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Offline andersson  
#1 Posted : 01 January 2023 21:12:00(UTC)

Joined: 02/05/2022(UTC)
Posts: 11

In my internet searches I stumbled upon a site with info of how to build a capacitor discharge unit. It's a quite interesting site I think so here is the link: www.talkingelectronics.com

I thought it could be a good idea to build a CDU as it sends a short strong impuls to a turnout and in the same time protect the solenoid against prolongued activation/overheating. You can read up on it here: www.talkingelectronics.com/projects/CDU-2/CDU-2.html

I have figured out the schematic but one diode, so if anyone can help me explain why it's there I would be thankful.
The diode next to the LED pointing upwards connecting plus and minus, what purpose does it serve? It states on the site it will protect the transistor and capactitor from back emf. I just can't see how?
D2 and D3 are protection diodes that prevent any back emf from damaging the transistor or capacitors.


Any insight would help before I solder it together.

thanks 1 user liked this useful post by andersson
Offline EMD_GP7  
#2 Posted : 02 January 2023 15:00:01(UTC)

Joined: 23/11/2010(UTC)
Posts: 185
Location: U.K. Midlands
This looks like a "Flywheel Diode"
When a DC solenoid is turned off the collapsing magnetic field generates a voltage spike at the opposite polarity of the driving voltage.
The diode in this position will short-circuit this spike and protect the driving electronics.
In my working life with electro-pneumatics it was usual to fit a diode in the coil circuit of DC coils for this reason.
A traditional automobile ignition coil works on the same principle of a collapsing magnetic force generating the spark voltage when the contact breakers are opened - amplified by the secondary coil - in this case the high voltage is wanted.
Hope this helps. Colin.
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Offline andersson  
#3 Posted : 02 January 2023 20:48:14(UTC)

Joined: 02/05/2022(UTC)
Posts: 11
Thank you for a good explanation! I must admit I do not fully understand (grok) but I understand enough to trust it now. Still things to learn, always things to learn..

Edit: after sleeping on it I do understand it now. All I had to realize was the differences of the systems states, charged capacitors vs looped with solenoid.

Edited by user 03 January 2023 11:21:46(UTC)  | Reason: Not specified

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