voltage clamp 1

[perts]

I have battery of 12v and i need to clamp the voltage so that it doesnt go above max 16V because of a jump start.

how would i go about handling this.

thanks

 

[nbucska]

How much current the circuit uses? (max/min )

<nbucska@pcperipherals DOT com> subj: eng-tips

 

[perts]

about 25 ma

 

[nbucska]

What is the lowest voltage your circuit can tolerate?


<nbucska@pcperipherals DOT com> subj: eng-tips

 

[perts]

it needs 12 v to run

the voltage cant go above 16

 

[nbucska]

I assume this is a mobile application, so the battery
undergoes discharging and charging.

In this case I suggest making a switching regulator which
can tolerate large input variation.


<nbucska@pcperipherals DOT com> subj: eng-tips

 

[perts]

cant i just put a 16 v zener from battery to ground

it will clamp it to 16 if batt goes up 16v

 

[buzzp]

What are you trying to protect?
There are many components that would not allow the voltage to go above 16V. If this is only a concern when jump starting, then you should look at MOV's, TVSS's, or zener diodes. Pay attention to the energy these devices can handle. Also, the speed at which they react might be considered. A large zener diode is likely the solution. You may choose to just get a big one and try it since it seems your not equipped to perform any complex analysis on the energy requirements of such a device. Thats where I would start. Good luck.

 

[nbucska]

The battery voltage is changing: it may be possible to simplify the protection circuit but it needs detailed specification.

Zener alone won't work unless it can sink about 1000 A
abnd dissipate tens of kilowatts...


<nbucska@pcperipherals DOT com> subj: eng-tips

 

[buzzp]

Nbucksa,
Your assumption about the required energy dissipation is valid.
I guess I can't imagine why a properly operating charging system (the car doing the jump starting) would ever get to 16 volts, especially under increased load.

 

[nbucska]

Hi perts:

ENGINEERING is the science of impossible.

Every engineering task is impossible (as defined )
-- the most important part of the design is deciding what
to neglect or add to the specs to make it practically
possible and still be good enough to MEET THE ACTUAL
NEEDS.

For example :let's suppose you have to design a 1 inch iron cube.

Easy, isn't it ? Well, on paper...

If you accept 99% iron and .02 inch tolerances it is a nickel each.

If you accept 99.9999% Fe and .00001 inch tolerance it may be a few thousand bucks each.

If you add a few 9-s and 0-s, you better start budgeting a second Manhattan project and hope for the best...

Just a few more and it will be absolutelly and forever impossible.
-------------------------------------------------------
An actual example:
In a unique test circuit a young EE specified a $1500.-precision power supply to be used as the source of the 2V pulldown voltage for the output of an ECL comparator to achieve maximum speed.

Noticing that only the leading (raising) edge was used I replaced the 2V supply with a 10 Kohm 10% resistor to the -15 V with the saving of $1499.95 and much improved
performance...

So if you need a 1 inch iron cube, SPECIFY IT or else you are beyond help.


<nbucska@pcperipherals DOT com> subj: eng-tips

 

[Comcokid]

Contrary to what many people think, the battery and charging system of a car, truck, or 24V system of a semi can be one of the nastiest environments for powering a circuit. It's like trying to power a circuit from a lighting bolt!

There a vehicle electrical system, there are things like "load dump" and co-inductance spikes, in addition to the typical inductive kick-back from relays, ignition coils, and electrical accessory motors. And, of course, there are the issues of jumping a dead battery, which for a 12 Volt car may entail being jumped from the 24 Volt system in the tow truck. Voltage spikes may be positive or negative, and can easily reach 40 Volts or more even in a properly operating vehicle.

For your circuit, try a 10 to 20 Ohm series power resistor into a power zener to protect your circuit. Additionally, you could use a Raychem PTC device in series with the resistor as well. A series fuse is always a good idea.

Since you only need 24ma, you could use a small regulator set for 12 Volts which has a maximum voltage rating of 32 Volts. Protect the input of the regulator with the zener or a automotive rated MOV. The regulator will operate out-of-regulation with insufficient Vin-to-Vout, but will protect you circuit when the voltage is high due to a jump-start.

 

[logbook]

Further to Comcokid’s posting, make sure you pick a "low dropout" regulator, that is one which can operate with a few hundred millivolts input to output "head room". An old regulator like a 7812 may give a 1V drop at 12V input (actual figure is uncertain).

 

[felixc]

Instead of clamping the power to ground to provide a protection, which needs all sorts of failure analyses to find out which one works best in your application, you can instead go the "serial switch way". Look for a high-side switch. These devices withstand high input voltages, and they have a logic enable input which you can easily drive with a voltage limit detector. This way, any overvoltage can e just turned "off". Look at the web site of International Rectifier for such switches. They are available in SMT and thru hole packages. I designed two automotive products with them, and it works great! That's what they're made for.
Felix

 

[OperaHouse]

Don't forget the lowly light bulb. Had a friend in lamp division and he always went nuts when I said light bulb, he says they are lamps. These arr wonderful non linear resistors that have faily low resistances till you start passing normal amounts of current through them. Then the resistance goes up many times. Use a 12V low wattage dome lamp with a 15V 5W zener. This will be short proof and insert negelgable resistance. Normal vehicle voltage should never get above 14.5V.