[GreenKeys] GreenKeys Digest, Vol 91, Issue 74

[John Nagle]

On 8/28/2011 9:01 AM, greenkeys-request at mailman.qth.net wrote:
> Message: 7 Date: Sat, 27 Aug 2011 23:43:54 -0400 From: "Ralph
> Mowery"<rmowery28146 at earthlink.net> Subject: Re: [GreenKeys] Looking
> For Bits To: "Green Keys"<greenkeys at mailman.qth.net>
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>
> As long as you get perfect print, there is no reason to go to a high
> voltage.
>
> There are two reasons I know of to go to a higher voltage.
>
> 1.  The older equipment like the Model 15 needs it to keep the oil
> and dust burnt off the keyboard contacts.

     You do need to burnish those once in a while, using a contact
burnisher.  And enough voltage and current to burn through an oil
film is needed.  This is called "wetting current".  The wetting
current for a contact varies with the material used for the
contact. For most contacts, a few mA is sufficient. This
is a big issue in automotive systems, where many cars now
have contacts that are just sense inputs to a CPU.

     Arcing at the contacts is not necessary, and will pit the
contact material.  I run Model 15 contacts at 120V, but only
about 5mA, and snubbed to eliminate arcing.

> 2.  If there is enough inductance (coils) in the circuit, the
> selector will not operate fast enough.  This is mostly important on
> radio circuits where there are lots of things to upset the timing
> already.  The 70 volts should be plenty in your case.

     With the Model 15, this is a big issue, because the selector
magnet has a 4 henry inductance. That's huge.  It takes about 120V
to get a rise time of 2ms or less and pull in the magnet fast enough.
However, the holding voltage needed is only about 3.3V, because
the selector magnet's DC resistance is only 55 ohms.
E = I*R = 60 ma * 55 ohms = 3.3V.
The usual power supply for loops is a 120VDC supply through a
2K resistor.  So 97% of the loop power goes into heating up
the resistor.

     Modern inductive coil driving is far more efficient.
Impact printers have been using better circuits for decades,
charging up a capacitor and dumping it into the coil.
I still want to build my circuit design that should be able to
power a Model 15 selector entirely from a USB port, using a
switching mode charger to get the needed 120V for 2ms.  It's
a surface mount part design, though, and is tough to assemble.

				John Nagle