Basic Circuit Design: Converting 12V to 24V

[tarmon.gaidon]

Hey everyone,

At risk of sounding dumb here I go. :uhh:

We are designing a basic electrical system that will have a regulated 12 volt, 10A source (A stator on a small engine) and I need to convert it to 24 volts to power two clutches. I am thinking about using two http://www.mouser.com/ProductDetail/Cincon/CHB50-12S24/?qs=sGAEpiMZZMtwaiKVUtQsNZk%2fyhBUJKktmTV1yafdAHw%3d" [Broken]. I am not very confident in the dc/dc converters I can find in our price range and I had a few questions.

If we use the converters to charge two 12 volt batteries wired in series and then powered the clutches off of the batteries are the converters at risk of pulling to much current. The converters should be able to handle the current requirements of the clutches but I just need to make sure that there is zero possibility of them failing. So in some freak accident where the clutches pull a bit more than they should, how will this affect the current going through the converters?

Hopefully that isn't to confusing, if you have any other questions don't hesitate to ask.

Thanks,
Rob

 

[Studiot]

The DC you will get out of your generator will not be suitable to operate DC/DC convertors directly.

Are you intending a dynamo or an alternator?

Why can you not generate the 24 volts directly from your shaft?

Do you need the batteries noted to supply the clutches even if the engine is not turning?

 

[tarmon.gaidon]

Why is the regulated DC from the stator not able to power the DC-DC converters?

 

[Studiot]

Because the output provided by a generator (dynamo or alternator+rectifier) is not true DC in the sense required by electronic circuitry. The conversion to true DC is achieved by the battery.

You might get away with using the two batteries and a voltage doubler circuit if you have an alternator. You have to put in about 2.5 times the output current to achieve this.

 

[Okefenokee]

It says in the data sheet "Continuous Short Circuit Protection". That means it must have a current limiter. Don't worry about drawing too much current. Disclaimer: that's not to say that it couldn't fail in some other way. I don't know the product but it looks like they're going for something rugged.

Also, I'm not sure what Studiot is getting on about. If the output of your generator is "regulated" by actual power electronics components then I would think you're ready to go.

 

[Phrak]

What's the application?

 

[pantaz]

Because the output provided by a generator (dynamo or alternator+rectifier) is not true DC in the sense required by electronic circuitry. The conversion to true DC is achieved by the battery.

How is power coming out of the rectifier-regulator not suitable for "electronic circuitry"? Circuitry that can't handle a little AC ripple is pretty poorly designed.

 

[tarmon.gaidon]

I agree, by all means I need to know if it won't work but I would think it should be able to handle the output of the stator.

 

[Studiot]

Well since everyone knows all the answers why ask the questions?

I did ask a couple of questions myself, to which I received no answers, but I tried to help anyway.

My questions were intended to get additional information needed to provide a more detailed answer.

Did you understand my response in post#4?

 

[Bob S]

The DC-DC converters you reference are regulating converters, and should be able to convert the 3-phase rectified output dc output signal from the alternator (Is this the "stator"?) to a true 12 volts dc. IF the dc-dc converters have an isolated-ground dc output, then you could parallel the two converter inputs, and series the two converter outputs to get 24 volts dc.

Bob S

 

[tarmon.gaidon]

Well since everyone knows all the answers why ask the questions?

I did ask a couple of questions myself, to which I received no answers, but I tried to help anyway.

My questions were intended to get additional information needed to provide a more detailed answer.

Did you understand my response in post#4?

Hey Studiot,

By know means was I trying to assault your intelligence or your statement. I was making an assumption and was rather confused. I value everyone's input and definitely need to look at every aspect of this.

Thanks,
Rob

 

[Studiot]

By all means go buy the convertors I really hope they work for you, but I saw nothing in the spec to suggest they will.

Of course my queries were not about the convertors, but about the nature of your 12V 10Amp supply.

You still haven't told us.

I asked because it is usual for motor driven generators to provide what is really a pulsing unidirectional supply, without a smoothing reservoir. This type of supply is normally fed direct to a battery and is better for battery charging than a true DC.

I think those who responded are under a misaprehension about the nature of 'regulators' as used in conjunction with generators. Normally they are not regulators at all, just voltage cutouts.

I also asked about the supply because it is possible you may be able to save the money on the convertors with a bit of rewiring.

 

[tarmon.gaidon]

I will go ahead and see if I can find some more information on the source regulator. I am working with a team and I need to make sure I have the right part numbers.

Thanks, I appreciate the feedback.

 

[Studiot]

See the section labelled WARNING in this site.

An auto alternator is designed to have a battery connected across it, not a DC/DC convertor.

Also read about the operation of altenators if you like.

http://www.bcae1.com/charging.htm

 

[tarmon.gaidon]

So it looks like the stator is "regulated" at 12 volts and that is going to be hooked up to a 10 amp alternator.

Briggs and Stratton Part Number #695466. I really can't find any addition information on it though.

 

[Studiot]

So did you read the warning?

 

[tarmon.gaidon]

I did, thanks. Do you have any suggestions for the best way to handle this?

 

[Studiot]

Difficult to say as you have told us so little about your application.

But I also asked before about other options.

Do you have to use this alternator or can you change it for a 24v one?
Do you use the output for anything else or could you run it at double speed through a gear?

Using the 12V regulator are you prepared to either rewire the rectifier stack or tap the ac off and use a different rectifier to feed batteries connected to form a voltage doubler?

 

[sophiecentaur]

An alternator may have problems feeding a DC DC converter but a fat car battery across the circuit would remove that problem.

Failing that, why not use two 12V alternators (cheaper than one 24V, I bet) with a long drive belt and charge two 12V batteries in series. You might have to go to a bit of trouble to 'float' the alternator supplying the 'top' (12 to 24V) part of the supply but, if you are happier with mechanical things than with electrical, arranging to drive two alternators wouldn't be too hard. You could have insulated mountings and just connect the 'Earth' of the upper alternator to the +12V of the lower one.

 

[tarmon.gaidon]

Alright I will try to outline everything a little better.

1) We have a small Briggs and Stratton engine with a stator producing a "regulated" 12V source.

2) Then we have a 10 Amp Briggs and Stratton alternator mention earlier.

3) From here I need to find a way to power two electromagnetic clutches, each requiring about 1.5A at 24V DC.

I don't see any reason as to why we couldn't use a different alternator if I can find one that fits our application and is within our budget.

By the way this for an SAE Baja Car.

 

[tarmon.gaidon]

An alternator may have problems feeding a DC DC converter but a fat car battery across the circuit would remove that problem.

Failing that, why not use two 12V alternators (cheaper than one 24V, I bet) with a long drive belt and charge two 12V batteries in series. You might have to go to a bit of trouble to 'float' the alternator supplying the 'top' (12 to 24V) part of the supply but, if you are happier with mechanical things than with electrical, arranging to drive two alternators wouldn't be too hard. You could have insulated mountings and just connect the 'Earth' of the upper alternator to the +12V of the lower one.

I hadn't thought of that. That may be the easiest solution to our problem and well within our budget.

 

[Studiot]

You said this was a team effort.

I suggest you discuss what you have found here with the rest of the team, there should be plenty to think about.

I see Sophie came up with yet another approach I didn't think of -just proves how many alternatives you have.