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Generate Negative Voltage

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Dc5e

New Member
Hi, I have +5v and i want to change it to -5v, how can i do this? Whats the most simple way of doing this?
 

Ravi

Member
Hi,

One of the easiest ways of making a practical voltage converter is to use either readily available components or special purpose IC's such as the ICL7660 voltage convertor. A DC voltage can easily be converted to reversed polarity by using the DC supply to power a free-running square-wave generator.The schematic given below is an example of such a ckt which operates at 3KHz and drives a voltage-doubler output stage also.
 

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Dc5e

New Member
i just tested the circuit and i had no luck, i used a 2.2k 10k, and 3 1microfared caps and 2 diodes and have no luck. I found some other sites with this type of negative voltage generator and i still have no luck. I'm using a 556 timer, (Dual 555)... this should work right?
 

Dc5e

New Member
ok, never mind, i got it working. But now i have a problem, my goal was to use the -5v to pull down a contrast line on my LCD. Everytime i hook up the contrast to the -5v, it becomes dark and then dimms to nothing. So the -5v is not strong enough. Is there anyways i can get a stable -5v able to pull down my contrast?
 

Roff

Well-Known Member
A Google search for "lcd contrast control" will get you a lot of hits, such as this one. Some of the other sites may be better.
 

Dc5e

New Member
i think i need something stronger than the timer circuit... or my combos aren't right, is there any other way to get negative voltage? and how hard would it be to make?
 

Ravi

Member
The easy way of getting (-) voltage is using the ICL7660 which is a dedicated voltage converter IC specially designed to generate an equal value negative supply from a positive source. If powered from a +5V supply it generates a -5V output. It can be used with any +1.5 to 10V DC supply, and has a typical voltage conversion effciency of 99.9 per cent :!:
when its output is unloaded;when the output is loaded it acts like a voltage source with a 70 ohms output impedance, and can supply maximum currents of about 40mA. It is housed in an 8pin DIL package and also houses a very efficient square wave generator that operates (without the use of external components) at about 10KHz and has an output that switches fully between the supply rail values. It also houses an ultra-efficient set of logic-driven multiplier 'diodes' that, when used with two external capacitors, enables voltage-doubling to be achieved with near-perfect efficiency.

(I tried this chip and the infor. I've got is from various text books)
 

Phasor

Member
Everytime i hook up the contrast to the -5v, it becomes dark and then dimms to nothing.

The 555 circuit is generally not able to supply very much current, unless you use some fairly hefty size capacitors.

This is probably why the display is dimming.
 

Ravi

Member
Quote
"The 555 circuit is generally not able to supply very much current,"

555 has a high currect output that can sink or source 200mA. Am I correct?
 

Dc5e

New Member
do you guys think it would work if i used 2 or more timer circuits to supply more current?

And is it possible to have the timer to switch a transistor so the capacitors recieve more current to make the negative voltage?
 

Ravi

Member
Hi Dc5e

I was wondering whether you could use a 7905 negative voltage regulator.Perhaps you may have to do a little bit of wiring.Does this method serve your purpose?
 

Ravi

Member
Sorry, I've overlooked my basics.Yeah,vincentng,7905 stb. will require of about 6V-7.5V absolute minimum to sustain stabilisation.
 

b.rad

New Member
the 7809 needs a -ve input voltage.

i have a similar problem and i haven’t found a easily available solution.

i have a 24V dc bus, and i need a +15V and -15V rail for some opamps....how do i create a -15V rail?

i found a chip pt6645, but its no available :(.
 
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Hero999

Banned
This thread is nearly three years old and I answered your question in other thread.
 

appsman

New Member
I Know this an old thread, but if anyone needs a beefier version of the ICL7660, the MAX660 is such a beast. With a 5V input, it will output -4.3V at 100mA load. The negative output will look like the inverse of the input supply sourced through 6.5 Ohms (the ICL7660 looks like about 80 Ohms) so you can figure the droop at smaller loads. And, yes, I work at Maxim.
 
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