# Converting voltage to resistance

1. ## Converting voltage to resistance

Hello
I hope someone could help me. I suppose somebody with some experience can see easily a simple solution.

My input is a 0-5 Volts signal
I need to replace in a circuit, an usually manual potentiometer of 300 ohms, with a circuit who receive that input (0-5 volts) and act as the potentiometer varying inversely from 300 to 0 ohms.

I thought in an AD converter, but it is not clear for me as I need an analog circuit who accepts, and atenuate, the current wich voltage is between 15 and -15 volts.
I thought in a transistor and resistor array who go bridging a resistors serial as the transistors saturates, but the precision was not good.

Well, I hope somebody could help me,

Sincerly thank you
Luis Jiménez I

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2. ## Re: Converting voltage to resistance

Hi,

please clarify: What is your aim? 0.300 Ohms or 15V ... -15V.
Creating a "voltage dependent resistor" is much more complicated than outputting 15V ... -15V.

Do you need galvanic isolation?
What currents?

Best if you could provide any existing schematic.

Klaus

3. ## Re: Converting voltage to resistance

The voltage in this terminals is between this range: 15 to -15 V. The aim is to attenuate it with the resistor.
I have a signal 0 - 5 volts with wich I wish to control an optocoupler - triac circuit.
Before this optocoupler, I have a two 555 PWM circuit and this last is controlled, if it were manually, with two resistors, the fixed one in series with the potentiometer of about 300 ohms.

This is because I only need to move the range of power in the higher 20%. This circuit is able to be controlled with that potentiometer, so If I could accept the 0-5 volts as an input to control an electronic potentiometer, I have finished that.

I'm trying to send the image of the circuit and it says the image was uploaded but it doesn't show a "that's all" or "finish" button or text

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I hope this image become visible

4. ## Re: Converting voltage to resistance

It's much easier to design a pulse generator with voltage controllable pulse width than a voltage controlled resistor.

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5. ## Re: Converting voltage to resistance

look at the yellow square in the image, this is the potentiometer I'm talking about

6. ## Re: Converting voltage to resistance

Maybe, but in some way thi is s voltage controlled puse width generator, the only thing is that this is manually controlled.
I'm thinking in an AD and then something like the simplyest Digital potentiometer IC. Some idea or IC you know?

7. ## Re: Converting voltage to resistance

thi is s voltage controlled puse width generator
Your schematic has the 556 IC which contains two 555. The 555 timer IC can alter duty cycle of its own pulse train, if you apply a variable voltage to its pin 5 ('CTL'). Does that describe what you want?

Frequency is affected too.

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8. ## Re: Converting voltage to resistance

No, 5 pin is an Output and I have to get a specific freq. because modulation depends on it, so once determined, has to be the same. Try to make it simple, you have a 5-0 Volts input and is needed, maybe including a R2/R convert this to resistance, maybe with transistors, comparators (only 4 bits) or OpAmps. 16 steps are enogh, even 8. Don't need serial data, memory or latch. Maybe you know somebody who has some experience with something simmilar.

9. ## Re: Converting voltage to resistance

I thought in a transistor and resistor array
Yes, a transistor can be biased so it conducts a little or a lot of current. The concept might do what you want, if adapted properly.

Notice that load current is proportional to the controlling volt level.

The diode makes it easier to adjust for the range you want.

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10. ## Re: Converting voltage to resistance

Many people confuse PWM with phase angle control. Your circuit is an example of such confusion.

Unless you synchronize the pulse width with the AC line's zero crossings, all that you are getting are randomly timed pulses which, when coupled to a Triac's latching characteristics, the only result will be a jerky and erratic control.

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11. ## Re: Converting voltage to resistance

I agree with schmitt trigger. It appears you are trying to produce your own 60Hz signal (U1A) then control the pulse width with U1B. It will work as planned but the resulting signal will randomly fire the triac. You either have to lock your 60Hz to the line frequency and phase or use the line frequency directly.

You can probably replace most of the circuit with a variable resistor and fixed capacitor.

Brian.

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12. ## Re: Converting voltage to resistance

Thank you to all who answered, I'm reading what S.T. told me to read, to try understand it.
So, if I generate a lot of pulses (compared to the 60 hertz of line, maybe 10 times on) and could control the width of them with an alike circuit, is not enough to control power? (I'm thinking in an AC 1 h.p. motor at 220 V) Do this fires the triac, no matter the power it be specified to?

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I'm sending the proteus8 file compacted in rar for if it could be useful.
You can see there how the pulse width varies as you change values in the 300 ohms potentiometer.
I'm yet reading the articles to differenciate between phase control and pwm.

13. ## Re: Converting voltage to resistance

Originally Posted by ljille
(I'm thinking in an AC 1 h.p. motor at 220 V)

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I'm going to ask you a question....exactly what type of AC induction motor are you planning to use?

Induction motors of that power range will be either split-phase, or capacitor-start. As the motor starts and obtains speed, can you hear a click?
If so, it has a centrifugal switch which either disconnects the split phase or the capacitor start once that a certain speed is reached.

If you lower the speed low enough such that the centrifugal switch re-connects them again, your motor will burn out.
Split phases and/or starting caps are rated for intermittent duty only. They will burn if they are permanently connected.

There are some motors, usually fan motors, with a permanent capacitor. But even then phase control does not work well because of the changing phase angle as the motor's speed and loading change.

The ONLY AC motors you can reliably control with phase control are either shaded pole and/or universal. But all I've seen are in the fractional-HP range.

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14. ## Re: Converting voltage to resistance

Expanding what was said in posts #11 and #12, you have to understand how a triac works. It isn't a controlled resistance as you originally quoted and it isn't an 'on or off' switch in the conventional sense. It is probably easiest to image it is a triggerable switch, when you fire a pulse at it, the switch turns on. The complication is that it doesn't turn off when you remove the pulse, it keeps conducting until the current passing through it is externally removed. As the circuit is working on AC, the voltage will be reversing in polarity, half a cycle in one polarity then the other half in the reversed polarity - which by implication means it passes through zero twice on each cycle. The current will also be passing through zero twice per cycle although slightly out of time with the voltage because of the inductive nature of the motor.

If you trigger the triac at any time while there is voltage across it, the conduction will continue until the next zero crossing of the current (when it equals zero). What you are trying to achieve is control of how much of each AC line cycle is fed to the motor and your circuit attempts to do that by using different pulse widths to trigger the triac. The reason it wont work is the pulse width, within reason, is irrelevant, even a very short pulse will turn it on, it is the time into the AC cycle that matters. As you have no synchronism between your own 60Hz oscillator and the line frequency, the pulse, no matter how wide it is, could turn the triac on at any time in the AC line cycle. Obviously that means you have no real control over the power you allow to reach the motor, the triac will fire almost randomly and always remain conducting until the next zero crossing point.

Brian.

15. ## Re: Converting voltage to resistance

PWM would only work if you have a switch that can turn on and off with the input pulse, such as a transistor, and probably the only ones robust enough to do that for a 1HP motor would be insulated-gate bipolar transistors (IGBT).
But, as has been noted, doing that is problematic for a standard 1HP induction motor.

16. ## Re: Converting voltage to resistance

So I did a circuit oriented to phase control, as you recommend me, thinking in an AC 1 h.p. induction motor to be operated only over 70% of its capacity. In order to have some control I added an 8 bit AD coupled with a digital potentiometer made with discrete components. (you can see it in simulation, so as the control input oscilates the right leds get on) The resistors in the input are thought for a 5 V range, and the output are 0K to about 50 K, but somebody could change that to meet his needs.

That would be the fine control, together a base level (a manual potentiometer preset) wich in conjunction with a capacitor, they make the RC timing of phase control. I added too an optocoupler but I'm not shure it works fine with this circuit.
Do you think it is O.K. to drive this motor?
I couldn't simulate the power stage as proteus doesn't have a monofasic AC motor model, Do you know how with a capacitor and a resistance could I simulate the effect of the 1 h.p. motor? (Or happily maybe someone has the spice model to be added to proteus 8)
Do you think it would be better to make a frequency oriented circuit? (Please don't say it is "difficult" because this word can be bad lucky. It is better silence or a good idea)

In another item, If someone have the 6N136 spice model, I would appreciate it very much if it is shared.

How is climate there?

Best regards.
Luis

17. ## Re: Converting voltage to resistance

I haven't got a clue what that schematic is supposed to do but it looks very dangerous to me.

I think the input stages are window comparators, the logic in the middle is something to do with making LEDs light up and the output stage is eight BC547 transistors running from an AC supply trying to control DC to an optocoupler.

I'm still not sure if you are trying to use phase control or starve the motor of current, either way, this doesn't look like a sensible solution.

Brian.

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18. ## Re: Converting voltage to resistance

Hi,

Wow, what huge effort to design that complex circuit.

And all because you refuse to accept our recommendation not to go the "voltage to resistance" way.

*******
On the one hand I admire people going their own way, but on the other hand I wonder why they go the hard way 50 years back.
I'm sure there are relatively simple target oriented solutions.....but I still don't know what's the target of your circuit.

Klaus

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19. ## Re: Converting voltage to resistance

Excuse me, I have not been more clear.
See the image.
I'm trying to put this three concepts together:
The bc547 bridges a series or resistors, depending on wich is on, should give a correspondant sum of resistors. As the voltage in the input grows up, the resistance will be less, in 7 steps. This resistance is in series with a manual potentiometer, with wich I define a minimum, that is, the minimum velocity of the motor, supposing that as higher it be, longer the time to light up the triac. So, the AD drives the potentiometer, who is the RC circuit of timing for phase control.
I didn't used ICs because I didn't find one digipot with parallel comunications, wich carries us again to serial, and it would be more circuitry. I tryed different comparators and I saw they work the same. I used the cmos buffers because the comparators work as off and 0, then I never have the 0 and 1, so the TTL doesn't work. The first column of leds will be eliminated, but on the second at right is possible to see which transistor have to conduct, so I will see how the system is working. So the bc547 are intended just as switches to make a bridge, wich defines a longer or shorter series of resistors (digipot). Now, the problem is that certainly they would be conducting from the 220 AC, trough a 3K resistor, so surely is needed another resistor between the series of resistors and the first collector, so I don't burn them. I supposed (Am I wrokg?) that this RC just defines a time and so it is possible to put the optocoupler in the middle, but maybe I have to restringe to the original circuit who have not this protection. What do you think about?
Thank you very much for taking time to see this spider net.

Sincerly
Luis J

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The target is to define RPM between 70% to 85% range aprox. of the total velocity of an induction AC motor with just a signal of 0 - 5 volts generated by a dedicated circuit or by a PIC, in a cheap solution.

I suppose that a "big" specificated triac most work a lot of tima without problems with a 1 h,p. motor in that range, what do you think about?

I was wondering if the site doesn't have a repository of tested circuits who could help others, because, for instance, when I have this working, I'd share the solution, and this could be used not just for a motor, and for different input and output series of resistors, including not linear (although few steps). If somebody need fast response, this circuit can work fine because doesn't have serial communications. The components are simple to find, cheap, and I think reliable.

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Something happened, my last answer with an image didn't appear. I received the message in my emai, I followed the link and it showed a message "to communicate this to administrators".

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20. ## Re: Converting voltage to resistance

Sorry but most of your assumptions about how this works are quite wrong.

Firstly, if I understand your motor, it is synchronous to the AC feeding it. You can control the speed to some degree by limiting the power to it so the load 'holds it back'
(increases the slip) but that implies the motor loses power and it's ability to maintain output under differing load conditions. See https://en.wikipedia.org/wiki/Induction_motor for more information.

Second problem, you are attempting to limit the power using a triac which is either going to be fully conducting or not conducting at all. There may be a border condition where it randomly fires but that will not control your motor speed and could even damage it.

Third problem, you seem to be trying to control the current through your optocoupler LED using transistors working from an AC supply - that will never work for two reasons, the transistors and LED only work on DC and in any case you want the LED to be fully on or fully off, not set to different currents.

Fourth problem - I can't see how your logic works, even if you did want to make the worlds most complicated ADC and DAC, you appear to limit it to only one of eight possible output steps when it is possible to have 256.

CMOS logic and TTL work exactly the same, if you really did want the outputs of the comparators to work the other way around, just swap the inputs over!

Back to reality:

If your motor can be phase controlled, all you need is a zero crossing detector and a variable time delay before firing the triac. Like 'Control por fase' but NOT using an optocoupler as the variable resistor!

If it can't be phase controlled you might be able to drop complete cycles to limit the power available to it.

If both of those methods wont work, you need to consider a VFD circuit. That will allow you full power while still letting you adjust the speed but the circuitry is more complicated.

Brian.

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