MAX232 for Negative DC Voltages

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 !

MAX232 Chip

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.

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 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.

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!

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.