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protection for chip

I need to protect a 555 timer circuit that I am going to install on my motorcycle. I'm not using a voltage regulator with this circuit. It will have to run at the vehicle system voltage. This is because the circuit uses a couple of constant current sources that require a certain amount of headroom. I tried a scheme using a couple of common diodes on the power line (one diode placed across the line followed by a diode in series) and a zener across the chip's power pins, all to no avail. I fried two chips. What kind of buffering or protection can I use to block the electrical transients? I am thinking of an RC filter next. I don't think this circuit draws more than about 15 mA, so if I use a resistor of a few dozen ohms it won't drop much voltage. What kind of capacitor should I use? Also looking for other ideas.

Reply to
kell
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I'd use a series diode followed by a capacitor or two across the rail. A couple of hundred uF electrolytic in parallel with a couple of hundred nF ceramic should do. If that's not sufficient then add a suitably rated one of these

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If you can tolerate the volt drop then a resistor in series with the diode may help. If you remove the diode then your circuit may go haywire when you start the vehicle.

Reply to
Edward Rawde

I think you were on the right lines with the diode and the zener. The problem is that the resistance of the vehicle power supply is very low, so when the zener tries to clamp the voltage it just cannot sink enough current. This is easily fixed by adding a resistor before the diodes.

You may find this application note useful:

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Reply to
Gareth

Here's how I solved the problem: Today I remembered that in my box of stuff I have some tiny 1mH chokes (they look about like 2 watt resistors). So I used the choke, changed out the old 1 watt zener for a much fatter one, and a ceramic bypass cap. Works now.

Reply to
kell

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