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positive/negative voltage from positive regulator

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henrymonkey

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to get a negative voltage ref I can use two batteries in series and take the center point to ground, correct?
my question is can I do the same with two 7805 regulator circuit, instead of batteries
 
May be. The devil is in the detail.

If you have two separate unregulated supplies with a common 0v, then yes you can.
But if you want to split one supply with two 7805s, then there will be problems.

JimB
 
every thing on my board is powered by a single +12 source with a common ground, I break off a +9 volt for one part of circuit and two 7805 for others( use two because current draw for everything using +5 is too much for 0ne 7805) now I have one op amp that I need a negative source for, and I have only a 7915 -15volt, only need -5 volt for only one opamp. will 7915 give -5 with +5 in specs don't show it will work, or is there another way to get my negative for one op amp I have 5.1 zener diodes can they be used? thank you.for your help. I can order a 7905 but it will be a week coming
 
No, without using a switch-mode inverter of some sort (voltage doubler, voltage inverter, etc, etc, etc), you can't get a -12v from a +12v source.
If you have a +12v source, all you have is a +12v source. Just +12v and 0v. Not +12v and -12v.
If you need a small amount of negative current, check out the ICL7662 capactive voltage inverter. It won't provide much current, but might be enough to bias the negative rail of your opamp.
But without knowing what this whole circuit is supposed to do (eg. how about some detail huh? some kind of state secret or what?), well, we can only guess and lead you in 20 different directions...none of which will be correct...
 
thank you for your reply, it isn't a state secret, I just didn't want to write a book, I am working on interfacing a motorized wheelchair with arduino, the' analog read' reads 0-5 volt, using a single side supply op amp I get about 3 volt useable signal to the 'analog read'( because weight of hand on control sensor)im thinking if I use a negative /positive opamp I can bias from the negative to 0volt (to compensate for weight of hand) and then have the 0-5 volt range to use in processing. will 5.1 zener in series work without loading down current draw
 
I don't know if will , just thinking something series from +12 volt to grnd where I can tap off a negative potential about 5 volt for op amp . but not a series that will load down the whole circuit,
 
Right...where is the negative being generated at? Re-read post#4 again. And again. You've got nothing below 0, nothing below ground, therefore you can't "tap" anything anywhere and get anything in the negative region.
 
Hi,

I dont know if this is what you are looking for but, with two 7805 regulators you can create a plus and minus 5 volts from +12 volts. It's not the best idea but it should work.

The idea is to connect the first 7805 as usual, which creates a +5v supply so now you have +5 and +12. Next, you connect the second 7805 the same way as the first 7805 except instead of running the ground lead to ground you run it to +5 volts (the output of the first 7805).
This configuration gives you +5 and +10 (and the original +12). To get +5 and -5 you use the output of the first 7805 as ground and the original ground as =5 volts and the original +10v as +5v.

The drawback is that these kind of regulars usually just source current they can not sink current, so you have to make sure that if the lower 7805 has to sink current that there is enough of a resistive load connected to it at all times (the -5 volt supply now) so that any required sink current will run though the resistor not the regulator (because the regulator cant do that). If there is not normally enough resistive load there then you need to add some more resistors to get the resistance down to the required value.

Depending on the required current level, op amp have been used for this purpose too.
 
thank you. that is what I was thinking of doing to start with, but didn't want to waste my regulators trying it. what was the last line about an op amp? I have only one op amp to drive with a 0-5 volt out and not much current as it is just going to an analog read pin on an arduino.
 
thank you for your reply, it isn't a state secret, I just didn't want to write a book, I am working on interfacing a motorized wheelchair with arduino, the' analog read' reads 0-5 volt, using a single side supply op amp I get about 3 volt useable signal to the 'analog read'( because weight of hand on control sensor)im thinking if I use a negative /positive opamp I can bias from the negative to 0volt (to compensate for weight of hand) and then have the 0-5 volt range to use in processing. will 5.1 zener in series work without loading down current draw

The 3 volt you get is 1-4V? And you want it to be 0-5V? Is that correct?

With 10-bit ADC, 1-4V range will give you approximately 600 readings compared to 1000 readings in 0-5V. Do you really need such reasolution? Is the output from the sensor so accurate that 600 readings is not enough to capture the value?
 
Ok, I get it now. I was under the impression that the O/P required the +12v in addition to the negative voltage.
Ya, basically, you're creating a 'virtual ground'. Might not work if you're going to plug that into an Arduino though, unless you've got the Arduino plugged into the same virtual ground as your power supplies. Depending on how you wire it up, you might end up smoking the Arduino and/or not getting any readings out of the ADC.
 
the 9 volt regulator is what im using to power the arduino, I have 12 volt from chair and two 7805 +5 volts one powers 9ICs, and the other runs the other ICs all have a common ground with the 12 volt. is it possible to stack in series an addl 7805 on one of the two already used to get a virtual ground to get vcc/vee for a lm324 op amp. I really need all 1000 steps on analog to get the resolution im looking for. As I have it now with a single side supply on the lm324, with my hand on the sensor im using about half of the analog read with the weight of my hand.
 
Then maybe you're going about it the wrong way.
Maybe you need to shift and/or amplify/attenuate that signal coming out of the hand sensor instead of trying to get a negative voltage at the opamp to do the job of bringing the signal range down to something you can read with fairly decent resolution.
As was asked before, do you REALLY need all 10 bits of resolution, all 1024 counts? Can anybody feel the difference between, say a 500 count and a 600 count?

The way I see it (and I still don't see a schematic anywhere), you're likely fighting a losing battle trying to get this to work from a single supply without some sort of negative rail generation...or a better understanding of why you might be letting the magic smoke out of a lot of parts unintentionally.
 
I really need all 1000 steps on analog to get the resolution im looking for.

You can get a 12-bit ADC, which will give you 4 times more counts.

If you want to simply shift the range (as opposed to stretching it rail-to-rail), you can simply subtract some fixed voltage by adding/tweaking resistors around your op-amp.
 
I have an adc0804 but don't understand interfacing with arduino, that is why I wanted to use the built in adc in arduino,It looks like I just need to add a 7905 to get my negative, I don't have a schematic I can link just a bunch of notes on a pad, the sensor im trying to use is a force sensing resistor "pololu corp #1645"..technically im trying to subtract the tare weight of my hand and get 0-5 volts or 1000 resolution into an ardunio, I thank you for the help , I will look at the tweeking you mentioned
 
the sensor im trying to use is a force sensing resistor "pololu corp #1645"..

If you look at the data sheet, it says the sensor has 5% relative accuracy and 0.5% resolution, so 200 counts would be enough to accurately capture everything that the sensor has to offer. Everything above that will be just noise, so 600 counts from a simple 10-bit ADC is more than enough.

technically im trying to subtract the tare weight of my hand and get 0-5 volts or 1000 resolution into an ardunio, I thank you for the help , I will look at the tweeking you mentioned

You can post your schematics here. I'm sure people will suggest mutiple ways to map the sensor output to the ADC range without using negative voltage.
 
thank you. that is what I was thinking of doing to start with, but didn't want to waste my regulators trying it. what was the last line about an op amp? I have only one op amp to drive with a 0-5 volt out and not much current as it is just going to an analog read pin on an arduino.


Hello again,

Using an op amp you can create a virtual ground. Connect two resistors in series to the +12v supply, the center tap is about +6v. Connect the op amp as a voltage follower amplifier, the new ground is now at the output of the op amp. The output of the op amp is at +6v but calling it ground then the +12 is at +6v and the old ground is at -6v. If you need it regulated then you also need a regulator or two, the LDO type, but we'd have to know more about where your source for the +12v comes from.
 
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thank you mr al, im looking at that , the 12 v is from the bus battery that runs chair, don't want something that will short the circuit. was thinking about maybe a capacitive series instead of resistive, have one palm and one digit to function the force sensor
 
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