74HCT154 sink power limit ?

Hi,

Part of a project I'm working on involves lighting crackers and fireworks from the Arduino. I use a 74HCT154 to select which output I should trigger. Basically, "lighting" means sending unrestricted current from a battery to a 10ohm resistor, which, of course, instantly burns.

I can do this by just connecting an array of reed relays to the outputs of the 74HCT154, but I thought I could maybe avoid such a wirework. Since the enabled output of the 74HCT154 goes to LOW state, I thought I could use only one "general" relay to the battery, then simply sink current from the selected output through the 154.

But I haven't been able to find the maximum current it can sink without damages (For the HCT family. It's available for the HCU). The triggering involves relatively high current (I assume a 1/4W resistor will require a few watts to burn *), but during a very short amount of time (My experience with electrical detonators showed that the firework start and the circuit break were almost instantaneous)

Thanks

  • This leads me to another question : Is there a way to know of the maximum current a circuit used ? Common multimeters' can't do such things, since their update frequency is slow (a few measurements/second)

It's 25mA output sink current. This isn't enough for what you want, you want to use a FET or motor driver to boost the current capability. I used to use the bulb from a flash light for this sort of think. I would remove the glass by tightening in slowly in a vice until it broke. This only required about 300mA to light powder.

Measuring fast changing current is normally done by putting a small resistor in line and measuring the voltage across it with an oscilloscope. This is fine if your scope has a differential input capability or one end of the resistor is at earth potential. If you need to measure the current in a line at a higher potential then connecting the earth to one end will short out your circuit. The old trick of removing the earth from the mains lead of the oscilloscope is frowned upon in these safety conscious days, your better off getting a better scope. For high currents you can get a hall effect current probe but those are a bit pricey.

The flash bulb is a good idea, but going to be a bit expensive, I'll stick to the resistor option... I don't know how much current they need to burn, indeed, as scopes are still a bit expensive for me. I'm going to try with FETs and find the right value by experimenting.

Thank you very much for this very precise and clear answer.

OK
have a look at what the model rocket community uses, I think their igniters use about 3A, make sure your FET can cope with this on the peak current (most largish ones can).