Training board DC circuit protection suggestion

[eagle1109]

Hi,

I've been working on repairing this power electronics training board for several months, I changed series resistors, voltage regulators, H-bridge MOSFETs, the power transistor and Op-amps.

So apparently, the faults are happening within the DC circuits, there are AC circuits, which are the 1-ph and 3-ph but they are protected with a fuse for each one. And the fuses work fine, there haven't been any problems with the AC lines, the fuse get broken every time and we change it and problem solved, but the real problem in the DC circuits.

And I just realized that this afternoon and I thought why can't I include a protection component, and save myself the time of repairing the boards every time ?

This is the board:

So, for 1-ph -> F1 and 3-ph -> F2. But the DC power supply has no protection. Most the problems comes from +10/-10V.

This is the schematic for the power supply part:

So, the problems happen with the +10/-10V and +15/-15V.

And this is what I have to deal with !

The burns eat the PCB, so I might be able to fix it now, but next time the PCB would be harder.

So I thought of finding a way to protect the DC main lines. What you think of ways to do ?

I thought of including a circuit with a RED LED and a buzzer, so when the trainee do a mistake and short something, the LED lights up and the buzzer rings so one can undo this particular experiment wiring.

 

[eagle1109]

The design principle is good but you can make it more versatile by using the other parts of the LM339 to monitor the other supply rails at the same time.

I will but first I want to be sure that the circuit for one stage functions OK, then I will copy and carry on for the rest of rails.

If you use a potential divider BETWEEN the supply rails rather than to ground you can make the comparator sense out of range voltages on all the rails while still keeping the measurement point above ground potential. Just make sure the resistor values are chosen so the negative supplies still produce a positive voltage at the resistors junction. Also remember that you can swap the + and - inputs of the comparator to reverse it's output state for a given input.

I think this could be another design route. Let me now proceed with using each comparator of the LM339. Or if you can point me with a design from the web for example.

I would be careful with BUZ1, if you put it in the transistor emitter it will have to pass the relay coil current and it will also increase the voltage needed to make the transistor conduct. It would be better placed across the relay coil with the emitter grounded.

Brian.

I tried and moved the buz to the collector side and it's not working. It's ok I can try to get it to work eventually. But my issue now is the circuit I'm working on is to a point ok and works well, but the issue is that the flip flop is set already when I launch the simulation, as there is nothing triggered the flip flop, and that causes it to activate the transistor and then the relay as there is no short at the load section. This a snap of the circuit:

 

[betwixt]

The flip flop has nothing to set its start condition. The solution is simple, add a capacitor across the reset switch so it sees it a being closed until it charges up.

I'm not sure what U8 and U7 do, the output of U7 will be a replica of Q on U6A. You can drive R8 directly from Q and still connect an LED plus series resistor to Q and /Q if you want to show on/off state.

Brian.

 

[eagle1109]

The flip flop has nothing to set its start condition. The solution is simple, add a capacitor across the reset switch so it sees it a being closed until it charges up.

I added a capacitor and a pull-down resistor, but the same issue. I took 2 snapshots for moment 0.0 seconds and 0.05 seconds in the simulation, I think it's a simulation problem, the main circuit didn't even received any voltages, of course that courses the voltage at the + input of the op-amp to be 0V and hence; the op-amp output is LOW.

Next moment, the voltages are ready:

So at first glance, the op-amp isn't activated,

I'm not sure what U8 and U7 do, the output of U7 will be a replica of Q on U6A. You can drive R8 directly from Q and still connect an LED plus series resistor to Q and /Q if you want to show on/off state.

Brian.

I arranged them to suit the logic of the circuit, I know I might get rid of them, but when I reset the flip-flop the circuit work perfectly.

 

[eagle1109]

Hi,

Can I use a polyfuse on each line of the DC block ?

Like these:

Are they suitable for the training board ?

Also, if I can use them, how to choose the specification to fit each DC line ?

 

[eagle1109]

The flip flop has nothing to set its start condition. The solution is simple, add a capacitor across the reset switch so it sees it a being closed until it charges up.

I'm not sure what U8 and U7 do, the output of U7 will be a replica of Q on U6A. You can drive R8 directly from Q and still connect an LED plus series resistor to Q and /Q if you want to show on/off state.

Brian.

Hi,

How about using polyfuse instead of this circuit ?

Considering the main DC schematic in #1, I'm thinking of inserting a polyfuse + a diode in series of each DC line. What you think ? would that work ?