LEDs & Power
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Connecting the transformer to the TC420

Firstly, I'd like to define what I'm talking about in this section, as this device seems to be called by many names on eBay and when people ask me questions. All the names are correct, though each implies a slight technical difference.

  • Power Supply Unit or PSU for short is a general term that means a device that is used to supply power to another device, for example a PC. The way it supplies power can vary from batteries to, in our case, mains voltage.
  • Transformer is a term for a PSU the converts one voltage to another, for example mains 110/230 volts to 12 volts via an electromagnetic inductor and windings. With our transformers, the current is also rectified from AC to DC.
  • Drivers are not a power supplying device in themselves, they are current management circuits. Sometimes drivers are built in to transformers and sold as Drivers or Transformer/Drivers. So there is not confusion, I will never talk about drivers in the context of a power supply.

To be clear, if I talk about transformers or power supplys/PSUs, I am referring to the device you plug in the wall that outputs a constant DC voltage. For LED strips, this will be 12v.
If I talk about drivers, I'm meaning actual driver circuits used to provide a constant current for high power LEDs.

Selecting the right transformer and setting it up

The transformer connects to the input v+ and v- terminals on the right side of the controller. It supplies power to both the TC420 and the LEDs, so you must get a transformer with enough power for these devices. Ideally, you should get a power supply with a regulated voltage, so no matter what the current draw, it will be a consistent 12 volts.

In the previous sections, I showed you how to calculate the amps for your LEDS. To calculate the size transformer you need, you must add the amps for all 5 channels together and add 10% as a safety margin. This will give you the smallest transformer size in amps. In the following example, I'll be using the following amp values for channels 1 to 4: 2, 1.5, 0.6, and 1.1.

amps = (2 + 1.5 + 0.6 + 1.1) x 1.1
amps = 5.2 x 1.1 (Multiplying by 1.1 adds 10% to the amps)
amps = 5.72

So we would need a 12v transformer rated at 5.72 amps or above. If the transformers are sized in watts, we must multiply the amps by the voltage.

watts = amps x volts
watts = 5.72 x 12
watts = 68.64

So we would need a transformer rated at 70 watts or above.

You can calculate your transformer using this calculator

amps watts

The power supply needs to be rated for at least 0 watts or 0 amps.

Questions - Drivers

Coming soon...

Questions - Lights off

I've been asked about power supplys, what happens to them when the lights are off, and if it's a good idea to have the power supply on a wall timer; the idea so that the power supply is off when the lights are off and rely on the TC420's battery backed up memory to keep time. First I'll start with is it possible, then I'll tackle is it worth it.

Is it possible?

I'm going to start with the battery backup, as if this can't keep time for the hours that the power is off, then this whole setup is doomed to failure. The average aquarium light is on for 6-10 hours a day, depending on what you are keeping. For testing the extremes of the battery, we'll go for a 6 hour set-up which would mean our PSU is off for 18 hours, so that will be the goal for my test. So far, I've had a TC420 disconnected from a power supply for 12 hours and it was fine. I will test out the full 18 hours when I get a new TC420 in a few weeks time.

I'm fairly confident that it will easily last the 18 hours. It most likely uses circuitry similar to that of an electronic wall timer, and I've had those things go without power for months and still be correct within an acceptable margin. My biggest concern would be the longevity of this, and if the battery would be happy with daily charging and discharging like that. However, for now I'm going to say yes, this is possible.

Is it worth it?

To me, this is the more interesting part of the question, and this time I'm pretty sure the answer is going to be no. Power supplys do not constantly produce the levels on the casing, that is their maximum level and will only supply what is being asked of it, plus about 10% which is lost in the efficiency of the transformer its self. When no current is being drawn, a power supply should only be ticking over at around 0.25 watts; the more efficient the power supply, the less it will be.

Test plan

I am going to measure the watts of an unconnected power supply, then connect only the TC420 to see how much that draws. This will give us an approximate figure for the TC420 in, what I'd call, standby mode, which is when it's connected but no LEDs are on. Once we have this figure, I'll measure some normal wall timers and see how many watts they use. In the case of a mechanical wall timer, it has to operate a motor to turn the time dial, and a digital timer is likely to have a circuit very similar to what is in the TC420.

The questions are:

  • how will a TC420 in standby mode's power consumption compare to a normal wall timer?
  • Will it be enough of a power difference to make any significant financial difference?
I'll get these answers here soon, so stay tuned.