EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: tigrou on November 06, 2023, 09:32:52 pm
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I came across this schematic (power supply of a old Pinball). This is the secondary side of the main transformer :
(https://i.imgur.com/WeVXxmil.png) (https://i.imgur.com/WeVXxmi.png)
What is not clear to me :
- Why do we have a resistor across some of bridge rectifiers ? Is it to protect them ? I would have say this to discharge some caps but that part is used to power the left and right flippers coils directly.
- Additonally one of there is (AFAIK) a variable resistor. Is it used for some kind of tuning (since it's not self regulated, unlike a SMPS)
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Best I can think of is that maybe the diodes were getting fried by inductive kickback from the flipper coils? Perhaps the few K resistance is much better than the many megaohms a backwards diode looks like. But there are better ways of doing this. No the diodes in their existing config will handle anything that arcs through the switch/contacts.
:-//
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My guess would be to provide a very crude way of regulating voltage. The variable resistor can adjust your peak output. Someone is bound to correct me though, just taking a guess. Is it just a coil connected to it? Or is there some switching occurring in between. Is it an analogue switch? Relay? Transistor?
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It's just a minimal load to keep the output at a safe level.
Transformers can reach way higher voltages than nominal in open circuit, a 12V secondary could easily output 18-24v.
The varistor provides further protection, as it's resistance decreases with voltage, thus increasing the load as the voltage rises.
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My guess would be to provide a very crude way of regulating voltage. The variable resistor can adjust your peak output. Someone is bound to correct me though, just taking a guess. Is it just a coil connected to it? Or is there some switching occurring in between. Is it an analogue switch? Relay? Transistor?
Here is the link to the full schematic (https://www.ipdb.org/files/762/Bally_1981_Eight_Ball_Deluxe_English_Manual_Model_1220_includes_paginated_schematics.pdf) (page 49). That file is available publicly that is why I included it here.
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Why are there red boxes on your schematic?
They obfuscate both the RefDes and the values of those resistors. :palm:
Also, VR1 is not a resistor.
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Why are there red boxes on your schematic?
They obfuscate both the RefDes and the values of those resistors. :palm:
Also, VR1 is not a resistor.
Sorry about that. I have uploaded a new picture.
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Also, VR1 is not a resistor.
I was waiting for someone to call that out. I was about to. :)