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Discussion Starter · #1 ·
I have done some further research on these power panels, and all I can say is I wish they had a 7.2v output.


I need to make a power panel with 1.2-1.5v out, as well as 7.2v out. The objective being that I can then disregard all of the batteries from the field kit andrun the glow starter and 7.2v rotostart all off the 12v battery (tethered by cables of cause).


So the bottom line is, as a minimum, I need to work out the curcuitthat will step 12v down to 7.2v.


Do the electornics wizards have any thoughts?


Obviously I need some resistors, but my mind doesn't recall the calculationsto to work it out. I am sure once upon a time I knew this. I also can't remember the effect on current. Alternatively, I could build some type of DC to DC transformer, but that all sounds to hard......


In a nut shell - 12v in and 1.2v, 7.2v and 12v (pass through) out. Then I can think about the nice bits like LEDs and meters after that.


Thanks,


Chris
 

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Interesting... was wondering why you needed the 7.2V!



Its not hard to build a glowplug supply from 12V DC, all you need is a voltage-regulating circuit capable of feeding 2-3A max.


But the rotostart probably sucks more than a few amps, you'd probably need some serious power transistors (time base switchmode?) to drop the voltage with high amps and they won't come cheap.





What about thinking laterally?



540 motors will work fine at 12V, main concern is they'll run faster. Can your engine hack the higher speed??


Or replace it with something with a higher wind. There exists 540-size motors with 70+ turns (stock is 27 turn), they are either built for rockclimbing electrics (high torque, low rpm) or for ppl to run their pitlathes off a 12V supply (lathes usually run off 4.8V for the correct speed).





Or... since voltage regulation don't have to be spot on, how about just wiring a 12V 21W lightbulb in series with the rotostart. Might just work.
 

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V=IR


Voltage = Current x Resistance.


You could work out the required resistor if you could measure the current useage of your roto start. Then go to Jarcar and get the one that can take that current.


EG: for 3A load:


12V = 3 x R


R = 12/3


R = 4ohm (but you will need a resistor that can handle at least 3amps, preferably 10A)





Check with Jaycar, but I think this is correct.
 

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yep your Ohm's Law is spot on.


But you're not trying to put the whole 12V thru the resistor, you only want the difference... 12V - 1.5V = 10.5V. Close enough though.


However you also have to consider the 'Power' to be dissipated by the resistor at your chosen current - that is how you choose which model resistor to use.


P = V x I


= 12V x 3A = 36W


40W is quite a bit of power to dissipate - imagine how hot a 60W lightbulb gets!!!You're gunna have to rig up a pretty large bank of resistors (max is 10W on ceramics usually) onto a large heatsink.


And that's just for the glowplug...






Nah, there's more elegant ways of designing a powerpanel circuit. Do a search on the web, there's quite a few to choose from out there.
 

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The other thing youcould do isget a 5A adjustable voltage reg and set it to 7.2V. This will work, as long as you starter doesn't draw more than 5A, and you don't adjust the trim once its set. A good idea would be to put a small volt meter pannel on the output so that you can see whats going on.


Using one of these would be handy as you can adjust the output to be used for whatever you need. Sorta like you own mobile electronic testing station.


Here is one from National Semiconductors:


http://www.national.com/ds/LM/LM1084.pdf


You may even be able to order free samples if you want to wait for them to arrive.


Hope this helps.
 

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I'd be very hesitant about using any TO-220 regulator chips for anything to do with motors. Best only use them for powering eletronic devices where the draw is known & regular. Motors are too wild for that, need a lot more overhead.


A 540 can draw about 2A just free-running.


Put load on it... 50-70A is not uncommon for short periods (race ESCs are rated up to 400A+). The closer you get to stall speed (stall = max torque) the more the motor draws, that's when it works the hardest.





A rotostart is like a cordless drill, right? It doesn't get a chance to spin up then engage via a clutch; its just directly geared to the output.
 

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Discussion Starter · #7 ·
Zoomer,


You are spot on. Hit the buttom and it is direct drive into the engine backplate, and therefore to the engine.


This is the motor inside of a rotostart:





Don't know if it looks familiar to you?


Regards,


Chris
 

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Hey, that PLUG looks familiar, where have I seen it from before...






If the pinion can be taken off, recycle it into your TLT-1.



Put a high-torque low rev unit in that will run off 12V... you can find 540s rated for 24V too but they're a bit rarer than the usual 12V beast.
 
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