Why is there a resistor across those bridge rectifiers ?

[tigrou]

I came across this schematic (power supply of a old Pinball). This is the secondary side of the main transformer :


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)

[Whales]

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. 

 

[Floopy]

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? 

[DavidAlfa]

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.

[tigrou]

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 (page 49). That file is available publicly that is why I included it here.

[Doctorandus_P]

Why are there red boxes on your schematic?
They obfuscate both the RefDes and the values of those resistors. 

Also, VR1 is not a resistor.

[tigrou]

Why are there red boxes on your schematic?
They obfuscate both the RefDes and the values of those resistors. 

Also, VR1 is not a resistor.

Sorry about that. I have uploaded a new picture.

[Veteran68]

Also, VR1 is not a resistor.

I was waiting for someone to call that out. I was about to.