Simple voltage limiting circuit

[nivaOne]

Hi,

I'm trying to make a voltage limiter for my kid's motorcycle. The engine has been replaced. It came with a different alternator. And the voltages are way too high now.

The rectified output varies between 15 and 26v DC. I'm trying to design a circuit which provides an output of max. 14,4 V for charging the lead acid battery.

The idea is to dissipate all the excess power over a transistor ( + heatsink ) while limiting the output voltage to 14,4V using a 15V zener diode to set the transistor correctly.


Please feel free to check the drawing and provide me with comments/suggestions.
Optional; would a resistor for limiting the current be required? (Rc)


Many thanks,

NivaOne

 

[Fish4Fun]

niviaOne,

I don't know anything about motorcycle specific alternators, but alternators in general use a voltage regulator that controls the current through the field windings. By controlling the current through the field windings, the output voltage in the stator windings is regulated. This 'sensing" is typically done post rectification and requires some minimum load, or a battery to ensure stable operation.

In general, the upper limit of the current an alternator outputs is a function of the resistance in the stator coils, while the "open circuit voltage" is a function of the field strength produced by the field coils & the RPM. Most alternators retain some residual magnetism in the field cores causing the alternator to "self excite" enough to produce voltage/current in the stator windings without an external power source; typically enough to begin the feedback process to the field windings so that the alternator will ramp up fairly quickly to full power.

It seems plausible to me that you have a "bad regulator", or a regulator that is not connected properly, or not suited for your alternator. BUT, like I said, I know NOTHING about motorcycle specific alternators. Before I built a shunting circuit, I would consider carrying the bike to a service technician.

Fish

 

[Fish4Fun]

Double post removed.

 

[nivaOne]

Hi Fish

Many thanks for your advice. The motorcycle dates from 1984 as well as the spare engine that came from a different brand. Both are not on the market anymore.
I have visited multiple service points, without success however.

The load regulator present today just can't deal with that 'higher' output voltage coming from the alternator. I can't modify it as it resides within a resin filled box.
So my only alternative is to build one myself.
( I assume that the present simple regulator, consists solely of a simple rectifier and a capacitor ).

So my question remains, would my circuit do the trick?

 

[Fish4Fun]

I think you may have calculated your wattages incorrectly.

26V - 5V = 21V ==> 21V = 330ohms * I ==> I = .0636A

P = I^2 * R ==> .0636^2 * 330 = 1.335W (resistor wattage)
P = IE ==> 21V * .0636 = 1.335W (Zener wattage)That is, the current flows through the Resistor AND the Zener and for all intents and purposes, both devices combine to drop the 21V. While this changes the device's power ratings, it does not affect your voltage regulator's fundamental design.

Next, the BD135 has a maximum collector current of 1.5A, a rating I find woefully low for Lead-Acid battery charging. Additionally the BD135 has a total device power dissipation of only Ta = 1.25W and Tc = 12.5W, again, ratings I find woefully low for Lead-Acid battery charging (<1A).

Transistors really are not my specialty, but it would appear to me that you have a constant emitter-base voltage of 5V, and with no current limiting shown, should place the BD135 in full conduction all the time (effectively turning it into a low value, heat dependent resistor).

So, no, I don't think your circuit will do the trick.

Now, back to solving your problem. Is it possible your new motor/alternator is designed to operate with a headlamp in series with the battery? Assuming a 100W bulb at a nominal 12V we would find:

100W/12V = 8.33A

12V = 8.33 * R = 1.44ohms

If we assume the resistance of the bulb is 1.44ohms, and the nominal voltage of the battery is 12V then:

12V + 12V = 24 ==> 26V -24V = 2V

The 2V will be "clamped" by some combination of the Lamp and the Battery with a current limit supplied by the 1.44ohm headlamp as follows:

26V = 1.44 I = 18.05A

This would mean that your headlamp would act as a resistive current/voltage regulator for your charging system. I would suggest you give this approach a try prior to ordering any components for your circuit.

Fish

 

[vk6kro]

Hi,

I'm trying to make a voltage limiter for my kid's motorcycle. The engine has been replaced. It came with a different alternator. And the voltages are way too high now.

The rectified output varies between 15 and 26v DC. I'm trying to design a circuit which provides an output of max. 14,4 V for charging the lead acid battery.

The idea is to dissipate all the excess power over a transistor ( + heatsink ) while limiting the output voltage to 14,4V using a 15V zener diode to set the transistor correctly.Please feel free to check the drawing and provide me with comments/suggestions.
Optional; would a resistor for limiting the current be required? (Rc)

Many thanks,

NivaOne

That circuit would give 14.4 volts as you describe, but you really need a current limiter to initially charge a battery and then a voltage limiter to cut off the charging when the battery is fully charged. That circuit will try to give 14.4 volts out, even into a flat battery. This may destroy the transistor or blow a fuse.

If you go to Google and click on "images" at the top, then search for "car battery charger", you will see just how complex a battery charger has to be to work properly.

This is not a trivial project and motor bike batteries are expensive, so you really need to take the bike to an auto electrician to get it properly diagnosed and repaired.

 

[Adjuster]

Hello nivaOne,

Yes, this is not a trivial project - and you really need to consider safety issues here: failure of a regulator could cause serious problems such as fire, the battery bursting or leaking, or engine failure. Such outcomes could lead to accidents, possibly injuring your offspring or others, as well as damaging property.

Can you be sure that any changes made to the bike will comply with the relevant laws or regulations, and with the requirements of any insurance policies which may be applicable?

I believe that repair work of this kind is best left to the experts.

 

[Bob S]

Two things:

1) The difference between an alternator and a dynamo is that the dynamo has a permanent magnet rotor, and the voltage output is proportional to the RPM, while an alternator has a wound-coil rotor which can be used to regulate the output voltage.

2) This 3-transistor adjustable-voltage regulator has built-in current limiting:

http://www.radio-electronics.com/in...urrent_limiter/transistor_current_limiter.php

Bob S

 

[nivaOne]

Thank you Bob. It definitely is a dynamo. The higher the RPM, the higher the voltage output. There is no feedback path, hence no possibility to control it otherwise then via extra hardware behind the output.

 

[nivaOne]

Thank you, Fish, v6kro, Adjuster.

I have learned a lot from your remarks.

A few extra remarks:
This project is indeed not a trivial project. The cycle is actually a moped which we consider a very lightweight motor cycle (sorry for the confusion, English is not my mother language )

We have been looking for professional help, but if there is no standard solution available then there is also no real willingness to help us out. So this road leads to nowhere.

I have learned in the mean time (reading between the lines), that charging a lead acid battery is not something which should be done with a constant output voltage. Allow me to study the behavior (internal resistance) first now and I will return with an adapted design.

This will include: fuses ( safety first), a different but more ideal output voltage which does not hurt the battery but still is able to feed the lights.

This voltage should be lower than the classical initial voltage which one typically sees when an empty battery is charged (14,4 V) but higher than the final trickle charge voltage (13,6 V).

The battery is solely present in the moped to provide light, etc.. while idling, driving at low speed,... It does not require a battery actually to function otherwise. Consider it a 'buffer' battery, which I will try to charge slightly will driving.

I hope that this might do the trick without 'killing' it due to overcharging it constantly.

I hope to be back soon here.

Thanks all again

NivaOne.

 

[nivaOne]

Hi Fish,

Can you please have a look at your calculations again? I'm wondering where the 5V came from, I assume that you should have used 15V instead (26-15 = 11V)

regards

NivaOne

 

[Fish4Fun]

I assume you are talking about my second post and this equation:

26V - 5V = 21V ==> 21V = 330ohms * I ==> I = .0636A

The 5V is from your Zener. Your resistor drops all of the voltage except the 5V your zener drops. So, @ 26V, 21V is dropped across the resistor and 5V is dropped across your zener. (I mis-calculated the zener wattage in my previous post).

Anyway, try this link:

http://www.treatland.tv/universal-moped-voltage-regulator-p/moped-voltage-regulator.htm

I am NOT endorsing the product, just seemed like it might solve your problem.

Fish

 

[vk6kro]

The idea is to dissipate all the excess power over a transistor ( + heatsink ) while limiting the output voltage to 14,4V using a 15V zener diode to set the transistor correctly.


The Zener is 15 volts. This is mentioned in the text and also on the diagram, although in the diagram the 1 is more like an up-arrow symbol.

 

[Fish4Fun]

The Zener is 15 volts. This is mentioned in the text and also on the diagram, although in the diagram the 1 is more like an up-arrow symbol.

WOW, ok, I really did miss the zener being 15V. I assumed the ^ was just a spurious mark. MY BAD.

My apologies.

Fish