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High amperage DC-DC down conversion

Discussion in 'General Electronics Discussion' started by DieselTwitch, Jan 26, 2013.

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  1. DieselTwitch

    DieselTwitch

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    Jan 26, 2013
    I'm in the middle of building a 100Ah 48V Mobile Hybrid Power pack. but I'm having some problems in the design of trying to find an efficient way to run a 48V battery pack and still use large 12V loads such as truck engine starting, ham radio operations, 12V lighting systems.... i want the 48V because it will allow me to run a much more efficient charging system and inverter.

    I've thought about using a high amperage Solid State relay that has a 0-10V control input and regulating the power that way. but that will only buy me 30A per relay.
    Crydom CKM0630
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    A solid state relay is definitely not the way to step down a power supply. It's meant to turn on or off, nothing else.

    You need a step down converter. You could build one, but at that power I don't recommend it. Or you can buy one like this . There are quite many sources, ask Google.
     
  3. donkey

    donkey

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    Feb 26, 2011
    I can only gooff current info and I assume you are confusing a relay (looks like a coil and a switch) with a transformer (2 coils)
    [​IMG]
    relay
    [​IMG]
    transformer

    a transformer IS used to alter voltage/ current is that what you are thinking of?
     
  4. davenn

    davenn Moderator

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    He's talking abouot battery packs, donkey, DC doesnt go through transformers ;)

    Dave
     
  5. donkey

    donkey

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    Feb 26, 2011
    I know that, you know that... its the only thing I can think of as to why he is thinkinga relay will convert voltage/current. yes they both have coils but work in different ways.
    to step down DC voltage you can get step down convertors and regulators.
    relay is definately not the way to go as it is simply a switch. as dave pointed out a transformer ONLYworks on AC so its out.
    http://www.powerstream.com/dc48-12.htm is ok... but starting a truck is high current. maybe an independant system for that would be best
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Maybe the OP is thinking of using a solid state relay for some kind of PWM?
    We won't know until he responds.

    Sorry donkey, I hope that doesn't make your nose bleed :D
     
  7. BobK

    BobK

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    I suppose you could also use a solid state relay as the switch in a buck converter, but it would not be ideal, unless it was basically just MOSFET.

    Bob
     
  8. DieselTwitch

    DieselTwitch

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    Jan 26, 2013
    No worries Donkey, Your translation of my question is totally understanable. its not a standard ON/OFF relay. It uses a 0-10VDC signal to control the output of the relay. lets say I have a 48V pack that I run a load through the relay (and relay is really a bad term here). If i give no signal (0V) then there relay is off. However if i give it 5V then I would get 24VDC out of the relay. 10V i would get the full 48V. What I'm trying to figure out is this; Is a DC-DC converter nothing more then a Solid state relay with some type of voltage regulator built in? if so then I dont need a DC-DC converter as my control system will be able to regulate the out put using a Voltage sensor on the output and PID loop in the program. Hope this helps to clear up my initial post
     
  9. davenn

    davenn Moderator

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    No, there are no SSR's in a DC to DC voltage converter
    DC-Dc convertors come in two main flavours ... Boost and Buck

    Boost converters.... take an input voltage and boosts ( increases) the output voltage
    Buck converters... take an input voltage and reduces it to a lower level output

    These types of voltage regulators are generally more efficient than the normal linear voltage regulator like a 7805 or 7812 etc. The higher efficiency results in less power being lost as heat in the regulator

    Dave
     
  10. DieselTwitch

    DieselTwitch

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    Jan 26, 2013
    Ahh..

    So now my question is if I need 100 or 200 amps of 12V out of a 48V pack. whats the best way to go about this?
     
  11. davenn

    davenn Moderator

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    Thats out of my league mate, not used to working with anything over ~ 40 - 50 Amps

    Dave
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The only way is a buck-convertert (also known as step-down converter). Anything else would create terrible loss.
    But 100 A - 200 A thats out of my league, too.
     
  13. Kiwi

    Kiwi

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    Jan 28, 2013
    The truck starter motor requires very high current. Could easily be 200A - 400A depending on the size of your engine.

    I can think of two ways that could work;
    1 - Fit a 12volt battery bank to start and run the truck.
    2 - Connect the truck to 12volts on the 48volt battery bank.

    Option 1 would need a battery charger running off the 48volt batteries to keep the truck battery bank charged.
    Option 2 would need a "charge equalizer" to monitor the voltage at the 12volt tap and keep it at 25% of the total voltage of the 48v battery bank. If the bank is being charged at 56v the equalizer will maintain the 12v tap at 14volts. I do this with 24v trucks running 12v loads. Never tried it with 48v vehicles. Don't know where you would get a 48v/12v charge equalizer.
     
  14. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Running 100 A - 20o A through a conductor requires a massive busbar. A simple wire as used in a car or in a household will melt.

    I think a better option would be to have multiple step down converters for different uses, e.g. 1 for the TV, one for the refrigerator, one for the radio etc. This way each converter and the cables have to bear only a few amps which is much safer than having a 100A construction. Also in case of a failure not all consumers are off at the same time and you can easily switch the most sensitive devices to a working converter.
     
  15. BobK

    BobK

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    I also am not well versed in high current, so take this with a grain of salt, and I hope others will comment.

    A buck converter is basically a PWM signal applied through an inductor. Your starter motor is an inductor, so all you need is the PWM. If you could use a PWM signal with 1/4 duty cycle off your 48V source it should apply the equivalent of 12V to your starter motor. No?

    Of course this does require massive swtiching transistors (MOSFETS or IGBT), probably many in parallel.

    Also the simple equation of Vout = Vin * duty only applies if the converter is working in continuous mode, i.e. the current never falls to zero, so you would have to verify that this is the case and adjust the duty cycle accordingly.

    Bob
     
  16. DieselTwitch

    DieselTwitch

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    Jan 26, 2013
    You're right that a that many amps would pop even a 1/0 gauge wire after a short time. However I got to thinking about how I do jump a truck. and most of the time I let the pack sit on the truck for 20-30 min before trying to start. So it occurred to me why couldn't I do the same thing here. I think Kiwi hit on it a little. I would break the pack in to two voltages. The Alternators, Primary Battery Pack and the Inverter will all run 48 volts. Then using an equalizer or charger have it maintain a smaller 12 volts pack, say 30Ah.

    Thoughts?
     
  17. DieselTwitch

    DieselTwitch

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    Jan 26, 2013
  18. BobK

    BobK

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    Might be, but not enough amps for your application.

    I hope some knowlegeable people will comment on my suggestion, which is likely to be the least exensive way to do it.

    Bob
     
  19. DieselTwitch

    DieselTwitch

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    Jan 26, 2013
    Now that I don't need the down converter to supply the full cranking amps and only maintain the charge in a smaller 12V battery pack then 50amps should plenty
     
  20. DieselTwitch

    DieselTwitch

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    Jan 26, 2013
    Here's a quick diagram of what I'm doing.
    After thinking about it, trying to get power out for lights, and other DC items that mostly use 12 volts would be a pain. So i re-did every thing to a 12V system. I lose about 5% eff on the inverter but Its losses I can live with
    [​IMG]
     
    Last edited: Jan 28, 2013
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