# Engineering Challenge

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Thread Starter

#### markinnh

Converting 24vdc to 12vdc is complicated by the need for the circuit to carry at least 500 amps

(This is a 24vdc to 12vdc bet and experiment between a few of us 'drunken' (no so that one would notice) sailors

Any help or ideas would be great.

#### PhilCorso

Markinnh... which circuit is to carry the 500A?

Regards, Phil Corso

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#### curt wuollet

Switching downvertor 24V at 250A to 12V at 500A. It would be spendy but doable. The magnetics would be interesting. The phone companies actually do this sort of thing at 48V.

Regards
cww

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#### markinnh

The source is a battery pack, 24vdc limited to 500 amps....the load is 12vdc 500 amps...do I use a carbon pile?

invert/transformer/rectify?...this is the quandry.

#### PhilCorso

Markinnh... Being sailors, how important are cost and efficiency?

Regards, Phil Corso

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#### Ken Emmons Jr.

There are companies specializing in low-voltage, high-current drives for
electric drag racing and electric vehicle conversions. I don't know if
this falls into their specifications but it might be a good place to
start looking for a product. If you aren't familiar with high power
electronic circuits I would not approach a home built current controller
of this magnitude.

KEJR

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#### Markinnh

I was thinking dynamic braking resistor....
.024 ohm $850 for the build (wound open resistor) in a vented box 26"X13"X6"....make quite a toaster We have a source 'wagon' for jumping large diesels at 24vdc....we want to be able to jump smaller boats that use 12 volts without adding yet another piece of equipment on the dock or quay...ease of use, footprint, possibility of modifying existing unit, all are factors. #### PhilCorso Markinnh... As you probably know I have a hangup regarding Anonymity. So, if you want the solution contact me off-list. Regards, Phil Corso (cepsicon [at] aol [dot] com) C #### curt wuollet Yes, you could use any old 6000 W carbon pile that you have lying around. Or power resistors. But the switching approach would be much more efficient and could be regulated. This is how things are done these days. Even some of your wall warts are switchers, the ones that don't get hot. Check some of the old line PS companies. Regards cww M #### M Griffin Is there any possibility of simply changing the connections between the cells so you have 12V instead of 24V? The output voltage is determined by how many cells you connect in series versus how many you connect in parallel. The question will be whether the battery packaging give you the necessary access to the cells. Are they actually 24V batteries, or are they 12V batteries connected in series? Don't go modifying things unless you are sure you know what you are doing though. There is enough energy present to cause fires and explosions. Also, modifying certified equipment without getting it re-certified may cause regulatory or insurance problems. C #### curt wuollet The problem with a resistor is that the voltage delivered to each type of starter would be different because they would draw different amounts of current. To have 12 volts for a bigger engine you might toast a smaller one. I have seen this sort of thing done with diodes as their drop is more constant with current, but 12 V worth of 500A diodes would be spendy and require large heat sinks. regards cww P #### Peter Butler Stack a bunch of power diodes until the voltage drop is what you need. They will get warm. Use a fan. K #### Ken E Just for curiosity, what do you use to switch 24V - 500A? Do you string a bunch of MOSFETS in parallel on a massive heat sink, or are there higher power devices available? I've never attemted such an application so I haven't poured through the semiconductor offerings. KEJR C #### curt wuollet I would assume VFET, HexFET, etc., but with 24 V you can tolerate some VceSat. And things are still changing rapidly with high power devices, so I'd do at least a quick survey of the field. But the ease of paralleling FET devices would probably win out _if_ they can switch fast enough without really exotic drive. The freq. would have to be fairly high to keep the magnetics size and cost down. The average current at 24V would only be a little more than 250 A for 500A at 12 V out. Regards cww #### PhilCorso Markinnh... did you and your Navy buddies accomplish your goal? Regards, Phil Corso #### PhilCorso Markinnh... you had posted that cost was a major concern. How about your labor as a minimum, plus some$100 - \$200 for optional bells & whistles?

Regards, Phil Corso

#### PhilCorso

Markinnh... are you sure battery capacity was(is) 250 A, and not 250 Amp-Hrs?

Regards, Phil Corso