SN7407: Not getting the proper output

Hi Dinesh,

Currently, your circuit is connected like this:

As you can see, there's a voltage divider between the 10kohm and 1.6kohm (coil) resistances, so when the open-collector output is off (no current flows into the driver), then you will expect to see about 3.3V at the output. (Vout = 24 * 1.6k / (10k + 1.6k) = 3.31V)

Additionally, this circuit is _always_ using up power at the output -- either current is flowing through the coil, or current is flowing through the driver, but either way the 10kohm resistor is using up power.  You _could_ make this work by reducing the 10kohm resistor to a much lower value, thus increasing the current flowing -- but you might not be able to get enough current this way.

A better solution would be to change your configuration to this:

In this configuration, when the driver is off, then the relay coil is also off and there is no power consumption from this part of the circuit.  When the output is on, then it sinks current and the coil works as expected (~15mA of current through the coil).

Hi Dinesh,

TI doesn’t offer discrete diodes, so I can’t really suggest a particular device.

I don’t see a problem with your original selection – I just moved the coil + diode in the circuit. You can probably get away with a much smaller diode -- it's only going to be seeing about 15mA of current through it and only needs to protect against 24V reverse voltage.

Hey Dinesh,

You may want to post these kinds of questions to a forum with more of a DIY type base (DIY Electronics on Reddit or the Electronics Stack Exchange are quite good in my experience) -- this forum is really dedicated to helping engineers developing products with TI's parts who encounter challenges. Not that I mind helping, but I would give quite different advice if I weren't a TI employee and speaking on behalf our company, if that makes sense. I can't really be seen to recommend other company's products or provide advice beyond the products I support.

That being said, you can connect an LED + series resistor directly to the output of our device. If the comments I read on Amazon were accurate, then the LED you are using already includes a resistor, so you don't need the 10k resistors. You'd just hook the cathode of the LED module to +24V, then the anode to the output of the '07.

* Don't take my word for it though -- check into the datasheet for the LEDs to be sure they won't burn up if connected directly to +24V. *

Hey Dinesh,

I thought of something that might help.

This is the appropriate circuit to drive an LED from an open-collector output:

You need two pieces of information to determine your R value, which are the forward current and forward voltage of your LED.

Using KVL you get this equation: 24 = V_OL + V_LED + V_R

And from Ohm's law (and KCL) we know that V_R = I_LED * R

You can rearrange to solve for R, but in your case I'm betting that the LED housing already includes a resistor, so all you really need to do is connect it as shown above and remove that 10kohm resistor.