24 to 12 vdc converter needed

[bob g]

to get exactly one have the voltage off a 24vdc nominal buss

and scale it as exactly as possible back to one half or 12volts nominal

most of what i see commercially take 24vdc input and output a regulated 13.6 vdc

output

what i need will track and have an unregulated output, so if i have 24.4 input

i get 12.2 output, 26volts input i get 13 output etc.

i need a max of 8amps output capacity.

what i am trying to do is use the much more common balmar mc612 with a prestolite

110-555 alternator (both of which are 12volt nominal) and use them together to charge a 24volt bank directly.

i got it to work by centertapping the 24volt battery bank, but the loading on have the bank to cover the balmar and excitation current of the alternator causes issues with getting a balanced charge across both halves of the bank.

i suppose i could use a dpdt relay to alternate between halves of the battery bank so that each time i charge i borrow the overhead power from alternating halves and over time maintain a balanced charge.

it would be nice to just step down what power i need from the full string and sample at centertap (sample needs are far less than a half amp)but to do that i need

a converter that scales 50% of the total voltage,, i could even deal with some other split "if" it stays at that % over the full operating range, such as 55% or whatever.

i just need it to always be the same percentage of the full voltage every time

or very close to the same.

with all the electronic gurus out there i figure someone might have a simple solution, if it were AC to AC i would just use a transformer.

any idea's?

bob g

[terry5732]

I have used a 555 to drive a MOSFET for partial duty. It generates an on/off pulsed DC output. After running through a series of smoothing caps it reads like normal DC at a reduced voltage.

[Opera House]

Any switching regulator chip could do that like a TL494 in a buck converter circuit.   These normally reference the voltage to an onboard 5V precision reference.  You could instead reference that  with the input voltage.  Use a resistive voltage divider to set this voltage to approximately 3V.

[oztules]

Is there any reason not to modify the voltage regulators in the alternators, and run them at 24v (28v) directly, or perhaps throw the regulators away altogether and use a variable resistor to control output voltage/current...

Seems the only sensible way forward.

........oztules

[bob g]

the balmar mc612 is an advanced microcontroller, programmable externally mounted regulator,, further it is potted in epoxy of some sort so alteration is out of the question.

i could do with a reostat if i wanted to babysit charging, but this is being integrated into a fully automated trigenerator, so that it will autostart if any of several parameters are met, and shutdown on a number of others.

so i need to keep the controller in place and unmodified if possible.

i have a couple of other options

1. the stator has AC outputs the voltage of which will track with the DC output voltage, so i could seperately rectify that after using a stepdown 3phase transformer (2:1 step down), or
   
2. use a voltage divider to devide the 24 volts, but i need a max of 8amps
   
   

so the resister would dissipate ~80watts,, (i could tolerate that) not very elegant

and certainly wasteful of energy.

at this point i may give option 1 above a shot, i think i have a suitable 3phase transformer/rectifier pack in my collection of goodies that is rated at 10amps output.

bob g

[Opera House]

It's 3am and I am not exactly reading for comprehension.  Just noticed you have two 12V alternators.  Why don't you dedicate one for each battery.  Insulating an alternators mounting is tricky and there are live frame issues, but that would better maintain each 12V battery.

[bob g]

thanks for the thought, i have considered that and i can insulate both easily because the positive and negative posts of each alternator are insulated from ground/frame.

the holdback is i have other work for the 12volt alternator, chargeing the starting battery, running 12volt ancillary stuff like water pumps, fans and electronics.

having slept on this issue i think i have found why the system wont work fully

when running the 12volt side gets up to 14.4 (with the oem regulator) so when it feeds power to the controller the controller see's the 14.4 and says "whoa" and shuts down part of the controller,, even though the sense line is below the set points.

would be easy enough to test the theory, all i need to do is disconnect the 12volt alternator for a bit from the 12 volt battery so the voltage will drop below the setpoint of the controller and then all should fall back into operation.

if that turns out to be the case all i would need to do is insert a diode between the 12volt battery and the power wire of the controller,, the diode voltage drop would be just enough to get under the setpoint and make the controller fully functional,,, might work.

thanks guys, for helping me think this through

bob g

[ghurd]

I do not really get the whole picture.

You have too much voltage, and need to get rid of some.

Need to regulate at 28.8V, with a signal of 14.4V.  That is half.  I don't see 'half' as being the important number.

It means you want rid of 14.4V.  That is the only important number.

Why not a Zener - Transistor circuit to be rid of the 14.4V?

Whatever voltage is left goes to the regulator.

If the batteries are at 24.4V, the regulator sees 10.0V.

If the batteries are at 28.8V, the regulator sees 14.4V.

Seems simple.

G-

[bob g]

half is the important number

if the regulator see's 10 volts instead of 12.6  (which would be half of 25.2)

it would set the charge rate at a much more aggressive rate than needed.

like i said, i don't need exactily have, but i do need a consistant % across the range, whether it is 50%, 55% or somesuch, but it has to scale in a linear fashion

and stay at the same percentage of the total voltage across the string.

bob g

[ghurd]

I figured the charge rate was full speed ahead, until float at 14.4V.

G-

[tanner0441]

Hi

Why not find someone who would trade two 12V Alternators for one 24V or get hold of a 24V alternator. Seems a safer option.

Brian

[oztules]

Thats how I see it Ghurd. I see no point in running a motor at half the power for twice as long... or any other combination. I would have given it the works (or whatever I could comfortably afford) till float, and then shut down.

Bob obviously has good reason to make this particular regulator part of the system. Personally I'd throw it away and build my own ( http://www.fieldlines.com/story/2006/6/10/13381/2616 ), but I may not be seeing the bigger picture.

Good luck with it Bob, you appear to have found your answer anyway.

............oztules

[rossw]

Bob, I'm seeing lots of comments from people telling you to trade in what you have, or other ways to re-use what you have that may or may not be what you want, but nobody actually answering your question.

As I see it, a simple 2:1 divider is what you want, but the excessive losses of it are something you DON'T want.

How about this for a fairly simple and fairly efficient solution?

A simple squarewave generator (1:1 mark/space ratio). This part is important - and perhaps easiest to achieve with a simple oscillator followed by a flipflop (guarantees you exactly square output).

Drive a decent power transistor or power fet with the square wave to chop your (approx) 24 volts. With a reasonably high frequency and a small filter after it, you should have a reasonably smooth output at exactly half the input supply (give or take the drop across your switching element).

Being a fully-saturated switch, power dissipation will be minimal, efficiency high, it'll track the input as you want and doesn't require modifications to any of your existing material.

RossW

[bob g]

RossW:

that makes sense, i like it!

you got a favorite IC to generate the Square chopper?

For the rest that have asked:

there is good reason behind the use of this controller, building something with half

the capability would be outside the abilities for most folks, also not having to design and build a 3 step programmable regulator leaves me with more time to design and build other aspects of this unit where no good off the shelf options exist.

believe me there is lots to design and build as it is.

thanks everyone

bob g

[brokengun]

That seems like it would be pretty inefficient and might waste a lot of your power as heat in the potentiometer.

[oztules]

A TL494 will probably do this nicely. Lock the two error amps fully on, and use the dead time control to derive the 50%duty cycle. (can be referenced by the internal 5v supply to determine the voltage on pin 4 (I think) to give you the 50% ratio locked). Only a cap and resistor for the oscillator, two resistors for the divider for the deadtime.... and maybe a pair of trannies for buffered output to your favourite fet.

........oztules

[rossw]

You could use just about anything, but since you asked... my favourite old mainstay for this was the CD4060.

This chip will make a free-running osc with the addition of 2 resistors and one cap, but then has a 14 stage (IIRC) divider.

Great for long timers etc.

In your application, just a .001uF cap, a 10K and a 100K resistor and pick one of the outputs thats at about the frequency you want.

Being CMOS, it'll run from 5 to 15 volts, and runs on almost nothing. A diode from your 12V supply, and an electro to smooth out the lumps and bumps would be good enough for the supply. Running at that voltage, it should be able to switch a FET directly, or if you want to use a p-channel fet, there isn't much problem to interface it.

The other thing you could do if you wanted to go a little upmarket would be to get a little PICAXE 08M chip. It has the internal osc and all the tricks, can generate squarewaves out in much the same way (has PWM output if you want, so you could actually tweek your voltage if you wanted to). Upside: less bits. Probably about the same price. Downside, it needs 5V to run so you would need an extra regulator.

[bob g]

thanks

i am working with the bs2 stamp and as such am using a puter power supply for my 5vdc needs anyway,

never thought about the stamp generating square waves, hmmm cool, something new to learn.

another question if i may,

i have access to the stator AC output, there are 3 posts coming out of the alternator that connect directly to the stator.

could i not just take one of the phases, and use a half wave rectifier

that way i would get for instance the top half of the wave in pulses?

gotta think that through a bit,

btw,, i also did a bit more testing and found that if i take the power from the 12volt side to power the controller and provide for excitation, and then also attach the keyswitch wire to the centertap of the 24volt battery string with the sense wire,, i can get the controller to work just fine and be able to program without placing an undue amount of loading on the centertap.

i want to do a bit more testing to make sure that this will work, if so it will be much easier than added circuitry.

the balmar controller is an awesome piece of engineering, fully programmable for all the different battery technologies, all voltage setpoints can be altered, time to start, soft start ramp up, temp sensors for alternators and batteries (provides protection for the alternator(s) and temp compensation for charging, 3 stages bulk, absorption, float and also manual equalization, amp manager to tailor the hp requirement so i can optimize for the available hp and gear ratio,, and a plethora of other cool stuff.

the bottom line or where the rubber meets the road will be in finding out whether or not i can get the efficiency i am looking for, i think i can anyway.

bob g

[Ungrounded Lightning Rod]

These solutions seem enormously overdesigned.

If the regulator has a voltage sense input that doesn't draw a lot of current, separate from the connections that get power to run the field windings, you could add a high-resistance external voltage divider to scale down the regulator's idea of what the voltage is by a factor of two.

Indeed, if the regulator already has a voltage divider from that voltage sampling input to ground you can rescale it by adding a series resistor of the correct value.

You may have to do some hack for the field excitation switch, too, so it will work safely with the higher voltage.  But again you can do a hack that doesn't waste a lot of power.

[Electron Skipper]

To keep things simple, why not just use a variant of National Semis' LM350 driving a pair of 2n3055's?

Cost less than a microcontroller, and a much simpler circuit.  If you need it to switch out below a certain voltage, use a MOSFeT wired as a Variable resistor as it will stop conducting below a point you can adjust by changing resistance values.  

By using the variable regulator you can adjust it for where you want your final voltage.  The only drawback is the circuit needs to see an input voltage 3 volts above your input voltage, but in the event that period of time between the begining of MOSFeT Conduction and the 3 volt minimum difference is short- it is not a factor to be concerned with.

If you need more than 200 Watts of dissipation, or are expecting a potential above 5 volts over set point at full current you can add additional 3055's until you reacht he balance point, or you can use another type with a higher current and power rating.  

It seems a much more simple aproach than trying to dumb down a microcontroller to do a linear IC's work.

[Electron Skipper]

Slight correction and addenda:

You can use this link for the PDF for LM 117/317 3 tab adjustable regulators,

http://www.national.com/ds/LM/LM117.pdf?

and scoll to page 14, and instead of using the expensive LM 195/295/395 precision transistors, use the 2N3055's instead.

Lower overall cost if you happen to have the parts in the junk box.

[bob g]

got to digging in my collection of parts and found a

3 phase piggyback transformer/rectfier unit

it was made by leece neville to convert a 12 volt nom. alternator to

provide for 24 volt charging (at 10amps max)while also being able to provide 12volts

for accessories at 80amps as normal.

with some connection switching i can convert it to inputing 24volt AC off the stator

and stepping it down 2:1 and rectify it to provide for the scaled 50% that i am looking for.

that would enable the alternator to output the 24 volt as needed and provide for 12 vdc power for the controller and the excitation needs for the alternator as well.

and do it across the total output instead of a centertap. should even be sufficient to charge the starting battery as well.

more options available than i thought i had

gotta like options!

bob g

[rossw]

If you'd read the original spec, he wanted something to provide ideally, a perfect HALF of the supply. If not exactly half, then close to but a constant ratio.

Thats the first part of the reason I suggested using a square wave to chop the supply - it would assure him of a (virtually perfect) half supply, and constant ratio.

Next was the shear power. Lets not lose sight of the fact he wanted 8 AMPS. At 12V (being the half of the 24V he was dropping) thats 96 watts of power to dissipate. Its hot, and it's wasteful. If the volts went up, it would be even more so.

Again, that was my reason for suggesting a square wave chopper. The series pass element (transistor or FET) would be fully saturated, or fully off. The dissipation will be minimal. A few watts - probably under 5W - which is MUCH better than simply burning it all up as heat.

2N3055 is only good for 60W, so even two of them are touch-and-go for the series dropping element you proposed. A regulator plus two such power transistors and various other bits would probably cost more than a single transistor and square-wave source!

[Electron Skipper]

Actually the 60 Watt rating is for the plastic cased 2N3055, the TO-3 metal is rated at 100 Watts.  Plus, I did indicate to use them paired, and just a few sentences down from that I had also written: If you need more than 200 Watts of dissipation, or are expecting a potential above 5 volts over set point at full current you can add additional 3055's until you reach the balance point, or you can use another type with a higher current and power rating.  

What he is after is "...12volts nominal" for the purpose of "...use them together to charge a 24volt bank directly...."

His own solution with the relay is the simplest.  I do believe he is better off forgetting the Balmar regulator because that series of Prestolite incorporates a regulator as part of the brush holder.  If he happens to have one that does not have the incorporate regulator, he can use my suggestion to increase the voltage in the armature, which he can then set his output voltage to any voltage around 29 volts he desires as his set point to charge the full battery as one battery.  Overall, it will be better to charge the full battery rather than just half at a time.

Less cost and less work too.

[ghurd]

A relay is almost never a good idea.

Do you use metal 2N3055s at 50W?  

I wouldn't, and don't expect Ross would either.

Plus wasting 96W and the power to make that 96W is a good % of the big picture.

[Electron Skipper]

If you are attempting to switch at a higher speed, a relay is not the best solution, but can work for a very long time if you filter across the contacts to reduce arc-over, but the old tube car radio "vibrator" really was nothing much more than a relay interupter, and they worked very well switching at about 120 to 150 Hertz.

There are many other factors to consider when looking at what case style/power rating is going to work best.  But ultimately, increasing the rotor potential in this manner is the more efficient approach.  Sure, he did find a piece that should accomplish what he wants to do with the alternator within his spec, what happens if that burns out?  Fatigue fails, or some other malady that often besets old equipment?  

My suggestion, which I probably should have explained a little further initially is his next best option for time efficient and materials efficient approach for what he want to do.  That circuit I suggest replaces the integral regulator of the alternator in order to get the alternator output up to a level to charge the 24 volt battery as a single 24 volt battery, instead of as a pair of 12 volt batteries joined at the center post.  He can also save the money by NOT buying the external regulator which is redundant for that alternator series.(or if he has that external regulator already- save it for another project.)

Additionally, if he is requiring an exact midpoint of the ouput of the alternator, if he has his current requirement met, 2 precision resistors in series tied from ground to the main power rail give him his reference voltage at the point located between the two resistors.

If the battery is being charged at the full 24 volts, there is no need for a relay, nor is there a need for finding a way to disipate the theoretical excess current. Given his original parameters in his first post, that is what he was sounding like he was wanting to do beside the switching aspect.  Besides, 2 TO-3 type 2N3055's in parallel have a 200 watt rating, and would be able to handle 96 Watts with plenty of dissipation to spare.  and I did also indicate in that post if he did not feel it was adequate, I did indicate just a few sentences below the mention of 2N3055's "...he could add more in parallel.  

Ultimately there is little problem running a transistor rated for 100 Watts, at 50 Watts power level, or a pair rated 100 Watts each at 100 Watts. IF you provide adequate heat dissipation, and are also within the voltage ratings of the transistor.  

The reason my suggestion works is if the alternator rotor is being run at a potential to generate the 29 volts nominal for charging a 24 volt battery, the armature is not likely drawing much more than about 4 to 5 amps at max charging rate of 165 Amps at 14 volts or 82.5 Amps at 28 volts, in both cases at approximately 4500 RPM.  The extra energy of the motion of the alternator is where that extra energy is coming from off the 3 phase stator.  The rotor also is normally at a lower potential than the output voltage of the alternator.  I have a pending project with a similar alternator that once it is done I can take the exact measurements and make adjustments for my application.  Most of the time the current drawn by the rotor/armature of a typical automotive/truck alternator should be in the area of 2 to 3 Amps.  

The beauty of the LM117/317 circuit is the adjustability of it.  For extra current drive, you add as many power transistors as you would need for your expected power handling requirement in parallel as described loosely on page 14 of the Application notes I cited.  If you needed to disipate 1000Watts, you either add 2n3055's in parallel until you are comfortable with the performance envelope, or you switch to a different type with higher ratings.  

2N3055 is a general type,  and is available as either a plastic version (with lower power ratings) that can fit the space requirements for a metal can TO-3, or is available as a TO-3, which is manufactured by a number of different manufacturers.  

So what is your hangup on this?  IMHO, It seems you want to nitpick on pointless minutae.

[rossw]

I'm not going to bother saying more than:

1. You missed the point,
   
2. You missed the point
   
   

and

3. You missed the point.

"There are none so blind as those who will not see"

[TomW]

Ross;

I would reiterate exactly the same point but you did it so eloquently, I will pass.

Tom

[Electron Skipper]

No,

1. you missed my point.
   
2. you missed my point.
   
   

and

3) you missed my point.

If you want to throw technology at a simple matter that can utilize simplicity, who am I to stop you from doing so. If you don't understand that alternators like the one mentioned can be made to produce other higher voltages within reason and limits, the problem is yours.  

[Electron Skipper]

The post is correct- None are as blind as those who fail to see.

Your ignorance must be truly blissful.