Voltage Regulator for a Small Motorbike, 6V AC Current

[alec_t]

Later, did a test with a bulb in series, with 13v DC 500mA transformer. In that test the circuit did function, reducing 13v to 11v.

Repeat that test, but this time measure the voltage between Reg and Gnd, the voltage across the 1k fixed resistor (R4), and the voltage across the bulb. Post the results.

Edit: Our posts crossed.
Looks like you've got the D and S transposed.
To confirm:
Connect the two D terminals to the Reg and Gnd lines respectively.
Connect the two S terminals to R4.

If this is on a breadboard, be aware the FETs could get very hot.

 

[sign216]

Okay, I'll make those corrections, and report back later on today.

 

[sign216]

Here's a revised version. Is it correct?

Note that the Source and the Gate terminals are both connected to R4, just at different ends.

 

[alec_t]

No. Like this :-

 

[sign216]

The image is very helpful. Thank you.

The 330n capacitor is new. So I should add that to the circuit?

 

[alec_t]

The 330n capacitor is new. So I should add that to the circuit?

Only if you intend using the Bulb By-pass circuit as well. The cap supresses voltage spikes arising from the by-pass SCR switching.

 

[sign216]

Success!!!
Voltage measured across + and - poles of the regulator is now regulated!

Trimpot (R2) dialed to 0 ohms gives 3.4 volt
R2 at 1,400 ohms gives 6.5 volt
R2 at 2,000 ohms gives 7.9 volt

What value is best for R2 in the final version?
Tested with a 12v DC 500mA input.
Any guesses what the bike produces at max throttle? 24v? Sometimes the bulb blows pretty dramatically.

 

[sign216]

Another data point:
9v battery as input
R2 at 0 ohms = 7.0 volt
R2 at 2k ohms = 8.2 volt

 

[alec_t]

That's more like it! Good news.

Trimpot (R2) dialed to 0 ohms gives 3.4 volt

That suggests the FET Vgs turn-on threshold is ~2-3V. So, within the datasheet range.

Any guesses what the bike produces at max throttle?

Couldn't say. I expect there are plenty of bikers who could give you an answer. Or you could stick an AC voltmeter on a bike and measure it .

What value is best for R2 in the final version?

Perhaps ~1k5? Whatever gives an RMS voltage close to 6V.

 

[sign216]

I'm travelling but when I return I'll try a 24v battery and report the results.
that should be close to the value at max throttle .
I've tried measuring max voltage with no success . sadly , the moped forums are little help on electrical issues .

which is why I'm here!

 

[alec_t]

I'll try a 24v battery and report the results.

Don't do that! I predict the result will be a blown bulb (you should have that in series with the regulator) and/or a red-hot FET. Don't forget the battery is DC, whereas the magneto is AC and would be operating at (relatively) high frequency at high revs/voltage. The coil's own reactance would have a greater current-limiting effect at high revs. Moreover, with AC the two FETs share the power dissipation; with DC one FET dissipates all the power.

 

[sign216]

how about I use a resistor instead of a bulb (what ohm? ) and connect the 24v momentarily to get a reading ?
remember the bike's going to be ridden at high throttle often .

 

[alec_t]

With AC, the coil's reactance limits the voltage that the brake bulb experiences. If the magneto generated 24V AC (open circuit) I doubt the brake bulb would ever see even half that, or it would have blown before the bike got out of the factory!
Since the reg is now operating at ~6-8VDC when tested with a 12VDC source and a test bulb to limit current, it would do the same with a 24V source and a suitable resistor in place of the test bulb. I don't see that giving you any further useful measurement data. However, if you're determined to experiment, let's choose a test current of 0.2A for argument's sake. With the reg set to 8VDC (roughly the peak of 6VAC RMS) the voltage drop across the resistor would have to be 24-8=16V. For 0.2A current that would mean a resistor of value 16/0.2=80 Ohms (82 Ohms is the nearest standard value). The resistor would dissipate 16x0.2=3.2 Watts, so a 5W type would be advisable for a prolonged test. For a very brief test a 1W type would do.

 

[sign216]

I apologize for the delay, work sent my to Ferguson, Mo (USA) for a few weeks.

I corrected a loose connection and intersecting wires in my breadboard mock-up, and did the tests again. This time with a 100 ohm resistor instead of the lamp. Two sets of tests, with a 9v "transistor" battery and a 20v battery from a portable tool. All of the tests produced uniform results; 6.1 to 6.7v. This despite varying the trimpot (0-2k) put in for the resistor (R3 or R2, depending on the diagram).

So the circuit is working better than ever! Although it's odd that varying the trimpot from 0 to 2k ohms didn't effect the outcome much.
But the outcome is correct, either way.

 

[alec_t]

it's odd that varying the trimpot from 0 to 2k ohms didn't effect the outcome much

Odd indeed. Have you verified the trimpot pinout/connections, to make sure the resistance is actually varying and you're not simply using just the two ends of the resistance track?

 

[KeepItSimpleStupid]

You beat me to it. You actually want the wiper and an end, but the terminal arrangement depends on the variable resistor. Usually, you tie the wiper and one of the ends together when using it as a two terminal resistor to increase reliability.

Make sure the two terminals of the potentiometer your using are varying.

 

[sign216]

Fixed it. When I tightened the connections, a diode was reconnected incorrectly.

New figures:

9.4v battery R3/R2 0 ohm = 3.7v and 2k ohm = 9.1v

18v battery R3/R2 0 ohm = 3.8v and 2k ohm = 10v


Looks like approx. 1k ohm is correct for R3/R2. I'll get a fixed resistor and try that out.

 

[alec_t]

That's more like it .
What sort of heat-sinks do you have for the FETs?

 

[sign216]

I was going to mount them in a heavy die-cast container, and use the container walls as the sink.

I'm thinking of going 1,500 to 1,800 ohm for the resistor, to keep the volts up and lights on strong.

 

[KeepItSimpleStupid]

You can get a di-cast box with a gasket. Now, remember that only one of the FET's tabs can contact the case. The one connected to ground.

Let's hope you purchased the mounting kit that includes silpads, so you don't have to use the silicon grease. the kit also includes bellville washers which keep a constant tension on the tabs.

I'd probably tap holes and use Loctite 222 or the one that doesn't require heat for removal and make a stud stick out on the inside. Then cut the screw flush.
Attach using the silpad, Bellville washer and nut on the inside. You'll keep the water integrity this way.