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KrisBlueNZ
Sadly passed away in 2015
You should stick with a 24V-coil relay if possible. The current needed by the circuit will be halved so you'll have a much better safety margin.
You've increased the 1 uF cap. Have you checked the power dissipation in the zener?
CDRIVE, I agree there are flaws in the circuit. It would have helped me to understand your position if I knew what you meant by "awful".
I think the principle of the circuit is sound, though adding a bridge rectifier would have been a worthwhile design decision.
I made some edits to my post after I realised how easily the circuit could be improved, but unfortunately you saw, and quoted, my original post.
The only thing I specifically disagree with you on is the idea of putting the LED in parallel with the coil.
Regarding the LED, I believe it is better in series with the relay coil, as I said. This is simply because you're using a single current for both the coil and the LED.
The purpose of the resistor across the LED is to pass some of the current, so as to reduce the current that the LED sees. A red LED has around 2V across it over a fair range of current. With 2V across it, a 330 ohm resistor will pass around 6 mA. Assuming you used a 24V relay and the average coil current was 20 mA, that would leave around 14 mA for the LED. These numbers are approximate.
Paul. The current in the coil is limited mainly by the coil resistance. This resistance and the average voltage across the coil determine this current. If the LED is in series with the coil, it drops about 2V which is why I suggested that the zener should be more than 24V. Using a bridge rectifier would improve the average voltage too.
This is why I suggest using a relay with a 24V coil: the current needed to operate the coil is halved.
Assuming you put the LED in series with the coil, the LED current, and therefore its brightness, is affected by the resistor across it, and you can vary that up or down. If you open-circuit it, the LED will see the full coil current, which may be too much for it and/or unnecessarily bright. If you decrease it below 330 ohms the LED will become dimmer.
Got to go, dinner is ready
You've increased the 1 uF cap. Have you checked the power dissipation in the zener?
CDRIVE, I agree there are flaws in the circuit. It would have helped me to understand your position if I knew what you meant by "awful".
I think the principle of the circuit is sound, though adding a bridge rectifier would have been a worthwhile design decision.
I made some edits to my post after I realised how easily the circuit could be improved, but unfortunately you saw, and quoted, my original post.
The only thing I specifically disagree with you on is the idea of putting the LED in parallel with the coil.
Regarding the LED, I believe it is better in series with the relay coil, as I said. This is simply because you're using a single current for both the coil and the LED.
The purpose of the resistor across the LED is to pass some of the current, so as to reduce the current that the LED sees. A red LED has around 2V across it over a fair range of current. With 2V across it, a 330 ohm resistor will pass around 6 mA. Assuming you used a 24V relay and the average coil current was 20 mA, that would leave around 14 mA for the LED. These numbers are approximate.
Paul. The current in the coil is limited mainly by the coil resistance. This resistance and the average voltage across the coil determine this current. If the LED is in series with the coil, it drops about 2V which is why I suggested that the zener should be more than 24V. Using a bridge rectifier would improve the average voltage too.
This is why I suggest using a relay with a 24V coil: the current needed to operate the coil is halved.
Assuming you put the LED in series with the coil, the LED current, and therefore its brightness, is affected by the resistor across it, and you can vary that up or down. If you open-circuit it, the LED will see the full coil current, which may be too much for it and/or unnecessarily bright. If you decrease it below 330 ohms the LED will become dimmer.
Got to go, dinner is ready
KrisBlueNZ
Sadly passed away in 2015
I also don't agree that the circuit is unsafe, provided that the control signal is optocoupled.
If I was going to replace this circuit I would use a relay with a mains-voltage AC coil, and switch it using an optocoupled triac device like the MOC3063. The 3063 is rated to carry 100 mA RMS so it could easily drive the relay coil directly, and an indicator. Suppression across the relay coil and across the thyristor would be needed. This would be smaller and safer than that circuit, but probably not cheaper.
If I was going to replace this circuit I would use a relay with a mains-voltage AC coil, and switch it using an optocoupled triac device like the MOC3063. The 3063 is rated to carry 100 mA RMS so it could easily drive the relay coil directly, and an indicator. Suppression across the relay coil and across the thyristor would be needed. This would be smaller and safer than that circuit, but probably not cheaper.
CDRIVE
Hauling 10' pipe on a Trek Shift3
CDRIVE
Hauling 10' pipe on a Trek Shift3
Hello Kris. I re-read what I posted because sometimes, ..actually most times, what I'm thinking vs what I'm writing comes across as a bit demeaning. If you interpreted it as such it was not intended that way at all. This is merely spirited discussion that is typical among all technical and scientific fields of study. It's the food of the mind that keeps our dendrites firing.
Regarding placing the Orange LED (Led2) in parallel with the coil .. opposed to in series, as in the original circuit:
I recommended this change because it's the most orthodox method. You'll note in my last schematic I posted I placed it in series. If it weren't for this feeble power supply I wouldn't have, but this thing has no (current) overhead at all. It's being milked as it is; even with the original 24V/1.2KΩ relay coil. Speaking of the original circuit.. The original 24V relay coil could never see the 24V that it was intended to operate on. The 24V Zener won't let the + rail go higher than that; minus the drop across Led2 and its shunting resistor . When the contact current is very high it's imperative that a relay coil (pull in force) be optimal. So yes, I understand why the LED is in series, but is it good design? No, not in a good power supply that has overhead.
Regarding the resistor in parallel with the Orange Led (Led2):
It should be obvious that I understand its function. After all, I modified its value in my last schematic to reflect use of the 12V/300Ω relay coil. I lowered it to shunt more current around the LED and keep the LED current close to 20mA.
You will also note that I added a resistor in series with the coil to insure that the coil is dropping close to 12V.
Regarding a Bridge:
Certainly a Bridge would be an improvement, but how are you going to do that without a power transformer? To incorporate a Transformer-less Bridge we would have to place the negative rail of this circuit at 338V (Peak Voltage) above earth ground. On the other hand, if we had a power transformer we wouldn't have this circuit at all and would be dealing with conventional power supply design.
Finally, my dislike for this circuit, besides all of the above is this.. The dropping capacitor (C1) is a weak link. Transformers rarely fail and all UL rated transformers are internally fused on the primary. Capacitors, on the other hand, especially ones subjected to higher voltages, surprise no one when they fail. They don't usually open, but instead leak, or worse yet, short. In this case, if C1 broke down and shorted the mains voltage would run though this entire circuit like sh!t through a Goose! I don't think a single component would survive the blast.
JMHO
Chris
Regarding placing the Orange LED (Led2) in parallel with the coil .. opposed to in series, as in the original circuit:
I recommended this change because it's the most orthodox method. You'll note in my last schematic I posted I placed it in series. If it weren't for this feeble power supply I wouldn't have, but this thing has no (current) overhead at all. It's being milked as it is; even with the original 24V/1.2KΩ relay coil. Speaking of the original circuit.. The original 24V relay coil could never see the 24V that it was intended to operate on. The 24V Zener won't let the + rail go higher than that; minus the drop across Led2 and its shunting resistor . When the contact current is very high it's imperative that a relay coil (pull in force) be optimal. So yes, I understand why the LED is in series, but is it good design? No, not in a good power supply that has overhead.
Regarding the resistor in parallel with the Orange Led (Led2):
It should be obvious that I understand its function. After all, I modified its value in my last schematic to reflect use of the 12V/300Ω relay coil. I lowered it to shunt more current around the LED and keep the LED current close to 20mA.
You will also note that I added a resistor in series with the coil to insure that the coil is dropping close to 12V.
Regarding a Bridge:
Certainly a Bridge would be an improvement, but how are you going to do that without a power transformer? To incorporate a Transformer-less Bridge we would have to place the negative rail of this circuit at 338V (Peak Voltage) above earth ground. On the other hand, if we had a power transformer we wouldn't have this circuit at all and would be dealing with conventional power supply design.
Finally, my dislike for this circuit, besides all of the above is this.. The dropping capacitor (C1) is a weak link. Transformers rarely fail and all UL rated transformers are internally fused on the primary. Capacitors, on the other hand, especially ones subjected to higher voltages, surprise no one when they fail. They don't usually open, but instead leak, or worse yet, short. In this case, if C1 broke down and shorted the mains voltage would run though this entire circuit like sh!t through a Goose! I don't think a single component would survive the blast.
JMHO
Chris
KrisBlueNZ
Sadly passed away in 2015
No, not at all. And I am quite capable of posting barbed comments too. As you say, it's a disagreement about design considerations. I did find weaknesses in the design, and I should have thought about it more before I posted my initial response.
On the internet, it's surprisingly easy for disagreements to snowball out of control and turn into exchanges of personal insults, even between normally sensible and mild-mannered people. "Someone QUESTIONED my JUDGEMENT on the INTERNET! I have to write a cleverly but strongly worded reply NOW!" I see that won't happen here between you and me.
I do still disagree with you on some points though.
Right, it is designed to a fairly closely controlled current budget. That is a deliberate compromise, because increasing the available current increases the power dissipation in the zener, and the standby load power of the circuit as well. The input coupling capacitor shouldn't be any higher than absolutely necessary, and this means that the current budget has to be managed carefully. This is a design decision, and it's the reason why the LED was put in series with the relay coil, because the relay coil current is conveniently close to an appropriate LED current and putting it in series reduces the load power, because you don't have a series resistor wasting a lot of power.
We have both pointed out that the zener voltage is too low to get 24V average across the coil, because of losses in the 1N4007, the LED, and droop due to ripple. I regard this as an error in the design. Putting the LED in parallel with the coil will only fix one of those four issues, and the increased load current would deepen the ripple, causing a further reduction in average coil voltage that could easily outweigh the 2V gained by moving the LED out of the coil circuit. The design needs to be fixed, by increasing the zener voltage and possibly other changes. Then, having the LED in series with the coil will have the intended advantages of reduced load current and less wasted power, and it's the winning option IMO.
Sure. My explanation was for Paul's benefit.
I don't like the idea of using a lower voltage relay, because again, the required current is higher, and this circuit is designed with a specific current budget. Increasing the available current increases the dissipation in the zener, and the standby current of the circuit. I think 24V is a good compromise for the derived rail average voltage, and the relay coil.
The whole circuit already has to be isolated from ground. You can't ground the negative rail in the original circuit - it's connected to Neutral. The relay provides the output isolation, and the optocoupler (which I keep saying is necessary) provides the input isolation. As long as the input and output are isolated, the circuit can have its negative rail connected to Neutral, or just as easily, to the output of a bridge that connects to the mains (via the input coupling capacitor). The whole circuit needs to be physically isolated, which is why I suggested encapsulation, or partial encapsulation.
Those concerns are taken care of by the design. The feeder capacitor needs to be AC-rated - the kind that is used for suppression. With all the markings from UL and the other safety labs. The resistor in series with it will be a fusible type, so if the cap was to go short somehow, the resistor would pop its clogs in very short order, while limiting the peak current, and the zener would protect the rest of the circuit.
Likewise
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Ok friends.
Here is what I have decided to do:
Get an original 24v 1200R relay and just increase the 1u cap to 2.2u. In my sim I get 22v 18mA on the relay which should be enough. before it was about 18-19v and too low which fried the contacts due to heavy current switching.
Only 1 component change (and the replacement relay) and fingers crossed.
Does anybody see any problems with this?
The zener is a 1W zener FYI.
-Paul
Here is what I have decided to do:
Get an original 24v 1200R relay and just increase the 1u cap to 2.2u. In my sim I get 22v 18mA on the relay which should be enough. before it was about 18-19v and too low which fried the contacts due to heavy current switching.
Only 1 component change (and the replacement relay) and fingers crossed.
Does anybody see any problems with this?
The zener is a 1W zener FYI.
-Paul
Attachments
CDRIVE
Hauling 10' pipe on a Trek Shift3
Go for it but make sure the cap is AC rated. If you can tolerate a dimmer Org LED I'd also reduce the value of the shunt resistor too. The goal being to get the coil current as close to 20mA as possible.
Good luck,
Chris
CDRIVE
Hauling 10' pipe on a Trek Shift3
The whole circuit already has to be isolated from ground. You can't ground the negative rail in the original circuit - it's connected to Neutral.
Likewise
Well, I think we've covered it all, so no sense in repeating myself. I think mine an your statements regarding the above quote needs clarification though. Yes, I used the term ground when I should have said neutral. Fact is though, they're darn near the same potential. The only difference between neutral and ground is that neutral is a current carrier while ground isn't. Back in the distribution panel it's the same node. Whether we're talking neutral or ground doesn't change the fact that a transformer-less bridge would put his common rail 338 Peak Volts above ground and neutral, His current circuit is very close to ground potential. Neutral is only above ground potential by the amount of voltage drop on the neutral wire. Which is relatively small.
Chris
I found a 250V AC rated 2.2u Cap in my stash - is that a bit close to 240v for comfort? The original was 400V rated.
Otherwise, would this be suitable for AC? Looks the same
http://www.ebay.com/itm/Wholesale-2...739?pt=LH_DefaultDomain_0&hash=item257302130b
Otherwise, would this be suitable for AC? Looks the same
http://www.ebay.com/itm/Wholesale-2...739?pt=LH_DefaultDomain_0&hash=item257302130b
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KrisBlueNZ
Sadly passed away in 2015
You're not supposed to rely on that. Any circuit that is galvanically connected to the mains (whether to Phase, Neutral, or bridge rectified mains) must be considered to be potentially live and must be isolated.
Also, reducing the value of the shunt resistor across the series LED will make almost no difference to the voltage dropped by the parallel combination, unless you reduce it so far that the LED doesn't light up at all.
In response to Paul.
The capacitor you found at http://www.ebay.com/itm/Wholesale-2...739?pt=LH_DefaultDomain_0&hash=item257302130b doesn't seem to be AC rated and doesn't have any safety agency markings. Have a look at http://www.digikey.com/product-detail/en/B32674D8225K/495-2942-ND/1277706 but this one doesn't have any safety approvals either. I think I was wrong about the requirement for safety approval for mains current coupling applications, because if the capacitor fails, the fusible resistor will pop very quickly and this will ensure safety. I'm not sure about this though.
Regarding your plans. Mains voltage can in some circumstances be significantly less than the nominal value you've used in your simulation, and component values can drift with age. The smoothing capacitor will dry out if it's in a hot environment. The worst case scenario is that in some circumstances, the relay will not pull in fully and the contacts will arc, as has apparently happened before, possibly causing a fire. So it's important to design with a safety margin.
The factors that contribute to insufficient coil voltage are:
- Coupling capacitor value too low for load current - can be fixed by increasing the capacitor value, but check the zener's power dissipation in the simulator
- Zener voltage too low - can be fixed by increasing the zener voltage, but check the zener's power dissipation in the simulator
- Half-wave rectification - can be fixed by adding a bridge rectifier
- Droop due to smoothing - can be improved by increasing the smoothing capacitor value
- Voltage loss in 1N4007 diode - unavoidable unless you use a bridge rectifier
- Voltage loss in series LED
I've already explained the changes I recommend. In order from most to least significant these are: increase input coupling capacitor if necessary; use a bridge rectifier and remove the 1N4007; increase the zener voltage; increase the smoothing capacitance.
You should aim for the average coil voltage to be equal to the rated coil voltage, i.e. 24VDC, with the nominal mains voltage. Minimal ripple is also very desirable, hence my suggestions to use a bridge rectifier and to increase the smoothing capacitor.
Edit: I've repeatedly said that to be safe and legal, the whole circuit needs to be isolated and must have an isolated control signal, e.g. optocoupled, because the mains input is galvanically connected to the part of the product that is shown in the schematics. Any OTHER components that are galvanically connected to that circuit must also be contained within the isolation area. If that's not the case, don't use a bridge rectifier! (You said you don't intend to add a bridge rectifier, but I have to warn you just in case).
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CDRIVE
Hauling 10' pipe on a Trek Shift3
I'm not the one who suggested a bridge. I don't care if we're talking ground or neutral. Please show me how you would use a transfomer-less bridge in his circuit without his common rail ending up 338V above mains neutral or ground. I say not happening.
No argument regarding the shunt resistor. I based that brain fart comment on the 40mA that the 12V coil would have needed.
Chris
KrisBlueNZ
Sadly passed away in 2015
That's right, if you use a bridge, the common rail of the circuit is guaranteed to be live. My point is that the circuit should already be isolated for safety reasons, because it must be considered to be potentially live, because it is galvanically connected to the mains. NOTHING ELSE should be galvanically connected to that circuit. It is NOT SAFE to rely on the common rail being at ground potential! See my edit to my previous post.
CDRIVE
Hauling 10' pipe on a Trek Shift3
Bassmo, this whole thread is just bots! Tell us again why a transformer or yes, even a cheep wallwart can't replace this power supply. This is a whole lot of fuss over something that is so basic. I said at the get go that I thought that PSU sucked. I still do.
Chris
Chris
CDRIVE
Hauling 10' pipe on a Trek Shift3
Yes, I see a problem with it. The 1uF capacitor equals a resistance of 3183 ohm, giving an available rms current of around 66mA, this equals to 1,4W to loose in the zener, with the ON LED current subtracted.
If you use a 2uF2 capacitor the current will increase to 140mA and killing your zener. See edit.
The 250V rate for the capacitor is too low, you'll need at least 400V, since the peak voltage for a 240Vac mains is 340V.
TOK
Edit:
I just saw that this is not a fullwave recitfied voltage, only a halfwave. This in fact halves the rms values above and reduce the current to about half of what is described. the power loss in the zener for the first design is around 0.7W and inside the envelope for the max power applied to this component.
The 2uF2 solution still is way outside and will burn out the zener, at least over time.
The capacitor voltage comment still stands.
TOK
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CDRIVE
Hauling 10' pipe on a Trek Shift3
I messed around with the values of ZD1 and C2 until I developed an acceptable plot. I included notes and virtual meters on the plots to make it easier for you to decipher. The left side of the plots (ramp up time) start at zero time when this circuit is first connected to the Mains. The ramp up time that you see only happens upon power up and is not related to Q1 being turned on or off, which is clearly shown.
Edit: In case you want to stay with C2=220uF I included plots for it too. I didn't calculate the zener power dissipation with this value though. Something for you to do.
Edit: In case you want to stay with C2=220uF I included plots for it too. I didn't calculate the zener power dissipation with this value though. Something for you to do.
Attachments
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Hi all.
I like the idea of increasing the zener.
I'd like to make sure it isn't going to mess up the rest of the circuit too though like overload any of the other parts in other legs.
The output transistor is driven by a magnetic reed switch through a resistor and zener to a 4093B schmitt trigger. The 4093 is powered by yet another resistor + zener at about 5v I think.
There is a delay after the reed switch is triggered by the magnet till when the pump turns off by about 10 seconds. probably charges a cap connected to the schmitt.
I'll double check that none of those other legs are affected by the relay driving leg and see if I can post a diagram to show you all how it fits together. There may be something crucial I've overlooked. It's just a bugger reverse engineering a ready made circuit into a diagram though. It's easy to miss connections and overlook parts.
I'm very keen to solve this as the manufacturer charges $400 to replace the circuit and it seems to have a built in propensity for failure too. I bet the parts are worth about $40 all up.
watch this space
I've learned lots from you guys already and appreciate the discussions and different ideas.
-Paul
I like the idea of increasing the zener.
I'd like to make sure it isn't going to mess up the rest of the circuit too though like overload any of the other parts in other legs.
The output transistor is driven by a magnetic reed switch through a resistor and zener to a 4093B schmitt trigger. The 4093 is powered by yet another resistor + zener at about 5v I think.
There is a delay after the reed switch is triggered by the magnet till when the pump turns off by about 10 seconds. probably charges a cap connected to the schmitt.
I'll double check that none of those other legs are affected by the relay driving leg and see if I can post a diagram to show you all how it fits together. There may be something crucial I've overlooked. It's just a bugger reverse engineering a ready made circuit into a diagram though. It's easy to miss connections and overlook parts.
I'm very keen to solve this as the manufacturer charges $400 to replace the circuit and it seems to have a built in propensity for failure too. I bet the parts are worth about $40 all up.
watch this space
I've learned lots from you guys already and appreciate the discussions and different ideas.
-Paul
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