Need Help Identifying and Testing the Crowbar SCR unit in an SSL PSU

[Pigbenis27]

Hey there Everybody!!

So I've been working on the Power Supply Unit for an SSL 4056 G series Console. The PSU has gone done recently and the forums on this site have provided me with an incredible amount of help in fixing the PSU. I've gathered a list of possible problems within the PSU and how to identify/fix the parts that are broken or worn down. I've identified that is the +/-20 power rails on both console supply units that has gone out and it indeed resulted from blown transistors. I have replaced all 24 transistors between all 4 power rails in both PSU (6 transistors per 4 total rails) and I am currently in the process of testing the Bridge rectifiers to make sure they are solid as well. However; one piece of information that I am missing and cannot seem to find regards an electrical part other have called the Crowbar SCR unit. I have read about crowbar units and i now understand what it is they do but i have yet to find any info on identifying these parts or how to test them. I have a snaking suspicion that this is in fact just another name for the Bridge Rectifier although i'm not sure and the term has come up in enough forums that it seems important enough to warrant further investigation. If anyone can either validate my assumptions or set me on the correct path and possibly even give a detailed description of the unit and the protocol for testing it I would be greatly appreciative! Thank you guys so much in advance for your help!!!


Tony

 

[RuudNL]

A schematic diagram would probably help.
(Not everyone is familiar with the circuit you are talking about.)
Usually a crowbar circuit is a thyristor that starts conducting in an overvoltage situation, protecting the rest of the circuit and usually blowing a fuse at the same time.

 

[psych60s]

In 661G power supplies thyristors used for the +20V/-20V are usually 2N3898. Google that reference and you'll find pictures of what you're searching for. If you look at the inside of your PSU you can see your bridge rectifiers, your transistors, and then your thyristor with a diode beside it.

How to test it? You have to understand how it works. You can verify with the power supply turned off that there is no short between the anode and the cathode of the thyristor. You can see that it is a direct connection between the power rail and the 0V reference. As Ruud explained it, if there is an overvoltage situation, a signal is applied to the gate and the thyristor starts conducting, acting as a short circuit. The tricky thing is that a thyristor only needs a pulse applied to his gate to conduct, so you have to monitor the gate of the thyristor at power on, to check if there is a triggering pulse.

 

[RuudNL]

Set V trip adjusts the voltage at which transistor T9 starts conducting.
If T9 conducts, TY1 is triggered and shorts the output.

 

[David Kulka]

Typically a crowbar circuit is used to protect the load against an over voltage condition. An SCR is turned on and it shorts the output by blowing the fuse (and maybe damaging a few components too). You can temporarily wire a power resistor or even a lamp in series with the SCR to limit current so nothing very destructive happens, then trip the SCR by forcing the output over voltage, and see whether current flows through the SCR as it should.

 

[Pigbenis27]

Alright Perfect!!

Thank all you guys so much for the help that was absolutely exactly what i needed! All of it!! I think i got it figured out from this point onward and i greatly appreciate your guys' input!!! Thanks for taking the time to do that you guys' rock!!

Tony

 

[guy_4]

Hello Tony,
Do you have fuses installed in the +/- 20V rails ?
This is a modification from the original design, and I would highly suggest you to do it, if not already fitted.
Otherwise, in case of problem, some copper traces will turn to smoke... 
Best,
Guy