[GreenKeys] Loop supply voltage

[Mike Douglas]

As others have already replied, a high voltage is needed to quickly build coil current from zero up to 60ma so that the selector armature is fully seated well before the bit is sampled. As you noted, idle current is established with I=V/R, so 0.060a = 100v/(1467+200 ohms). So for a 100v loop voltage, you’d need an adjustable resistance centered around 1467 ohms.

The downside to this is that the 1467 ohm resistance is dissipating over 5 watts of heat. This a very large and hot resistor in terms of modern electronics. And of course, a 100vdc power supply to bite you is also not something a modern day hobbyist typically deals with.

An alternative is to establish the steady state current with a low voltage supply and apply a high voltage only while transitioning from 0ma to 60ma. Once 60ma is reached, the low voltage is fine. This can be done in a variety of ways, but one simple technique is to use a capacitor to capture the energy from the collapse of the inductor’s magnetic field during the on to off transition (60ma to 0ma), then dump this capacitor back into the inductor during the 0ma to 60ma transition. You can see an demo/explanation of an interface that uses this technique starting at 7:45 in this video https://youtu.be/fLkBb9MuTjE?si=9Z_ysve2yUf9-9-d

Mike D

> On Jan 18, 2025, at 9:12 PM, steve bennett via GreenKeys <greenkeys at mailman.qth.net> wrote:
> 
> 
> GKs,
> 
>    Yet another M15 question.
> Please check my math. I'll be really embarrassed if I screwed up ohms law.
> My selector magnets are in series and reading a total of about 200 ohms.
> This means to get 60mA I need to apply 12V?
> 
> I thought loop supply voltage was way higher than that, like up near 100v
> Am I confused? Wouldn't be the first time.
> 
> -Steve
> 
> ______________________________________________________________
> GreenKeys mailing list
> Home: http://mailman.qth.net/mailman/listinfo/greenkeys
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:GreenKeys at mailman.qth.net
> 
>>>> Jordan Spencer Cunningham's GreenKeys Search Tool: https://teletype.net/gksearch
>>>> 2002-to-present greenkeys archive: http://mailman.qth.net/pipermail/greenkeys/
>>>> 1998-to-2001 greenkeys archive: http://mailman.qth.net/archive/greenkeys/greenkeys.html
>>>> Randy Guttery's 2001-to-2009 GreenKeys Search Tool: http://comcents.com/tty/greenkeyssearch.html
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to deramp5113 at yahoo.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/greenkeys/attachments/20250119/8ad89807/attachment.html>