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View Full Version : How much current can I pull from the Transporter IR output?

cliveb
2007-11-09, 06:33
I'm planning to use the PowerSwitchII plugin, and am thinking of using a solid state relay as the switching device. It pulls about 15mA. Is the Transporter's IR output able to supply that much current?

Since it's primarily designed to be used with a device that has to run an IR LED, I imagine it should be able to supply that much, but just wanted to double-check.

fcm4711
2007-11-09, 09:06
Hello cliveb

The output voltage is 3.3 volts and there is a resistor of 100 ohms to protect the output, so I guess it depends on what internal resistor your solid state relais has.

Measuring current over a 150 ohm resistor as load results in about 12 mA.

Cheers
Felix

cliveb
2007-11-09, 09:30
The output voltage is 3.3 volts and there is a resistor of 100 ohms to protect the output, so I guess it depends on what internal resistor your solid state relais has.
Thanks Felix. The SSR's input impedance is 300 ohms. (So in fact at 3.3V it pulls 11mA - the 15mA spec is presumably for a nominal 5V operating voltage). So that gives a total load of 400 ohms, which at 3.3V I make about 8mA. So it looks like it won't do the job.

Guess I'll have to fall back on building your standard circuit that uses a transistor and a PSU to switch a traditional power relay. Shame - I was hoping a SSR would be a neater (and cheaper!) solution.

But now I'm confused. This output is designed to drive an IR Blaster, right? And that means it needs to be able to drive a LED. Doesn't a LED typically need about 20mA to operate?

Measuring current over a 150 ohm resistor as load results in about 12 mA.
Isn't 3.3V into 150ohms 22mA? Was that just a typo? Or am I suffering brain-fade?

EDIT: yes, I'm suffering from brain-fade. I now see that you're saying you placed a 150 ohm resistor across the output and measured 12mA current. Adding in the 100 ohm internal resistor, we have a total load of 250 ohms, which at 3.3V gives us 13mA. So that all seems to be within the bounds of sample variation.

cliveb
2007-11-09, 11:28
Just to wrap this up: I tested the TP's IR output with a 330 ohm load, and the voltage dropped to 2.48V, which is not enough to switch the solid state relay. So I'm definitely going to have to build the "standard" PowerSwitchII circuit with a transistor and PSU. Bah!

Hering
2008-01-05, 15:22
Hi!

I use a opto-coupled triac (BRT13H) to switch a relay. The relay has a 220V coil. So, no low voltage supply is needed.

I've put a 560Ohm resistor in series with the optocouplers LED. The triac can handle enough voltage and current to drive the coil of the relay. A glow lamp parallel with the coil protects the triac from overvoltage and indicates the relay status.

Cheers,
Hering