RCCrawler Forums - Power the Traxxas Rock Lights direct from rx

Electrician by trade here. It's taken me a little time to remember some bits about Ohms law and series circuits to get back to you, but I think I can help. It's important that your able to understand everything I'm about to lay out exactly. It takes a shockingly small amount of current or amperage to do damage to or kill a person, so if anything is unclear get clarification before continuing to wire this up. Danger warnings aside, lets begin.

We can figure the theoretical max resistance of all the lights Traxxas runs off their power supply using Ohms Law. We're solving for resistance, so R=V (volts)/I (amps). They state it runs 3v and .5A, so our equation looks like: 3/.5=6 ohms. Remember, that's the maximum resistance that any of the lights could be.

How do we know that? If we overloaded the power supply, the lights wouldn't work obviously. But there's another electrical rule at play here, Parallel circuits. For those lights to all glow evenly, they all need 3 volts. So the power supply is set up to supply all power to each load at once, instead of bringing the current flow through the light bar, then the body lights, etc. https://sub.allaboutcircuits.com/images/00086.png Its set up something like this. So each light set gets 3 volts, but they can only use a portion of the available .5 amps.

Also in parallel circuits, the more parallel resisters (in this case, led light sets) are in a circuit, the lower the total resistance of the circuit. It makes sense, there are more paths for the current to take, so it has an easier time flowing. That means your light bar can't be more than 6 ohms resistance.

Now we need to find the right sized resistor to hook up your lights and not fry them.

I don't remember if you said you're using the stock Traxxas ESC or a replacement, so I don't know how many volts or amps your receiver is getting. I'll calculate for the Traxxas ESC and show my work; if you don't use the stock ESC or use a BEC, you should be able to plug the right numbers in and get the answers you'd need. I already did the research on the stock ESC while looking into replacing my steering servo. "thumbsup" The stock setup supplies the receiver with 6V,1A; exactly half the light power supply is rated for.

Realize now that this is another parallel circuit scenario, so all the servos and the receiver share that 1 amp to function. So then lets not worry about getting .5 amps. (If it was designed well, Traxxas should have the light power supply be able to supply more amperage than the lights would need anyway.) Instead, lets get 3 volts for the light bar. To do that, we need to know about Series circuits.

Unlike Parallel circuits where voltage is a constant and amperage is dropped across loads, in Series circuits amperage is a constant and voltage drops. We can use this to get the voltage right for your light bar, but it will mean cutting one of the leads and soldering in a resistor.

To figure this at 6v is pretty easy. In any series circuit, the voltage dropped across all the loads adds to the total voltage of the circuit. We've got two loads, the first is our LED bar and the second is our mystery resistor. To split the voltage in half, we'd need a resistor that matched our LED bar as closely as possible, so a 6 ohm resistor. Easy right?

But I promised to show work, so let's look at a 7.4 volt situation. In that case we know we want 3 volts at our light bar, which should have 6 ohms resistance and be flowing .5 amps. From that we know two things about the mystery resistor: 1) we want it to take the remaining 4.4 volts from the supply, and 2) since it's in a series circuit, it's going to be using the same .5 amps as the LED bar. So we solve for resistance again: 4.4v/0.5a = 8.8 ohms resistance. So that resistor would need to be 8.8 ohms if it's getting 7.4 volts from the receiver.

WAIT!! We're not done yet! There's two more things we need to check: How much wattage will the resistors need to dissipate, and is our solution realistic?

Electricity is always trying to do work, so the voltage on our mystery resistor isn't just disappearing. The resistor is using that voltage to produce heat, which we can calculate as watts (P). If our resistors aren't able to dissipate all theheat they create the will (literally) burn up and fail. I'm going to use the best known numbers we have to figure the wattage our resistors will be creating, the volts we're trying to apply to them and the resistance itself. The equation is: watts (P) = V^2 (Voltage squared)/ R (ohms). For our 6v resistor, that looks like: 3^2/6 = 1.5 watts. For our 7.4v resistor, its: 4.4^2/8.8 = 2.2 watts.

The last thing to check is the realism of our solution. If we use the 7.4v solution, we should be ok. Pretty much any BEC or ESC that could supply that voltage could spare the .5a we figured we'll need. But the stock 6v solution is likely to cause problems. The stock BEC only gives us 1a after all, so this lighting setup would starve the receiver and servos of power and cause radio glitches, poor servo performance, maybe even ruin a servo. Basically the stock BEC is weak and easily overloaded, which is probably why there's a separate light power supply to begin with. :roll:

The stock thing is still possible, IF the resistance of the LED bar is different from what I calculated here. If it's got a lower resistance, it could use less amperage and then the whole thing is workable again. We'd need to plug our new numbers into all the equations again, but it's still plausible.

I hope this novel of a post helps.