Trakmate IR diode Voltage

[keven]

I bought an IR diode kit from TrackMate a while ago.
it came with four IR diodes and a Resistor (100 ohm).
I may have had a schematic on how it was to be wired, but I must have misplaced it?
I'm pretty sure it was to be wired with the provided resistor in series with the four IR diodes.
Just not sure what DC voltage this was to work at?

[keven]

Got an answer from Trackmate:

12-volts

[jmacartney]

I admit to "borrowing" this image from Steve Medanic's Siberia Racing tech pages.

IR schematic.jpg

[keven]

In the case of the trackmate DIY kit components, the current draw is 120 ma…
resistor is 100 ohms and dc voltage of 12 volts.

[Maddman]

A current draw of 120ma indicates to me that the LEDs are wired in parallel and not in series as shown in Jmacrtney's post.

[Axle]

In the case of the trackmate DIY kit components, the current draw is 120 ma…
resistor is 100 ohms and dc voltage of 12 volts.

Where did you get 120mA from? If you had only a 100 ohm resistor (no LEDs) connected to a 12V supply, that would pass 120 mA (12V / 100 ohm = 0.120A)

But 120mA through a single 100 ohm resistor would burn 1.44 W in the resistor. That seems excessive. How big is the resistor?

The diagram posted by jmacartney seems like the correct way to wire the LEDs if you were only provided a single resistor with the kit.

With all the devices wired in series (4 LED + resistor), and assuming roughly 1.5V drop per LED, the overall current would be around 60mA at 12V. The LEDs might be ok with that.

Also, 60mA through the 100 ohm resistor would be 0.36W for the resistor, so a common 1/2W resistor would be ok here.

If the LEDs were wired in parallel, it would be best to include an individual resistor with each LED, since LEDs wired directly in parallel do not share current evenly.

[Maddman]

At operating current of 120MA would require four diodes and four resistors. Each string would consist of one diode and one resistors. The four strings would be wired in parallel. Each diode would be operating at a current of 30ma. I feel that this current is excessive. I typically run my LEDs at 10ma or less. I also like to build my strings with the LEDs wired in series as shown on my Siberia Racing page. To provide redundancy I typically wire two strings in parallel so that each lane has two LEDs dedicated to it.

[RichD]

I looked up the specs for IR LEDs and found that the operating voltage was 1.2 and the maximum current is 30mA. If you look here and plug in those numbers: LED Resistor Calculator (hebeiltd.com.cn)
with four LEDs in series you would need a 270 ohm resistor with a .36 watt rating and with the LEDs in parallel you would need a 100 ohm resistor with a 2.16 watt rating. With the resistor that was supplied the LEDs should be wired in parallel. Be aware that with the LEDs in parallel if one should fail the others would see a higher voltage and could burn out. If the LEDS are wired in series and one failed they would all go out and you would need to figure out which was the bad one. If I was building a setup from scratch I would have the LEDs in parallel each with its own 390 ohm resistor.

[keven]

trackmate IR LED schematic.jpg
this is how trakemate told me to wire their kit.

[Maddman]

Assuming a 12V power supply and a 2.2V forward voltage drop for each diode the TrakMate light bridge circuit will draw approximately 30ma. Per my spreadsheet the resistor will dissipate approximately 0.1 Watt. If you are building your own light bridge you might want to check out this Tech Page on my website: http://siberiaracing.altervista.org/...ech_Bridge.htm