Unusual Way of generating negative voltages

Unusual Way of generating negative voltages

While performing different kind of experiments with analog electronics we come across situations where we require Negative voltage values to provide as reference in some circuits or as a current sink etc. In our carrier with electronics tinkering we come across many times to circuits where we need negative voltages.

The Usual Way !

So how every one out there generates negative voltages? There are many ways, most of the people uses standard negative voltage regulators like 79xx series Linear regulators. However such kind of regulators are available only for some fixed values. Another popular way of getting negative DC supplies is to use a switched capacitor circuit or IC like HT7660, LM2660, LT1054 or MAX660 there are 100s of such switched capacitor voltage converters are available in the market. But finding any of these Chips in one's local market is a pain ! It's simply hard to get these ICs. But still we need the negative voltage for our next experiment. There are many other ways but they either give you less flexibility in terms of output voltages or their parts are hard to find. Its time to look at an Unusual Way !

The Unusual Way !

We all use Micro controllers, aren't we ? Anyone who have ever used a micro-controller know this 16 pin chip called MAX232. Well this chip is normally usedfor signal conversion from TTL (MCU signals) into RS232 or EIA232. But nobody can stop us to use it in other ways. We can use this IC easily for a variety of negative voltage applications. The most promising factor is that, this IC is one of the most commonly available IC and its cheap also.

MAX232 Working Principle

If you have used MAX232 you might be knowing that this chip requires a bunch of external capacitors to perform its intended operation. What those capacitors do ? We all know that Capacitor holds charge, here also they do the same thing, What else :P But sometimes this property of capacitor can be exploited in beautiful ways. MAX232 exploits in one of such ways. MAX232 uses these capacitors as charge pumps to convert one DC voltage level into another DC voltage level. Basically A charge pump is a kind of DC to DC converter that uses capacitors as energy storage elements to create either a higher or lower voltage power source. Charge pump circuits are capable of high efficiency, sometimes as high as 90–95% while being electrically simple circuits.

From Wikipedia Charge pumps use some form of switching device(s) to control the connection of voltages to the capacitor. For instance, a two-stage cycle can be used to generate a higher pulsed voltage from a lower-voltage supply. In the first stage of the cycle, a capacitor is connected across the supply, charging it to that same voltage. In the second stage of the cycle, the circuit is reconfigured so that the capacitor is in series with the supply to the load. Ignoring leakage effects, this effectively provides double the supply voltage to the load (the sum of the original supply and the capacitor). The pulsing nature of the higher voltage output is typically smoothed by the use of an output capacitor.

Lets hook up the Wires!

MAX232 is pretty straight forward to use. From the 16 Pin diagram (shown in right) You can see that it has pins for some Capacitors, Supply voltages Vcc, Vs+, Vs-, GND and Transmit/Receive Lines. It has two pairs of level converters. RS232 signal levels are +3 to +15 for Logic 0 and -3 to -15 for Logic 1, While TTL signal is +5V for HIGH Logic and 0V for LOW Logic. Since MAX232 is a TTL/EIA232 converter. It can convert 5V to -3 to -15 Volts and 0V to +3 to +15 this is the property that we exploit in this experiment.

Now if you hook up the PIN 11 or 10 to Vcc through a 10K Pull Up resistor. Output pin 14 and 7 respectively will give -8.5V DC voltage output. This is the case when all the 4 Capacitor are connected, If you need some lower value (actually higher as we are dealing with negative Quantities ) of Negative Voltages just hook off Capacitor like C4 or C3 or Both. This will give a different Negative DC output. So Try it out on your own for negative DC voltages.
Another straight forward way of getting stable negative output from MAX232 is from Vs- or Pin number 6. This Pin will always give -8.5 Volts if C1 and C2 are connected.
Though the MAX232's datasheet mention that the minimum Supply voltage required for MAX232 is 4.5V but you can still provide some lower values of Vcc like 3.3V and can get smaller values of negative voltages like -5V DC from Pin 6 of MAX232. Try out experimentation with some Vcc values less than 5.5V to get the negative voltage output that suits to your requirement.

How much current !

Well i didn't checked the upper limit but i generated around -5V DC by giving ~3V at Vcc and connecting only C1 and C2. I tried to lit a 5mm RED LED using MAX232 pin 6 as sink for LED current. It was glowing bright. So it could sink current upto 20mA. But i am not sure how much will it be able to sink exactly. But it can be used for negative voltage references for sure.

Points to remember ! Do not give more than 5.5V on the Vcc of MAX232, that may fry the chip. You can probably use 10uF/16V electrolytic capacitors that should give same kind of results, However result with 1uF/16V are not tested.

Contact the author at devesh[at]electroons[dot]com