I'm planning on replacing the potentiometer in my 2000+ watts motor controller (arduino controlled washing machine) with a photoresistor that has a suitable resistance range and then glue a led onto it so i can control the speed via arduino pwm. The maximum current it has to endure would be 230ish VAC.
However the photoresistors i've got are only rated for 150VDC. On ebay i can't seem to find any photoresistors rated with a suitable voltage and i'm on a tight budget. So i was thinking i could just use 3 of them in parallel and see what happens. As their dark-resistance is 1MOhms and the potentiometer has a max of around 300K that should work out, at least in my head it seems to be less risky than just using a single one.
So yeah, any avdvice on whether that could work or where to buy suitable photoresistors, or any other workarounds so i could use the ones i already have would be great.
And just out of curiosity, what would happen to the photoresistor if i just used it way above it's rated voltage?
Edit:
Sorry i just noticed that this would fit better in the Motors section, however i can't delete this thread. Could someone please be so kind to move it?
What is the value of the potentiometer you are replacing ?
That is the FIRST fact to consider when choosing a replacement. You can make a voltage diivder with the photo resistor in the middle of two current limiting resistors but since the on resistance is relatively low , the current limiting resistors may need to be in the 10's or 100's of 'k" range.
raschemmel:
What is the value of the potentiometer you are replacing ?
That is the FIRST fact to consider when choosing a replacement. You can make a voltage diivder with the photo resistor in the middle of two current limiting resistors but since the on resistance is relatively low , the current limiting resistors may need to be in the 10's or 100's of 'k" range.
Hi,
The potentiometer goes from <1k to 300k and the photoresistors have their range from 8ish KOhms (bright) to 1MOhm (dark).
That's ok though as i am never going to need the upper half of the power output spectrum of that motor controller as it would probably tear apart the entire washing machine in the glimpse of an eye if turned up all the way.
Another thing that got me thinking is that the Potentiometer is rated at 3 Watts, but i don't think that any actual current would flow through it as it only controls the thyristor, right?
Another thing that got me thinking is that the Potentiometer is rated at 3 Watts, but i don't think that any actual current would flow through it as it only controls the thyristor, right?
If it is rated at 3W then there is a reason for this. I suspect that the current pulses would be too much for the design. I have used a light dependent resistor in the gate of a triac circuit many years ago ( it was in the 60s ) but those were rated for 250V AC. I would not advise you to use anything smaller.
Another thing that got me thinking is that the Potentiometer is rated at 3 Watts, but i don't think that any actual current would flow through it as it only controls the thyristor, right?
If it is rated at 3W then there is a reason for this. I suspect that the current pulses would be too much for the design. I have used a light dependent resistor in the gate of a triac circuit many years ago ( it was in the 60s ) but those were rated for 250V AC. I would not advise you to use anything smaller.
Thanks, yeah i don't feel like risking that. I ordered one of those now:
I have now sucessfully set this up, replacing the potentiometer in the SCR device using the above mentioned sensor. However there is one really big problem.
It seems that the resistance value of the photoresistor changes over time.
As an example in arduino i set the pwm output for the led to 25, which is sufficient to set off the motor with a test load in the barrel of the washing machine. The output voltage is at around 130 Volts. Then i can slowly hear the motor getting slower and looking at my multimeter i can see the output voltage getting lower and lower.
Now i'm pretty clueless as to what is causing this, is the resistance of the photoresistor changing over time?
Edit:
Could this be a malfunction being caused by exceeding the power rating of the sensor? (the potentiometer was rated at 2W, the photosensor is rated at 250mW)
Could this be a malfunction being caused by exceeding the power rating of the sensor? (the potentiometer was rated at 2W, the photosensor is rated at 250mW)
Very likely. It is probably heating up, which will change the resistance.
It doesnt seem to be heating up, at least not from what i can tell by touching it right after a test run.
Now it also suddenly raises the voltage, not only lowers it.
The self made opto coupler is encased in a black plastic case where no light can get in or out. I'm using a white SMD led with a 1kohm resistor, it is connected to an arduino pwm output and to gnd.
To even out the behavior of the optocoupler i covered both the surface of the led and the plastic encasing surface of the photoresistor in a thin layer of white paint.
Grumpy_Mike:
Some black plastic is transparent to IR, try wrapping it in foil but make sure it causes no shorts.
I doubt this one is, it's a reused plastic casing from a power supply i disassembled and the walls are about 1mm thick. I will try to wrap the case in tinfoil just to make sure. Thanks
Edit:
Could fluctuations in the power supply cause the led brightness to fluctuate enough to cause this behavior? The GND lead is a shared one which other sensors and low current actuators use aswell, also arduino takes it's supply voltage directly from the 5V+ lead of a Computer PSU and not over the Power supply connector, because applying the 12V lead from a PSU to it will cause the voltage regulator on the arduino board to get very hot.
You may consider some sort of feedback from AC side to help Arduino stabilize the output voltage (by dynamically changing the PWM rate).
You can put a small transformer across the motor (230V > 6V) and compare its (rectified and filtered) output with a variable threshold (0 - 5V, coresponding to the desired output voltage) by reading both analog values with Arduino.
someduino:
You may consider some sort of feedback from AC side to help Arduino stabilize the output voltage (by dynamically changing the PWM rate).
You can put a small transformer across the motor (230V > 6V) and compare its (rectified and filtered) output with a variable threshold (0 - 5V, coresponding to the desired output voltage) by reading both analog values with Arduino.
Oh now that's an idea i didn't even think of, thanks! However my plan is to wait until the hall sensors i ordered arrive and just measure the motor rpm directly so arduino can adapt appropriately. I think that's simpler to accomplish than to find an appropriate transformer, rectifier and all the L's and C's required to filter it. I only just spent countless hours some weeks ago removing (what i think is) all of the HF interference from the setup.
@Grumpy_Mike: I really doubt that it has to do with temperature, as this weird behavior can start only seconds after initiating a test run, there would have to be a lot of heat dissipation and it would have to come from either the led or the sensor and none of them seem to be any warmer from what i can tell by shutting it down right away, pulling the plug, opening it up and touching everything.
In the datasheet it does not say anything about temperature sensitivity. However it seems that what i previously interpreted as power rating was only the power dissipation? I'm a little confused now.
As a next step i will try to measure the voltage applied to the led to see if there is any fluctuation that could explain the fluctuation in the output current of the scr.
Sorry if i made any technical mistakes, i'm still only learning for my ham radio license and this is one of my learning-by-doing projects.
Edit:
Just for scale here is a picture of the opened up case with the optocoupler components:
If you have access to SCR circuit, you could also use a random-phase optoisolator triac driver (MOC3052) instead of LED-photoresistor. The Hall sensor it's also a good idea (I've used a transformer because I had a suitable one available!). Good luck!
In the datasheet it does not say anything about temperature sensitivity.
Yes it does it says:-
(2) Derate linearly to zero at 75°C.
That means the parameter goes from what it says at 25C and by the time it reaches 75C it has dropped to zero. That is very temperature sensitive.
What happens when it is in the dark?
How about posting your schematic so we can see the configuration and the component values you are using?
Little update on this, sorry for not answering in such a long time.
I'm sorry i can't post any schematics because there are none, i figure things out as i go.
In the meantime i tried replacing my selfmade optocoupler with a MOC3052, but that wouldn't work as it would only either switch on the SCR or switch it off completely, nothing inbetween.
Now there are two more plans in my head for figuring out a better solution:
Use a generic* Solid State relay with PWM, but that wouldn't work with AC as it doesn't do zero crossing detection, is this right?
Using a KBPC5010 rectifier to convert AC to DC and then switching it via a pwm driven solid state relay.
I was wondering if those 'generic' solid state relays are even capable of working with pwm to do voltage regulation, all i see is that most of those solid state relays are made to switch AC loads on and off.
If that wouldn't work i would probably go with some sort of power mosfet.
Grumpy_Mike:
Yes it does it says:-That means the parameter goes from what it says at 25C and by the time it reaches 75C it has dropped to zero. That is very temperature sensitive.
Hmm but isn't that related to power dissipation in relation to the power dissipation at 30°C?
You can not use PWM with any sort of SSR because once it is triggered on it stays on for the rest of the cycle.
The only way to get proportional control with AC is to detect a zero crossing and then trigger it on after a delay. It the. Stays on for the rest of the cycle. This is called phase angle triggering.
Grumpy_Mike:
You can not use PWM with any sort of SSR because once it is triggered on it stays on for the rest of the cycle.
The only way to get proportional control with AC is to detect a zero crossing and then trigger it on after a delay. It the. Stays on for the rest of the cycle. This is called phase angle triggering.
You can not use power FETs with AC
Thanks mike but i was referring to rectifying the AC using a KBPC5010 and then using either a FET with PWM or a SSD since i figured that a SSD wouldn't work with PWM with AC.
I was wondering whether a SSD used with DC would work with PWM for voltage regulation.
I was wondering whether a SSD used with DC would work with PWM for voltage regulation.
No most SSRs are AC only.
You sometimes get FETs packaged up as DC SSRs but they are rare. You could in theory PWM those providing the SSR is suitable. However you are better off just using a power FET.
