protection for chip

[kell]

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.

 

[Edward Rawde]

"kell" <kellrobinson@billburg.com> wrote in message
news:1124660684.718703.7180@g47g2000cwa.googlegroups.com...

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.

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
http://www.google.com/search?&q=transil

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.

 

[Gareth]

kell wrote:

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 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:

http://www.st.com/stonline/products/literature/an/3584.pdf

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.

--
-----------------------------------------------------------------------
To reply to me directly:

Replace privacy.net with: totalise DOT co DOT uk and replace me with
gareth.harris

 

[kell]

Gareth wrote:

kell wrote:
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 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:

http://www.st.com/stonline/products/literature/an/3584.pdf


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.

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.