Protecting 9VDC Circuit from Accidental 28VDC Supply Connection, 5-10mA Current

I have a 9VDC circuit I am trying to protect from someone accidently connecting a 28VDC supply. I've found alot of circuits for transients, but not much for constant overvoltage. The circuit normally draws 5-10mA. Thanks.

What is normaly used is a fuse in combination with a circuit called a 'crowbar'. It works by allowing voltage to pass through a zener diode which activates an SCR. This effectivly creates a short circuit which makes sure the fuse burns through. The output will be a greatly lower voltage then normaly so the components on the output side wont be affected by the highvoltage in this short period of time (a few microseconds). Here is a good overvoltage protection circuit example:
http://apowersupply.com/wp-content/uploads/2009/09/overvoltage-protection-diagram.gif
I know for a fact that this circuit is from a magazine called 'elektor', that is a well known magazine in the Netherlands and Belgium so it sould be good.

Thanks Boi. I'll give that a try.

With 5-10 mA you could use a resistor and a zenerdiod and a transistor to make a 9V out of 28V so it would work on either Voltage!
I'll make you a schematics if you need.
I suppose you have 28 V in the system maybe you would think of using 28 V on this circuit as well.

Good point Per. We use 28VDC on most of our circuits. I could put a few diodes in series to reduce it to 9V, but I am limited on board space and power. I would really appreciate the circuit. I'll try both. Thanks.

Here are some calculations to get you started on a basic zener circuit, see the attached, badly drawn paint picture for the schematic.
Because the voltage difference between the two is large (28-9=19V) you should use a 1.3 W type (those are pretty common), the voltage should be 9.1V (also standard). The maximum current through the circuit can be around 1.3W/9.1V=0.142A, the voltage over R is 28-9.1=18.9V so the minimum resistor value should be at least R=V/I => 18.9/0.142=133 ohms.
To calculate the maximum resistor value we will have to know the current and voltage difference, which we do. The voltage difference is 18.9V and the current is as you mentioned about 10mA. 18.9/0.01=1890 ohms.
To get the appropriate resistor we use the closest E12 value which appears to be 1.8k.
When using this resistor there will be a current flowing of I=V/R => 18.9/1800=0.0105 = 10.5 mA.
The power in the zener will be P=V*I => 9.1*0.0105=0.09555 Watt which is far below our maximum of 1.3 Watts. It is still a smart thing to get a zener this large, they are only 10 eurocents in the Netherlands so who cares

See circuit below

If you want to calculate how you should go about this then I’ve submitted this circuit for you so that you can see how you derive R , IZ , VOUT , I OUT

This circuit only uses one series NPN transistor and designed to operate from 14 volts to 28 volts allowing 1 amp out at 9.35 - 9.55 volts If you require more current then use the other circuit which incorporates a Darlington array

Calculations are on the sheet for you

Mark Harrington

www.harrington.force9.co.uk

Thanks for everything Mark.

Well if u use 28 VDC u could use a LM/MC7809 voltage regulator tre pin and two condensators.
With 10 mA Power would be 19*0.01= 0,19 W wich would work on any of those regulators.
Then you would have a good stabilized 9V . The complete set with your analog will use 0.14-0.28 W ( with 5 - 10 mA)
I googled this for your description cause I was to lazy to make my own !

http://www.circuitdiagram.org/9v-power-supply-lm7809.html

Welcome pal hope it helps you

Along this line, how about using a voltage regulator that can take 28V on it's input. With only 5-10ma, even at such a high dropout voltage (i.e. approx 17-19V) there would be, max, like around a 5th of a watt to deal with -- which is negligable.

Some suggestions:

* LM317L (40V + output voltage + 1.5V or so dropout)
* LM78L09 (30V [and more if there are rectifier(s) in series - e.g. a bridge rectifier will add around 1.2V])

Consider, though, that a "28VDC Supply" could deliver a higher voltage if not regulated.

Mark, What tool did you draw this circuit in?

Hi Cody

Protues Professional ( Labcenter electronics ) www.labcenter.com

Its not bad but ,,,, Its has for a few problems which I cant seem to get answers on

Some of these are Colpitts oscillator oscillators, flip flops, Xtal controlled cmos oscillators 4000 series, Hartley oscillator You have to try and trick the simulator into starting oscillation

The other problem is that they don’t have a Transformer component with say 240 in what ever voltage you want out which can be a real pain

Don’t ask me why but its brilliant for PCB’s you get a design package with this as well Annual fee is £25.00 per year but unfortunately on the down side I’ve been on at them for nearly two years and still cant get answers from these people in relations to issues that I have addressed

In general though it is pretty good for the money I don’t suppose you can expect too much for roughly £160 which is quite infuriating as it simply needs attention here and there other wise it’s a good little package shame they cant get back to you and provide answers at at the very minimum reply to email “Normal this “

This quite a gripe when it comes to software companies they start off well and then for some unknown reason we start to get this response from some of them Customer service just does not exists I think its wrong to be honest plus of course it prevents us from doing work which could be done and we could then submit designs so as to improve the economy but there you go what can you do