Author Topic: Need some help building a circuit involving switchs and relays  (Read 4911 times)

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Offline ShaydzmiTopic starter

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Re: Need some help building a circuit involving switchs and relays
« Reply #50 on: May 29, 2020, 08:14:53 am »
This is the big led that I promised to give you a photo of it:
 

Offline Renate

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Re: Need some help building a circuit involving switchs and relays
« Reply #51 on: May 29, 2020, 12:20:39 pm »
This is a cute little circuit that feeds the 24 VAC to the light bulbs.
It's "hard" (low resistance) in the sense that the triac is on most of the time.
It's "soft" because the rest of the time it's just a 470 ohm resistor.
A 50 mA bulb will cause a 12 V drop when the triac is off.
(Ok, the triac is off only in the low voltage part of the AC cycle so it doesn't generate that much, maybe a volt or two.)
The thermistor will reduce the current if too many bulbs are on.
There's no reason that a dozen bulbs can't be on at the same time.
The triac also can be a dimmer for all the bulbs, it's just a question of the voltage of the two zener diodes.
Edit: Graphic corrected, added sensing, added 24V supply

« Last Edit: May 31, 2020, 06:28:23 pm by Renate »
 

Offline ShaydzmiTopic starter

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Re: Need some help building a circuit involving switchs and relays
« Reply #52 on: May 29, 2020, 12:53:13 pm »
Where are the in and the out? And where are the 3 big resistors?
 

Offline Renate

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Re: Need some help building a circuit involving switchs and relays
« Reply #53 on: May 29, 2020, 10:43:26 pm »
I corrected and updated the drawing above.
As I said, the two stacked power resistors are bypassed with a solder blob.

The 24VDC supply is riding on top of the 24VAC (which is a bit unusual to see).
They have to do that so that they can measure the sense.
The "floating ground" is the minus side of the 24VDC supply.

 
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Offline ShaydzmiTopic starter

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Re: Need some help building a circuit involving switchs and relays
« Reply #54 on: May 31, 2020, 09:44:14 pm »
Hi,

I tried to de- bypass the two resistors, the bulbs almost would not even glow.

The zeners D2 and D3 have this: 1N5225 (if they have the same value).

The thermistor gets hot even if one bulb is on.

Does this information help?
 

Offline Renate

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Re: Need some help building a circuit involving switchs and relays
« Reply #55 on: May 31, 2020, 11:23:32 pm »
The 1N5225 are 3.0V Zeners.
That means that the peak voltage across the triac should be 3V before it fires.

It sounds like the thermistor has failed with a high resistance.
Measure the resistance in-circuit (it should be so low that the other stuff doesn't matter).
I would expect the resistance to be less than an ohm.
The point of the thermistor is for protection.

If you want to try as a test replace the thermistor with a 10 ohm 1/2 watt resistor.
This is not a safe or a permanent replacement.
(The resistor will burn up if you have a short in the feeder cable.)
Or you could just put in a 1A fuse.

You know, the triac is good for 16A, you said the supply was 6A?
There is no reason anything should be complaining at 50 mA.
« Last Edit: May 31, 2020, 11:26:12 pm by Renate »
 

Offline ShaydzmiTopic starter

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Re: Need some help building a circuit involving switchs and relays
« Reply #56 on: June 02, 2020, 08:46:17 pm »
I'm sorry I didn't see your reply until now.

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Measure the resistance in-circuit (it should be so low that the other stuff doesn't matter).
I didn't understand what you mean.

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The point of the thermistor is for protection.
To protect what? what are the things that may be damaged if no thermistor is installed?

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Or you could just put in a 1A fuse.
Maybe I'll do this, but I think that if the fuse blows up, the machine won't stop if a yarn is broken.

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You know, the triac is good for 16A, you said the supply was 6A?
Yes, actually the transformer supplying the board has 2 ouputs (24V 4A) and (24V 8A), but I don't which one is which, because no labels are on the transformer anymore.

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the triac is good for 16A
Can you please explain to me what does the triac do exactly in this circuit?


 

Offline Renate

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Re: Need some help building a circuit involving switchs and relays
« Reply #57 on: June 03, 2020, 02:27:49 pm »
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Measure the resistance in-circuit (it should be so low that the other stuff doesn't matter).
I didn't understand what you mean.
Take your meter and measure the resistance of the thermistor without un-soldering it from the circuit.
It should probably be almost a dead short so that the value of the other components hardly matter.

As I've said, the triac is the main piece that feeds the light bulb current.
If it's not working correctly, then the only current to the lightbulb is through the 470 ohm resistor.
That would make the bulbs glow dimly and not work at all when 10 bulbs were on.

I never said to leave the resistor or fuse or anything in circuit permanently.
I only meant to use it as a test to prove that the thermistor is bad.

Just measure the thermistor, ok?
 

Offline ShaydzmiTopic starter

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Re: Need some help building a circuit involving switchs and relays
« Reply #58 on: June 03, 2020, 03:13:06 pm »
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Take your meter and measure the resistance of the thermistor
It measures 103.9 \$\Omega\$ .

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As I've said, the triac is the main piece that feeds the light bulb current.
If it's not working correctly, then the only current to the lightbulb is through the 470 ohm resistor.
Probably that's the case. How can I measure the triac to know if it's working or not?
 

Offline Renate

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Re: Need some help building a circuit involving switchs and relays
« Reply #59 on: June 03, 2020, 03:55:03 pm »
100 ohms is far too high for a cold thermistor. It's bad.
Consider a single bulb on that (50 mA) it would drop 5V.
But it would also get warm and the resistance would even be higher.

The point of the thermistor is to protect if one of the feeders or the cable got shorted to ground.
You want to limit the current in such a case.

The triac is not broken open because the thermistor gets warm.
It might be shorted (but I don't think so). If it were shorted the lights would work, but the relay wouldn't.

As I said, replace it with a low value resistor or a fuse as a test.
The bulbs should be bright and the relay should trigger correctly.
 

Offline ShaydzmiTopic starter

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Re: Need some help building a circuit involving switchs and relays
« Reply #60 on: June 03, 2020, 04:22:41 pm »
I replaced with a 10 \$\Omega\$ resistor, burned out.
 

Offline Renate

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Re: Need some help building a circuit involving switchs and relays
« Reply #61 on: June 03, 2020, 07:15:44 pm »
Ok, then something is wrong here.
A 1/4 W resistor should be able to take a load of 3 lightbulbs no problem
3 * 50 mA = 150 mA
(150 mA)^2 * 10 ohms = 225 mW

With the control board disconnected, measure the resistance from the feeder wires to ground.
Since you know where they are spliced you can measure them individually.
They should both be infinite ohms with all the yarns in place.
 
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Offline ShaydzmiTopic starter

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Re: Need some help building a circuit involving switchs and relays
« Reply #62 on: June 03, 2020, 07:42:15 pm »
I don't think that there is a problem, I think the burn of the resistor is a cause of the fact that here were no yarn in the feeders, so their switches were closed all of them.

What I did after is making a short between the thermistor's legs, and tested 76 bulbs , and after that 152 bulbs, they didn't dim at all, however the triac got little bit hot.
I measured the current when 152 bulb on was around 5 amps.
« Last Edit: June 04, 2020, 09:46:08 am by Shaydzmi »
 

Offline Renate

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Re: Need some help building a circuit involving switchs and relays
« Reply #63 on: June 03, 2020, 11:33:15 pm »
You didn't mention that the yarn was out in half the feeders.

The thermistor might not have a very long life if it often sits there in overcurrent mode.
Why is this machine powered up when half the yarn isn't there?
Does it need to be on when you are setting it up?

This makes a good design a little bit more complicated.
All 168 bulbs at full brightness would be 8.4 Amps.
That represents a combined load of 2.857 ohms!
Throw in those 1.2k bypass resistors and you have an actual load of 2.041 ohms.
 

Offline ShaydzmiTopic starter

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Re: Need some help building a circuit involving switchs and relays
« Reply #64 on: June 04, 2020, 09:44:19 am »
I was wrong counting bulbs above, I corrected it.

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You didn't mention that the yarn was out in half the feeders.
It was out only when I replaced the thermistor not before.

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The thermistor might not have a very long life if it often sits there in overcurrent mode.
Usually ther will be less that 10 bulbs on, but in some cases half of them or even all of them, but this case is rare,for example when a new worker doesn't know what he is doing, or when the belt broke.
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Does it need to be on when you are setting it up?
Yes.

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This makes a good design a little bit more complicated.
All 168 bulbs at full brightness would be 8.4 Amps.
That represents a combined load of 2.857 ohms!
Throw in those 1.2k bypass resistors and you have an actual load of 2.041 ohms.

The brightness doesn't matter that much when most of the bulbs are on, we need it only when 20 or less are on .
Btw: Other brand machines when 7 are on automatically turned off, and blink once every 5 seconds or so.
 

Offline Renate

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Re: Need some help building a circuit involving switchs and relays
« Reply #65 on: June 04, 2020, 01:32:47 pm »
Well, the original circuit is not bad, as long as you replace the thermistor when it fails.
The good thing is that even if the thermistor fails the relay will operate correctly.

I think that that 3 pin connector on the edge that has a big solder blob on it
was designed to go to a light bulb on a panel to indicate "too many bulbs on".

Yes, you definitely want a circuit that can differentiate:
  • No bulbs on
  • Some bulbs on (less than 10 or 20)
  • Too many bulbs on
Even flashing the bulbs would take as much as 12 A.
If the circuit was DC this would be easier to supply a peak current.
Do you know if the models that flash use DC instead of AC?

 

Offline ShaydzmiTopic starter

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Re: Need some help building a circuit involving switchs and relays
« Reply #66 on: June 04, 2020, 03:04:33 pm »
Thank you for following this long thread!

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The good thing is that even if the thermistor fails the relay will operate correctly
Can you please explain to me in details what does this triac do in this circuit? Please consider that I'm not an electrician by any mean, I'm just a learner, so don't assume that I am knowing much.

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Even flashing the bulbs would take as much as 12 A.
Did you get this value from where?
152x50mA= 7.6 or am I wrong?

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Do you know if the models that flash use DC instead of AC?
Yes the wire to the bulbs is carrying AC, the board you know it it AC an DC, maybe, if you want, I can bring that board here also, and you can teach me how it works.
 

Offline Renate

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Re: Need some help building a circuit involving switchs and relays
« Reply #67 on: June 04, 2020, 05:27:33 pm »
Let's say that you want to build a weight scale that can weigh both a mouse and an elephant.
It will have to use a very light spring to detect a mouse.
But somehow you want a very strong spring that can tolerate an elephant.

The triac circuit is like that.
The 470 ohm resistor is the light spring.
If you draw 50 mA (light bulb) or even just 25 mA (the bypass resistor) you'll be able to see the effect.
(25 mA * 25 mA = 11.75 V)
But what if a couple of Amps want to flow through that?
That's where the triac steps in. It says "Wow, that little spring is being stretched all to heck, I better step in and start pulling."

12 Amps is all the light bulbs AND all the bypass resistors.

There are ways that we can cut down all the current when too many bulbs go on, but we also want to make sure that the operator can see that is what is happening.
 

Offline ShaydzmiTopic starter

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Re: Need some help building a circuit involving switchs and relays
« Reply #68 on: June 04, 2020, 10:36:36 pm »
Thank you.
Ok, I see, the triac then only feeds the bulbs, it has nothing to do with operating the relay. The resistor exists to feed the transistors driving the relay?
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12 Amps is all the light bulbs AND all the bypass resistors.
The bypass resistors shouldn't be considered, because they won't work as long as the bulbs are working, and we won't let all of them burn .
 

Offline Renate

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Re: Need some help building a circuit involving switchs and relays
« Reply #69 on: June 05, 2020, 07:12:07 pm »
The bypass resistors shouldn't be considered.
They must be considered or you will hurt their feelings! :P

The bypass resistors are in parallel with the light bulbs.
Current goes through them whether the bulb is there or not.

Some of my numbers were a bit off too, so let's do it again.
The light bulbs are 1.2W, 24V.
Dividing, we get a current of 50 mA.
The bypass resistors ar 1.2k.
24V / 1.2k = 20 mA
That's a total of 70 mA.
76 feeders, 2 circuits each = 152 * 70 mA = 10.64 Amps.
 
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