high current DC to DC - Page 1
Forum section: electronics / beginners

author: Prime73
author_rank: Regular Contributor
timestamp: September 04, 2013, 07:56:10 pm
content: Hello there,I'm trying to design a circuit which could deliver up-to 30A (max) at 6-7v from a 12V car battery. Plan to use LM317 as a reference and three TIP2955 to handle high current. Found a few circuits online. for example:I can't figure-out how to properly calculate values for R1 and R2 in the above circuit and is R2 really necessary there at all? I saw a few similar schematics without R2? Don't wanna just copy it without full understanding the schematics.thanks in advance

author: M. András
author_rank: Super Contributor
timestamp: September 04, 2013, 07:58:22 pm
content: why dont go with a switching controller and its implementation? at that current its gonna pass 90watts of heat

author: Zbig
author_rank: Frequent Contributor
timestamp: September 04, 2013, 08:04:25 pm
content: More like 180 - 250W. That's insane amounts of heat to dissipate, especially in the battery-powered setup. You're right that SMPS is definitely the way to go.@Prime73: what are you going to power with that?

author: M. András
author_rank: Super Contributor
timestamp: September 04, 2013, 08:06:10 pm
content: Quote from: Zbig on September 04, 2013, 08:04:25 pmyeah you are right i miscalculated ithttp://www.ti.com/lsds/ti/power-management/step-down-buck-controller-products.page#p451max=30;120pick one which suits you

author: Prime73
author_rank: Regular Contributor
timestamp: September 04, 2013, 08:14:52 pm
content: Quote from: Zbig on September 04, 2013, 08:04:25 pmI need something simple for a kid's toy car (the one a kid can drive around in). it had 6v battery which is gone and I have a spare car battery that fits perfectly into a battery compartment. Problem is - the car runs on 6V DC motor. 30A is probably an absolute maxim. I've measured current on a motor when the car goes up hill and it's around 12A. and yes I know I can just buy a 6V battery for a carbut I wanna learn something and It seems to me a good project to work on.

author: Zbig
author_rank: Frequent Contributor
timestamp: September 04, 2013, 08:28:10 pm
content: You do realize you're going to waste exactly half of the battery charge as heat? And I don't take motor efficiency into account here, you'll waste 50% of the battery charge just by lowering the voltage by half with linear regulator. Furthermore, don't underestimate the heat generated, you'd basically have to build a little trailer for just the heatsink requiredBut kidding aside, it's a really poor idea. You wouldn't want your kid to get burned once this bursts into flames or melts the plastic toy car. Imagine two 100W (old school, tungsten) light bulbs stuck in there if you need some food for thought.

author: edavid
author_rank: Super Contributor
timestamp: September 04, 2013, 08:30:56 pm
content: Quote from: Prime73 on September 04, 2013, 08:14:52 pmIt's not really such a good project. At that power level, either a linear or switching regulator could fail fairly dramatically (smoke, flames) while your kid was driving around.Of course you should also consider what would happen if your car battery spilled acid or exploded, as they sometimes do. Was the original 6V battery sealed?If you want to go ahead, why don't you try taking some plastic off the top of the 12V car battery? Maybe you can tap 3 cells and avoid the voltage conversion problem.

author: Prime73
author_rank: Regular Contributor
timestamp: September 04, 2013, 08:40:51 pm
content: Quote from: edavid on September 04, 2013, 08:30:56 pmthe original battery was sealed. Seems like I should probably abandon this idea and just find a new 6V battery or convert 12v battery to 6v and call it a day.thanks all

author: crisr
author_rank: Contributor
timestamp: September 04, 2013, 09:43:53 pm
content: As long as we are talking safety, opening up a 12V lead-acidbattery to tap 3 cells if probably not a good idea, especially an originally sealed one.On the other hand, a 6V DC motor can easily be driven from a 12V supply with 50% duty-cycle, no need for special voltage regulation. If the toy has no electronic circuitry or electronic throttle control, you could simply make a 50% duty-cycle square wave oscillator driving a N-channel low side MOSFET (at 30A a single cheap jelly bean IRFZ44N with 17.5mohm RDS-on would need to dissipate just over 8W @ 50% duty-cycle with a 12V gate voltage) to the motor negative (or what would be the original 6V battery negative terminal if there is a reverse switch). If there is some kind of throttle circuit (which would probably be some rudimentary PWM controller), it would be more complicated, but you would basically need to check if it would support a 12V supply (check capacitor voltage ratings, etc) and find a way to limit the motor switcher's duty-cycle to 50%.

author: alxnik
author_rank: Regular Contributor
timestamp: September 04, 2013, 09:57:52 pm
content: Given that the motor is an inductive load, wouldn't it work if you put a second PWM on the other side of the motor from the first one?Actually if the caps don't blow up and it has a PWM throttle, the 12V might work out of the box. You just have double the throttle response.Toy car turbocharging anyone? (just kidding, don't try it at home, especially with a kid driving...)Quote from: crisr on September 04, 2013, 09:43:53 pm

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 04, 2013, 10:37:37 pm
content: Just buy a ready-made DC-DC module. It takes some serious engineering to get reasonable performance at these power levels. And don't forget the fuses!

author: edavid
author_rank: Super Contributor
timestamp: September 04, 2013, 11:02:57 pm
content: Quote from: crisr on September 04, 2013, 09:43:53 pmHe said it was a car battery - they are not sealed, except for very expensive AGM batteries that he wouldn't be putting in a toy car.And yes, you wouldn't want to open the cells themselves... but if the connections are just hidden under some plastic, why not?

author: Psi
author_rank: Super Contributor
timestamp: September 04, 2013, 11:14:41 pm
content: Any ready-made dcdc solution that can do 30A at 6V is going to cost more than a new 6V battery.If it was me i'd probably buy one or two grunty 2S lipos from hobbyking and wire them up. Maybe with a large resistor and two relays + cheap attiny mcu.The mcu can control the relay to provide undervoltage cutout and could even dynamically bypass the high power resistor as the battery discharges and the full voltage becomes ok for the toy.For the absolute cheapest option i like the idea of tapping off the 6V inside the battery. A bit messy, and you'd want to be careful nothing could short or leak out if the toy is upside down. But assuming those issues were solved it would work perfectly fine electrically speaking.You could even have a large DPDT switch to use the 2nd part of the battery, reserve tank

author: crisr
author_rank: Contributor
timestamp: September 05, 2013, 01:21:43 am
content: Quote from: alxnik on September 04, 2013, 09:57:52 pmEven an inductive load needs current to flow somewhere; if the PWMs are not synchronized, that (putting a second PWM on the other side) would only work with a frequency of at least an order of magnitude greater the original circuit's frequency. Also, beware of the possibility of an e-brake, which could short your PWM output!And about just running from 12V without doing anything electrically, unless you physically blocked the throttle from going over 50%, and especially when used by a kid, the motor (not to mention the drivers) would overheat and probably die really fast (with twice the voltage power would be 4x greater).