Resistance Question

[holdthat1]

Im in the process of planning out my next build (SSC P7) but I kind of confused in some areas.

I plan on using about 2 feet of teflon wire (Trail Tech Coaxis from battery space). Will the length of this wire add a lot of resistance? Im debating whether too dd the p7 or use a driver.

http://img836.imageshack.us/i/ttmale6.jpg/


This cable will be attached to the battery pack. 3 d NIMH batteries




This cable would be attached to the back of the mag





Finally, this cable would be in the middle conecting the too. According to batteryspace.com, the switch would consume .14 volts.

If I chose to Direct Drive/Regulation the P7, which bin should I use DSWOJ or DSWOI. Would I need to use 4 D batteries if the length of the cable added too much resistance?

Any help would be greatly appreciated.


-Richard

 

[DIWdiver]

Yes, all that cable will add a significant amount of resistance, which will limit the current you can get with DD. I could estimate the resistance if I knew the wire sizes and lengths.

With 3 cells you could use a boost type regulator, but you may not be able to get low modes depending on the regulator, when the input voltage is higher than the LED voltage at low current.

With 4 cells you could use a buck type regulator, which is more common and generally more efficient, and you wouldn't have any issues with modes working properly. You might not get full brightness as the batteries are nearing end of discharge. Be careful to watch the minimum input voltage of the regulator. Some are higher than the 4.0V that's your min battery voltage at end of discharge.

D

 

[holdthat1]

Yes, all that cable will add a significant amount of resistance, which will limit the current you can get with DD. I could estimate the resistance if I knew the wire sizes and lengths.

With 3 cells you could use a boost type regulator, but you may not be able to get low modes depending on the regulator, when the input voltage is higher than the LED voltage at low current.

With 4 cells you could use a buck type regulator, which is more common and generally more efficient, and you wouldn't have any issues with modes working properly. You might not get full brightness as the batteries are nearing end of discharge. Be careful to watch the minimum input voltage of the regulator. Some are higher than the 4.0V that's your min battery voltage at end of discharge.

D

That total Length of the cable is about 2' and 18 awgExtruded TFE Teflon. Sounds like I'll have to use 4 d's. Will the driver from shinginbeam work well with this build?

 

[holdthat1]

bump?

 

[holdthat1]

Since the amount of wire will add a bit of resistance, I could remove the cable that has the switch and use the two cables with the connectors. I would dd the light with 3 D batteries or 4 if needed. I could plug the cables together when I wanted to turn my light on then uplug the cables to turn the light off. Now with 1 foot of teflon wire, will this cable lenght still add alot of resistance? What other options do I have?


Richard

 

[DIWdiver]

Since the amount of wire will add a bit of resistance, I could remove the cable that has the switch and use the two cables with the connectors. I would dd the light with 3 D batteries or 4 if needed. I could plug the cables together when I wanted to turn my light on then uplug the cables to turn the light off. Now with 1 foot of teflon wire, will this cable lenght still add alot of resistance? What other options do I have?


Richard

I've been offline for a couple days, sorry.

You can't DD with 4 cells, unless you have HUGE resistances. Well, huge compared to DD builds. With half an ohm you'd get around 2 amps, but that's pretty high resistance for wires and connectors.

Are you sure that's 18 ga? Looks much smaller, but pictures can be deceiving. If it is really 18 ga, it's about 0.00625 ohms per foot per conductor, or about .025 ohms for two feet of wire each direction. At 2.8A you'd lose about 0.07V. That's not really a lot, so maybe DD is okay with this wire.

I've never been a fan of DD for LEDs because the light output depends on the length of wires, quality of switches and connectors, and the condition of the batteries.

The shiningbeam driver looks like it might work pretty well with 4 cells, but it will get pretty hot without heatsinking, so you'll have to pay attention to heatsinking it if you want to run on high.

D

 

[holdthat1]

I've been offline for a couple days, sorry.

You can't DD with 4 cells, unless you have HUGE resistances. Well, huge compared to DD builds. With half an ohm you'd get around 2 amps, but that's pretty high resistance for wires and connectors.

Are you sure that's 18 ga? Looks much smaller, but pictures can be deceiving. If it is really 18 ga, it's about 0.00625 ohms per foot per conductor, or about .025 ohms for two feet of wire each direction. At 2.8A you'd lose about 0.07V. That's not really a lot, so maybe DD is okay with this wire.

I've never been a fan of DD for LEDs because the light output depends on the length of wires, quality of switches and connectors, and the condition of the batteries.

The shiningbeam driver looks like it might work pretty well with 4 cells, but it will get pretty hot without heatsinking, so you'll have to pay attention to heatsinking it if you want to run on high.

D

Thank you for all of your help. You definetly cleared up some things for me. Now that we have established a Direct Drive would be the best route, which bin should I use I or J?

 

[DIWdiver]

Thank you for all of your help. You definetly cleared up some things for me. Now that we have established a Direct Drive would be the best route, which bin should I use I or J?

I had to look up what that means. That's the voltage bin, with I being 3.25-3.5V and J being 3.5-3.75V. For this application your voltage will be 3.0-3.6V, minus some losses in wiring.

So if you use the J bin it is likely that you won't have enough voltage to drive the LED to its full capacity even when the batteries are fully charged, no matter how low you can get the resistance of your wiring, switches, etc. But if you use the I bin, there's some possibility of overdriving the LED and reducing its life.

Given those choices, I'd go for the I bin and make sure I have really good heatsinking, and not go all out for the lowest possible resistance.

D