4S MC-E direct drive on 12 NiMH cells

[marcopolo]

Is it a bit too risky? (single MC-E 4 dice wired series)

Fully charged voltage 12S-NiMH on the pack will be 16.8v for a few moments after power is applied, which will give 4.2 Vf per LED for a few seconds after the power goes on .Maybe in excess of 1500mA.

The voltage will quickly drop to around 15v say Vf 3.75 which would give around 1000mA for a further 10 minutes or so.

Voltage levels off for much of the rest of the discharge at 14.6 - 14.2 and spends most of it's time in this range. This should be 3.6V giving around 750mA.

I would guess some decent heatsinking is in order. It's just the first part of the dicharge cycle i'm worried about. Would a zener (decent wattage)across the load be neccesary?

any thoughts appreciated.

Marco.

 

[chimo]

My gut feel is that it would be too high.

However, if you really want to know, you will have to test your serial connected MC-E by driving it at the desired current and measuring the voltage drop across it. From that point you can determine the amount of voltage you will need to drop from your battery pack and insert the appropriate resistor to achieve it.

R = V(drop required)/I(desired current)

Wattage value needed for resistor = I*I*R (round up)



For example (don't use these figures):

Let's say your MC-E settled in at about 3.3V/die @ 750mA for an over all drop of 13.2V.

Your nominal battery voltage is around 1.2V/cell for a pack voltage of 14.4V

You would need to drop 1.2V at current of 750mA. The resistance needed to do that is 1.6 ohms.

The resistor should be able to handle (0.75*0.75*1.6) = 0.9 Watts.

You should round up, and you should also check that the max current (batteries fresh off the charger) is not excessive.

Good luck!

 

[marcopolo]

Thanks fo the info. Back to the drawing board!.

 

[chimo]

No problem.

I just checked one of my MC-Es. I measured 12.8V @ 750mA. (~3.2V per die).

 

[marcopolo]

Hmmmm 12.8v, 3.2v Vf per die, I might just order another Flex driver! It's quite a bit lower than 4 XR-E's.