[GreenKeys] Newbie BARTG ST5 Power Supply Question?

[gil smith]

Hi Brooke:

At 12:32 PM 5/24/2007, Brooke Clarke wrote:
>Can someone tell me the purpose of D12 and D13 in the ST5 SU?
>http://www.baudot.net/creed/docs-alan-hobbs/rtty-the-easy-way.pdf
>I understand that the 3k3 Ohm resistors that they shunt set the current 
>and they are back biased when the related transistor is open.
>I expect that they are for shunting the back EMF spike from turning off 
>the selector magnet and back on in the opposite polarity.  That would seem 
>to be a problem greatly slowing down the turn off time.

The diodes clamp positive-going voltage spikes (L*di/dt) resulting when the 
coil is turned off.  The diodes are simply clamped to the positive loop 
supply voltage (across C15).  It does not slow anything down.  I do it the 
same way in the TTY-Connect circuit.

When you do not have the HV supply in the same enclosure (ie: just a 
separate loop keying circuit), you do not have the luxury of this simple 
spike clamp, which is why you see RC snubbers and other spike-limiters in 
other circuits.  The diode to HV is very clean and accurate when the loop 
supply is available.

gil



>--
>Have Fun,
>
>Brooke Clarke
>http://www.PRC68.com
>http://www.precisionclock.com
>
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