January 22, 2009
about 2 years ago
I bought a couple of these to put between an arduino and some 12V automotive relays. Unfortunately they only are rated for up to 50mA/ch and the coil current of my relay is 75mA so I bought some of the next step up in the series the PS2502-4’s from digikey that can handle 160mA/ch.
I’m worried about the power dissipation that is 120mW/ch on both devices. The relay is going to draw 900mW (75mA * 12V) but to calculate the power dissipation of this device I just need to take Vce(sat) which is .3V on the PS2501-4 and 1V on the PS2402-4 and multiply it by Ic of 75mA right? So with the PS2402-4 I would have 75mA * 1V = 75mW and be well within the 120mW / channel rating right?
Is my reasoning here right? Is Ic just the current through the relay like I assumed? Is there any Vbe that I need to worry about (Not sure what this would be with a photo transistor).
Is it ok to drive the relay directly with the opto-isolator or should I just use it for protection and use it to turn on something like a TIP120 instead?
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