Max voltage /heat problem

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Dallas90 said:
Could someone help me learn here, by walking me through the operation of the circuit?
Click to expand...

Before the circuit above is powered up, all the capacitors will have zero volts across them. When 24Vac is applied to the input, it is rectified and it's peak voltage of 33V charges up the 470uF cap in a very short time (milliseconds). The 100uF cap charges very slowly through the 180K resistor and it is this charge time that sets the delay. Because the 100uF has zero volts across it upon power up, pin 2 of the OpAmp will have nearly 33V on it. Since this is much higher than any possible voltage on pin 3 (11-22V) pin 6 of the OpAmp goes to near zero. This forces the voltage at pin 3 to stay at 11V. As the 100uF cap charges, the voltage at pin 2 of the OpAmp will drop. Once it drops below 11, pin3 will now be greater than pin2 and pin 6 of the OpAmp will go up to nearly 33V. This will cause the voltage on pin 3 to change to 22V (due to the 100K feedback resistors) ensuring that the OpAmp switches with a snappy hysteresis action and stays on. It also supplies base current to the 2N2222 which turns it on along with the relay as well.
 
Where are you getting the 24V from?

Is it feasible to use a centre tapped transformer?

That way you could use an LM7812 and a bridge rectifier to get the 12VDC required for the 555 and a relay to switch the AC.

Another option is to use an LM317 which if fine as long as the difference between the input and output is under 40V.
 
As a sidelight from the simple/no heat/ cheap delay problem. I have a delay module that I threw onto the circuit board of my prototype and now I need help with that. It is a pass through module, or to say it is in between the 24v and the power to the relay, after 40 seconds the power is allowed to flow through and the relay closes. Problem is, with the mini relay, I'm getting chatter during the 40 second delay period. After some research, I threw a capacitor 100 µf 50V and a resistor 1w 100Ω on it (across the coil). It worked as it should (no chatter), but the resistor smoked about the time the relay opened. More research reveals that the coil (24Vac) has a resistance of 350Ω and draws 25.35mA. I did have a 390Ω 1W on hand and while it didn't smoke (at least not after a minute or so) it was getting warm so I shut it off. (too early?)
Am I way off on R or C, or am I leaving something out?
 
First off, post a link to the "pass through module".
Second, was the 100uF capacitor a non-polarized one, assuming you are connecting AC across it?
Third, you'll need a 2W resistor if you use the 390Ω resistor to have a margin of safety, though I have my doubts about the usefulness of a snubber network without seeing the circuit it is used in.
 
Here's the link **broken link removed**.
No, the capacitor appears to be polarized.
This, may not be the way to go ultimately, but it did stop the relay chatter. It was very light chatter, more of a light buzz, but the relay did allow some current flow.
 
KD102 set to 40 seconds. I've tried a few combinations of resistors and caps, I still haven't found a combo that does away with the buzzing coil but results in a manageable amount of heat.
 
Does you your relay buzz when operated from straight 24Vac without the KD102?
If the relay buzzes when it is supposed to be off, the problem may be that the KD102 has a minimum loading requirement. ie: It does not turn fully off because it draws power through the load (Your relay) when it is supposedly "off". If this is the case, try putting a 24V 50-150ma light bulb in parallel with the relay coil.
 
I like it. hopefully I'll get a chance to try it before I go on a short vacation. 16th - 30th. I'll report back either way. I was concerned that that's what could be happening.
 
Well, I had very little time (no time to go to the store) so I pulled the pins out of a (12V) automotive brake light and shoved the pins into the breadboard. It worked, the relay didn't chatter except for a millisecond or so at switchover.
Now, do I HAVE to use a lightbulb or is there a more elegant solution?
 
If your load draws 500mA or less, then you don't need the relay at all and can use the KD102 as the switch.
Your KD102 needs minimum of 20ma flowing through it when it is on and it leaks 2ma when off. That is why the extra load of the light bulb gets rid of the buzzing. You could try different resistors of lower and lower values (start at around 1.2K) instead of the bulb to see where the buzzing stops. You'll need a resistor of sufficient wattage though. P = E²/R so a 1000Ω resistor will dissipate 0.576 watts at 24V therefore you'd need a 1 watt resistor since the next lowest wattage is 0.5W. ( 24²/ 1000 = 0.576 )
 
The load draws 3 to 3.5 amps so I need the relay. Thanks for the clues on where to start with resistors. I'll try them when back from vacation.
 
A single 1.2K 1/2W resistor would be pretty close to it's power rating. I would use two 2.2K 1/2W resistors in parallel or two 1K 1/2W resistors in series instead. The advantage of using two resistors would be availability and cost. (Two 1/2W resistors are probably cheaper than one 1W unit)
 
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Well, here's the report for today. I'm off the the electronics store again to buy more resistors. the 2 x 2.2k 1/2 w in parallel still allowed the chatter although at a much lower frequency. The single 1.2 was about the same. As was 1.0 and 820. 820 was the lowest value I had, and almost caused the buzzing to stop but it also got pretty warm in the next few minutes after the relay switched. I'm now afraid that I'll have too much heat after I get rid of the buzzing. Very frustrating.
 
Now I'm trying various values in series. So far, if I get a value that stops the chatter in the delay period, it chatters when it supposed to be on.
 
Very strange. I'm kinda stumped here too. You might want to contact the manufacturer of the KD102 and ask them about your problem. Include the part number of the relay and how it is wired.
If it was myself trying to solve this, I would look at the switched AC waveform with a scope across only a resistive load and also with just the relay.
You may want to experiment more with the 330Ω RC snubber network. It would help to have a handful of non-polarized caps on hand for that as you'd have to experiment a bit.
 
I've been working in an unairconditioned garage and it was 102 today so I may not have been at my sharpest at all times, but I tried quite a number of combos. I'll try some more tomorrow, I don't have a scope so I can't look at the waveform, and I've been thinking of trying to get some different relays.
 
O.K. As of 1:00 CDT have a working solution. I get about 10 seconds of very light chatter (so light that it might not hurt the project) and a nice solid, on time switch over. I'm using the KD102 and putting a 1.0K 1w resistor and 2 non-polar 50v 10uF caps in series with the relay coil. There is no overheating after switch over. I would love to get rid of the minor start up chatter but at least I feel like i can make progress on the prototype.
Any ideas on the start up chatter?
 
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