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  • Author Author: fmilburn
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555 Timer Voltage Inverter

fmilburn

I still have work to do on my Kelvin (4-wire) Milliohm Meter project, one item of which is to improve the accuracy of the meter when measuring below 3 milliohms.  I suspect the problem may be that the instrument amp is too close to the ground rail.  To correct this I ordered a LM2662 which is a CMOS charge pump that can act as a voltage doubler or as a voltage inverter.  This will allow me to create a negative rail which hopefully will improve accuracy.

 

I was brushing up on how charge pumps / voltage inverters work and decided to do a little mini project this evening with a 555 timer (newer version which can operate down to 2V and frequencies to 2MHz) to create my own voltage inverter.  Here is the circuit:

image

Note:  Although I am not using polarized capacitors they are being indicated on the circuit diagram for clarification should they be used.  I connected it up on a breadboard using parts I had on hand and powered with 3V3 from my bench power supply.

    1. R1 = 1k
    2. R2 = 6.8k
    3. C1 = 100p
    4. C2 = 0.1u
    5. C3,C4 = 2.2u
    6. D1,D2 = IN4001

I need to order some Schottky diodes :-).  Lo, and behold, it worked (at least with no load).  Here is the output with the oscilloscope probing pin 3 of the 555 Timer and the -3V3 output.

image

The 555 timer is providing a  3V3 507KHz square wave with roughly 54% duty as seen on the yellow trace.  The calculated frequency for the 555 Timer output was much higher but I expect my breadboard is adding capacitance and throwing off the calculation.  The blue trace is the negative output measured to be -2.72V.  The DC output with 20 mV divisions is shown in the next oscilloscope screen shot.

image

The noise is all within a 20mV band.  And here is what the breadboard looks like:

image

The pencil is pointing at the LM2662 that I bought for the project.

 

It should be noted that the circuit with the listed components gives poor response unless the load is very light.  See the simulation by Jon Clift in the comments as well as new experiments by me.  The following table gives experimental results for different loads.

Load (Ohms) Pin 3 Square Wave Ouput (V) Circuit Output (V)
No Load 3.3 V -2.7 V
33k Ohms 3 V -2.1 V
3.3k Ohms 2 V -0.6 V
1k Ohms 1.8 V -0.2 V

 

 

Charge pumps and voltage inverters seemed like magic at first and it took me a while to catch on to what was happening (mechanical engineer here).  I may order some Schottky diodes and inductors to explore other topologies when I have a bit more time.  Comments always welcome.

 

EDIT:  2018-11-28  Realized I did not label all the pins on the schematic and updated it.  Showed polarity in case electrolytic or tantalum capacitors are used.  Added the load table from the Comments below.  Clarified 555 Timer version and Diodes used.

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  • The datasheet for the original 555 characterises it for 5 to 15V supply and up to 100kHz. It obviously works for you like this - just saying for anyone else who tries it and maybe doesn't get such good results.

     

    I had a go with it in the simulator. TI have a model for the TLC555, which is a CMOS version, but it's painfully slow so I moved to using a voltage generator set to 507kHz and giving a 3.3V square wave out like this

     

     

    image

    This is the result

     

    image

     

    That's with a very light load, it doesn't do quite so well if you take 10mA from it. The slightly lower voltage probably comes from me using different diodes (did you use some 1N4001s, or something like that?)

     

    Potential problems with operating it that fast are the turn-off time of the diodes, if you use rectifier diodes, and also that cheap electrolytics might be struggling a bit above 500kHz, though your tants can manage.

     

    If you want to go a bit further with the circuit you could try this following one.

     

     

    image

     

    Basically, I've duplicated the arrangement and sat one on top of the other in a nice ladder arrangement (it's a known circuit, I haven't just invented it, though it would more normally be drawn with the left side of C1 connecting to the junction of C3/D1/D2 to make it symmetrical up both sides of the ladder). That results in this

     

    image

     

     

    If you wanted to, you could continue in that vein adding additional sections. You can also reverse all the diodes and go off in the other direction (up in voltage rather than down) if you want to [though in practice you'd then take the first diode D2 to the positive rail so that the whole thing sat on top of the supply voltage rather than ground - otherwise all the first stage would be doing is giving you the supply voltage less the diode loss].

     

    Apologies if you already know all this, which you probably do if you've been searching around for info, but it might be useful to someone reading this.

  • in reply to jc2048

    Hi Jon,

    Thanks for simulating this.  Good point on the capabilities of the 555, I should have stated that I am using the TCL555CP which is good down to 2V input and frequencies up to 2MHz.  As you and Rod suspect, this circuit does not work well unless very lightly loaded - at least in the form I currently have it.   I used the IN4001 because I had them on hand.  Here are the results of tests with increasing load.

    No Load

    image

    33k Resistor

    image

    3.3k Resistor

    image

    1k Resistor

    image

    Summarized

    No Load:  3.3V square wave and -2.7V output

    33k Ohm:  3V square wave and -2.1V output

    3.3k Ohm: 2V square wave and -0.6V output

    1k Ohm: 1.8V square wave and -0.2V output

     

    This is pretty interesting stuff and I must return to it when I get some time....  I considered soldering my LM2662 up to an adapter and trying it out but need to save it for the other project.  Will definitely put some additional stuff in my next basket to explore some more.  Also need to start using the simulator more.

Comment
  • in reply to jc2048

    Hi Jon,

    Thanks for simulating this.  Good point on the capabilities of the 555, I should have stated that I am using the TCL555CP which is good down to 2V input and frequencies up to 2MHz.  As you and Rod suspect, this circuit does not work well unless very lightly loaded - at least in the form I currently have it.   I used the IN4001 because I had them on hand.  Here are the results of tests with increasing load.

    No Load

    image

    33k Resistor

    image

    3.3k Resistor

    image

    1k Resistor

    image

    Summarized

    No Load:  3.3V square wave and -2.7V output

    33k Ohm:  3V square wave and -2.1V output

    3.3k Ohm: 2V square wave and -0.6V output

    1k Ohm: 1.8V square wave and -0.2V output

     

    This is pretty interesting stuff and I must return to it when I get some time....  I considered soldering my LM2662 up to an adapter and trying it out but need to save it for the other project.  Will definitely put some additional stuff in my next basket to explore some more.  Also need to start using the simulator more.

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