Voltage help

Technical discussion on converting internal combustion to electric
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Canberra32
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Voltage help

Post by Canberra32 » Thu, 13 Dec 2012, 03:11

Hey guys do any of you know an easy way to step a 36v pack down to 24v?
I have been playing with a solar charge regulator to do it but I'm not sure it's the best way to go.

Any ideas?

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jonescg
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Post by jonescg » Thu, 13 Dec 2012, 03:43

Depends on the load, but a big resistor would work ;)
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Post by zeva » Thu, 13 Dec 2012, 04:33

How much power are we talking? A DC/DC converter is the usual solution, but can be expensive.

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acmotor
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Post by acmotor » Thu, 13 Dec 2012, 05:53

Many lower cost solar controllers are just PWM voltage regulators (DC buck converters) and are fine for a 36V to 24V stepdown. (and cheap)
The issue may come that many solar controllers do not have current limiting. They rely on the PV to limit the source current.
If your load does not self limit its current then the solar controller may get overloaded if running from a 36V battery.
Most solar controllers have no input/output isolation like HF transformer based DC-DCs. This may or may not be important to your application.

Also, the 24V will likely be 27.2V and may include a battery charge profile to 29V. If this is OK for your load then fine.

Full MPPT (maximum power point tracking) solar controllers will also work though money would be wasted on their higher cost in your application. Once again, they may not have current limiting so your load would have to have its own safe current limit.

Are you charging a 24V battery or running 24V devices / circuits ?
You may need a full DC-DC converter if isolation between supplies is required. Image
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Canberra32
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Post by Canberra32 » Thu, 13 Dec 2012, 10:54

Well I'm not talking much power at all.
It's running a 24v 400w brushless motor that only goes 2800rpm
I don't want it to be able to draw more than 20amps and in fact it would be great if it could limit the amps.
But also I don't want it to be overly expensive or weigh much.

....and if it could lay golden eggs too lol.

Just trying to find a way or ill have to just switch to a 24v battery it's just a shame because there is a battery shape and size that I'm trying to stick to

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Post by Tritium_James » Thu, 13 Dec 2012, 23:58

If the motor is run from some kind of motor controller (eg it's a BLDC motor) then if the controller can deal with it, just run it from 36V. Save the inefficiency, cost and weight of the DC/DC.

The thing that kills motors is either heat (too much current) or speed (too much voltage). But using a higher voltage to run a motor, and then not taking it to a higher speed than it's meant to go won't cause any problems. eg the motor can go to 2800rpm from 24V, then it would be able to get to 4200rpm from 36V. But as long as you don't spin it above 2800rpm, then running from the higher voltage will have basically zero effect on the motor.

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Post by acmotor » Fri, 14 Dec 2012, 01:28

Yep, that is the way to go.
Likely the existing BLDC controller can go to 36V battery pack anyway. Check it out.
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Canberra32
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Post by Canberra32 » Fri, 14 Dec 2012, 04:11

Huhh... Well that really fixes the do I don't I struggle I had today lol
Was just going to try the hook it up and see method :)

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Post by PlanB » Fri, 14 Dec 2012, 16:48

Speaking of voltage I'm having trouble finding a 3.3v 50mA reg that can handle the ~55v from a 48v fully charged scooter battery. A non linear half watter would be nice with a 60v input rating. Any suggestions?

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Post by Johny » Fri, 14 Dec 2012, 17:56

eBay item 280779490666

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Post by Johny » Fri, 14 Dec 2012, 17:59

Here's a smaller one - and cheaper.
eBay item 300810925667

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Post by PlanB » Sun, 16 Dec 2012, 03:35

You got good search skills chief, I must have spent half an hour on ebay with those LM2596 based cards & all mine had 35v caps on the input. Well done & thanks, I've ordered a couple.

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