Caps and FETs

[Ajj]

Hi Guys,

How do I calculate/determine the optimum value for external caps in a set up?

I also want to replace the caps and FETs in an ESC.  For the caps, again, how do I work out the optimum value per cap required?
With regards to the FET, same again, how do I determine optimum FETs. 
What is the hierarchy of attributes of component, caps and FETs, such as operating temp, ripple, current, etc.
Basically I want to replace mentioned components with better ones.

Looking forwards to responses.
Cheers, Ajj

[ziggy12345]

Hi Ajj

Stu Much will talk you through external capacitor banks for your car. I recently looked at external FETS and saw that a German guy built a very high poower ESC using a low power backbone and loads of external FETS. After research I found that the ESC makers have figured it all put so best to just buy a high power ESC.

That said I'm not an electronics guy so if you managed to add power to your ESC using external FETS I would be interested to see what you did

Best of luck

[Ajj]

Hi ziggy, the ESC has a high rating.  I wanted to see how much better the ESC would perform and try reduce ESC failure.

High amps aside, restrictors have been brought to my attention.  Whether or not the ESC could handle the motor.

Cheers, Ajj

[ziggy12345]

For motor, ESC and cells you need to go big, bigger, biggest..lol. so motor will be rated least power with ESC rated above this and cells rated above the ESC

[Ajj]

LOL, I'm not sure the ESC is going to fit - it's a bit big and heavy.  Still going to take a punt on it, as it's affordable.

So I brought some lipos for another car and went for the lightest packs, which were 3900mAh.
Can I use such low capacity or is there a minimum requirement?

[StuMutch]

Hi Ajj and welcome,

I cant comment on the FET's I'm afraid. But caps yes.
On the voltage try and give yourself 20-30% headroom, this will allow for any voltage spikes which you can occasionally get.
Go for several smaller caps rather than 1 large one. It helps with the ESR and also helps the caps to dissipate any heat more efficiently.
Try to use 8-10 AWG for the power as it will be better for resistance, and help the caps do their job.
16-18 mm diameter work great, the most important thing when selecting a cap is the ESR (equivalent series resistance) this is effectively the speed that a cap can react to any kind of ripple, get the lowest you can find.

I am happy to send you a couple of free 6 pot PCB's so that you can mess around at your leisure.
uf can sometimes be space dependant, but please PM me regarding uf.

Hope this helps

Stu

[Ajj]

Hi Stu and thanks for the welcome/info.

Thanks for the offer of boards, funnily enough I was having trouble finding suitable items.
I have so many questions with regards to electrics as they have changed so much since I last played and even back then I favoured nitro.

I have a question on batteries.  Does battery capacity have an effect on getting the most out of the esc/motor, is there a recommended minimum or do you just buy the lightest packs?

[ziggy12345]

A larger MAH will deliver more amps so yes

[Ajj]

so there's a balance here with regards to weight and power.
how do you determine the best capacity for the drive?

[ziggy12345]

Almost always the cars are way over powered. I'll post later

[StuMutch]

Go for high C rated packs, and high mah. A good high mah pack will also have less voltage drop.
Graphene cells seem to be very good at the moment.
Try not to get too hung up on weight at this point. The fastest 808 car weighs 12 pounds.

[Ajj]

Wow that's some weight.

Ok, so what's the maths to determine optimum capacity for current draw?
Or could you point me in the direction for further reading?