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 <[email protected]> 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:[email protected]

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 [email protected]