Zener & SCR shunt regulator help

[gman4dx266]

Hey guys. Hoping someone may be able to help. A friend of mine who dabbles in electronics helped me build this voltage regulator which rectifies 3 phase AC (from a gas engines stator coil) and then regulates it to ~13 or 14 volts. The device works when the engine is idling but as the AC output rises, the DC output falls. We had someone recommend the 300 ohm gate resistors as well as the 300 ohm "burn off" Zener resistor, but I'm not sure if the problem lies in the resistor value, or perhaps we are pushing reverse current through the diodes? (Not sure of the specs on the bridges). Thanks for any insight.

Also, I believe there was a Darlington array IC in place of the TIP 120 Darlington in this circuit, however I was advised it was simpler and more suited to use just one single unit.

[pebe]

Hi. I cannot identify which are the collector and base of the TIP120.

[gman4dx266]

Hi. I cannot identify which are the collector and base of the TIP120.

Pebe, the top most lead is the base, left would be collector. I have had several recommendations as far as upgrades, one was to change the TIP 120 for a TIP42. Another stated my capacitor may be too small, however I'm already using a 1000pf capacitor. The same personal also suggested that the gate resistor may be 50% too small. Hopefully any of those help you. Thanks for the response!

[pebe]

The emitter is connected to the gates of the triacs so it is more positive than the collector, So the circuit needs a PNP transistor, not the TIP120 which is an NPN. Even then, it would not turn off the triacs with that circuit.

I'll draw up a suitable circuit for you ASAP.

[gman4dx266]

Excellent. Thank you for the help Pebe. My one friend mentioned the same thing about the wrong transistor, which is why he recommended the TIP42 which is the PNP style. Im awaiting your reply for the updated format. Thank you

[pebe]

A couple more things. Can you tell me the type of zener fitted (its voltage), and the AC voltage when the engine is running at normal speed?

[gman4dx266]

I am currently using a 14v Zener. Idle input is 3 phase AC approximately 24v. It rises up to approximately 120-140 VAC over the engine speed rise.

[pebe]

Is the circuit arrangement around the diodes rectifiers and the triacs an original proven design, or something you have drawn up yourself? Because I am having trouble seeing how it works.

[gman4dx266]

Yes, Pebe, it is basically the same scheme used by a commercially available motorcycle or ATV voltage regulator/ rectifier. The SCR gates are controlled by a transistor Zener pair. The project works part ways but it seems like it is unable to latch at one output and maintain it. If you have found a successful way to maintain proper cycling of the triacs I would be very eager to see your method. Thanks

Edit: here is a commercially produced model of the same thing (albeit with burnt out diode bridges) which I've taken apart.

[pebe]

3PH Regulator.GIF (23.16 KiB) Viewed 31560 times

I sorted to a way to replace the TIP120 and was going to post it. However, I found two problems with the rest of the circuit so I am attaching your circuit with the problems outlined.

The two transistors (any small NPNs) work like this. With the V+ line below the zener voltage, TR1 is cut off so TR2 is on, pulling the 330 resistors low and energizing the gates of the triacs. When the zener starts to conduct, TR1 turns on and TR2 turns off; the triac gates then have no drive.

All Ok so far, but then I looked at the rest of the circuit and spotted the two faults.

1. Any time when voltage on phase1 is more positive than phase2, the two bridge rectifiers will conduct as shown by the red path, taking V+ to over 200V. The zener and triacs will not control that in any way, so it would be disastrous for a battery connected there.Similarly for phases 2-3 and 3-1.

2. When TR2 is on, all three triacs have their gates enabled, so as soon as there is a difference in their anode and cathode voltages (either polarity) they will conduct. Looking at the circuit, as soon as there is a difference between phase1 and phase2, T2 and T3 conduct putting a short circuit between those two phases as shown by the blue path. The same with T1.

Once triggered, a triac will remain on until there is no voltage between anode and cathode, so the only way to control it would be to switch it on just before the zero crossover point, then it would switch off at zero crossover, ie. pulse width modulation. But that would be a whole new ball game.

I have Googled for ‘3phase battery charger’ but all I have found have been large commercial chargers. So I don’t think I can help you.

Perhaps another member may be able to help.

[gman4dx266]

Pebe, thank you so much for the help you've provided! The 1st fault I see what you're talking about with the voltage surge between any given phase. I am not sure if that will be a real world issue or not as all 3 should be equal. For fault two where you mention the short circuiting, that is actually how the devices work (a reason stators have insulation breakdown and failure so often). I will try to build your design and see how it works out. I see you have downsized the capacitance value; how was that determined and what overall change does that make? Is it smoothing the input to the transistor control portion? Thanks again

[pebe]

I assumed a steady DC voltage was required at TR1 base, but in view of what you said it may be better to stick to the original 300 ohms and 1000pF. That would allow the gate drive to be removed quickly.

You will also need to wire the rectifiers in series with the feeds from phase to the triac. That would eliminate the short circuit between phases. Better still if you used a single diode for each triac, rather than a bridge type.

With the diodes disconnected from the + and - rails, you will need a neutral to connect the 0V to. Can I assume that your supply is 3phase plus neutral?

[gman4dx266]

Thanks for the response. I do not have a neutral line only 3 "hot" coming in. Can you illustrate what you're referring to as far as a single diode between phase and triac? Thanks again

[pebe]

I can now see how you can wire it up using bridge rectifiers. It's not a lot different from your original circuit.

I'll draw it out ASAP.

[pebe]

Here is the amended circuit. I have coloured in the current path when phase A is more positive than phase C. Red is current out through one pair of diodes in the bridge, through the triac to battery+. The blue is the return path from the battery- through the other pair of diodes.

The ideal float charge voltage for a 12v battery is about 13.8V. Allowing for the 0.7V drop across R1, you need a zener of 13.1V. That’s not a standard value but you can make it up by using a 7.5V and a 5.6V in series.

Note that is no current limiter (also none in your original circuit). It may be that the 3phase supply will limit the maximum current because it may have a fairly high source resistance. But if not, you can insert a small resistor in series with the battery+ lead. A value of 0.5ohms would limit the current to 6 to 8amps when charging a dead flat battery.

I hope that helps.