Need 120VAC constant to momentary relay or circuit

Thread Starter

likes_shiny_things

Joined Sep 11, 2011
24
Hello, I hope I'm allowed here - I know very little about electronics.

I have an LCD panel mount counter that works on 10-300 VDC and 20-300 VAC and I want to count the number of times my well pump (240 VAC) activates.

I connected the counter to one side of the well pump power (120 VAC) but in addition to the counter incrementing once when the pump turns on, it also increments 2 or 3 times when the pump turns off. The counter's specs say it will register 40 pulses per second. (If each pump cycle were counted consistently I could just divide the count to get my actual number of cycles - but sometimes it increments 3 and sometimes it's 4)

I added a solid state relay (control 120 VAC) thinking that would correct the problem, but when it did not correct the problem, I realized that the relay was just responding to the pulses just as the counter was.

I searched the Web for a constant to momentary circuit or relay and all I found were solutions for 12 VDC.

Does anyone know of a source for a solution that is a circuit I could build that would make the relay fire momentarily?

Or any fairly simple and inexpensive alternate way to achieve the same results?

I would prefer that the solution fit inside a single-gang wall box for simplicity, but if I have to add components outside the box that will be okay.

Thank you for your help.
 

vrainom

Joined Sep 8, 2011
126
Probably the back emf from the pump is causing the spurious counts. So I propose that you filter the voltage from the motor to the counter with a simple circuit like this one.



The 100kohm is a bleeding resistor so the pulse disappear after the pump has turned off

Anyone agrees?
 

mbxs3

Joined Oct 14, 2009
170
If I were in your position and looking for the easiest, quickest, and cheapest solution...I would use an old laptop power supply(they usually put out around 19 VDC or so), and I would connect that to where you currently have the counter connected. Then connect the output of the laptop power supply to your counter input.
 

mbxs3

Joined Oct 14, 2009
170
Probably the back emf from the pump is causing the spurious counts. So I propose that you filter the voltage from the motor to the counter with a simple circuit like this one.



The 100kohm is a bleeding resistor so the pulse disappear after the pump has turned off

Anyone agrees?
I was thinking the miscount was due to the fact that his counter "counts and displays the number of pulses that appear at its input terminal at a rate of 40 pulses per second (Hz)."(from the datasheet)
 

Thread Starter

likes_shiny_things

Joined Sep 11, 2011
24
I appreciate the help vrainom and mbxs3.

Does the pressure switch have an AC or DC voltage running through it's contacts?
AC

And can you draw up a schematic to illustrate your current setup?


The 100kohm is a bleeding resistor so the pulse disappear after the pump has turned off
If I understand correctly the theory behind this, I don't think this would work because the counter would still register the pulses both when the pump turns on and when it turns off.

...I would use an old laptop power supply
I understand how that would probably work and I may very well do that but I was hoping for a solution that would fit in a single gang wall box.

Is it practical to come up with something like the below example except for 120 VAC instead of 12 VDC? (image from http://www.the12volt.com/relays/page5.asp#ctm )

 

mbxs3

Joined Oct 14, 2009
170
I guess it all depends on what is causing your issue.

If it is a frequency issue, then the only solution I can think of is to rectify it to a DC signal so your counter isn't registering the 40Hz pulses but rather just counting the On/Off state determined when there is a either a High or Low DC input.
 

Thread Starter

likes_shiny_things

Joined Sep 11, 2011
24
...registering the 40Hz pulses...
I may be wrong about this, but when I see a term like "40Hz" or "frequency," I'm thinking that is referring to any of the many forms of the EM spectrum that are occurring 40 times per second (i.e. a sine wave) as opposed to the meaning in the context of the counter's specs, which I'm interpreting to mean that it would register a maximum of 40 pulses per second.

So I'm saying that the issue is due to the extra pulses when the pump turns off and that's why if I could build a constant to momentary circuit, that should solve the issue.

Here is what a Redington rep told me when I asked if this counter would work in my situation:

The unit should work in the application. The only caveat to this is that sometimes, motors and pumps will produce extra pulses when they are turned off, and these pulses may look like real signals that the counter will want to count. Thus, there will be an inaccurately high count. I'm not sure that this is very likely in the pump application as it would be with a free-running motor.
 

vrainom

Joined Sep 8, 2011
126
If I understand correctly the theory behind this, I don't think this would work because the counter would still register the pulses both when the pump turns on and when it turns off.
If I'm right this device counts the presence/absence of voltage either ac or dc. One cycle would be the presence and then absence of voltage at a maximum rate of 40hz.

The filter I proposed would just turn ac into dc through a 15k limiting resistor to filter out the pump's back emf, so it would activate the counter when the pump turns on and after the pump turns off and the bleeding resistor depletes the capacitor (around 1 second later) that would be the end of the pulse, so just one long pulse.

it's really cheap and simple and in case it doesn't work we can figure out something more complex.

Just remember that the input resistor and the capacitor's voltage are calculated for 120vac.
 

Thread Starter

likes_shiny_things

Joined Sep 11, 2011
24
If I understand correctly the theory behind this, I don't think this would work because the counter would still register the pulses both when the pump turns on and when it turns off.
In the time since I posted that I understood it in a different way and I was thinking I should change my mind.

So thank you for your explanation - it makes sense to me now that there is a good chance it will work and I shall try it.

Two questions:

1) If I cannot find a 15K resistor, is there a range you can recommend that I stay within and have it still work?

2) Same for the capacitor - if I cannot find a 10μF, are there other values that will work?
 

vrainom

Joined Sep 8, 2011
126
Well, the values are very common to find, but in the remote case you can't find them they're not very critical, just keep a ratio of around 1/10 between the input resistance and the bleeding resistance. And keep in mind that a larger bleeding resistance or a larger capacitor's capacitance value will longer the end of the pulse after the motor has turned off.

Also the 15kohms is 1 watt power disipation just in case there's a short circuit in the output and it has to absorb the whole power.
 
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