06-07 9:41 a.m., john larkin wrote:
>>>>> We have a box that has to never make any false outputs. Bad things
>>>>> could happen.
>>>>>
>>>>> Part of the fix is to have a solid powerup reset signal, to handle
>>>>> power brownouts or such. This looks OK:
>>>>>
>>>>> https://www.dropbox.com/scl/fi/cejyyhcrdph1bewn8a6nx/P800_Reset_1.jpg?rlkey=leky75poeerbojmd3xjat4z54&raw=1
>>>>>
>>>>> The 12 and 5v rails will have a bunch of downstream bypass caps. The
>>>>> dump resistors will discharge them.
>>>>>
>>>>
>>>> Did you allow for the voltage drop across the transistor in your Reset
>>>> Supervisor selection?
>>>>
>>>> Just asking...
>>>>
>>>> John :-#)#
>>>
>>> Sure. In normal operation it's inverted-state saturated, so the MAX
>>> part sees all the +5.
>>>
>>> If the +12 dips to about +10, the transistor base is +5, and the
>>> emitter is 4.4, and the MAX says reset.
>>>
>>> That transistor has an insane beta and a pretty hunky inverted beta.
>>>
>>
>> A saturation voltage drop of maximum 0.25V CE for the BCX70, BE
>> saturation is max. 0.85V...
>>
>> https://www.infineon.com/dgdl/bcw60_bcx70.pdf
>>
>> John :-#)#
> 
> Assume a lightly loaded emitter follower with +5 on the collector.
> Drive the base from some variable supply Vb. Ramp Vb up from zero.
> 
> Initially, the emitter will track Vb with about 0.6 volts drop.
> 
> It gets interesting when Vb exceeds +5. The emitter keeps following
> the base until the B-C junction forward biases, around +5.6. Then the
> transistor saturates in inverse mode. As you tease Vb, the C-E
> saturation voltage can be made exactly zero, or you can make the
> emitter go above +5 with a little more base drive. Backwards
> saturation.

Thanks for the explanation, I've never been that great with transistors 
- too much magic smoke! Too easily released...

I'm better with tubes, EM, and 8-bit vintage TTL for some reason.

> 
> I made a bunch of 16-bit DACS that way once, all discrete parts.
> 
> The Fairchild BCX70 has beta about 500 and inverse beta about 16, both
> measured at 2 mA. I like that part, but AoE rates it dead worst for
> Rbb at 760 ohms. Hurt my feelings.

So complain to Winfield!

John :-#)#



-- 
(Please post followups or tech inquiries to the USENET newsgroup)
                      John's Jukes Ltd.
        #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
          (604)872-5757 (Pinballs, Jukes, Video Games)
                      www.flippers.com
        "Old pinballers never die, they just flip out."