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Sherlock Ohms

Control Switch Saves Shelf Lighting

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William K.
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Re: Cool Runnings
William K.   1/22/2014 1:50:01 PM
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That is certainly true, at the least. Of course now it is approaching being a switching regulator, which the OEMs could have used. That would have cost a bit more but made the device truely universal.

armorris
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Re: Cool Runnings
armorris   1/22/2014 12:33:55 PM
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I just realized that you can't wire the SCR in exactly the same place you would connect the zener diode if the zener is wired in parallel with the output capacitor. You would need a diode between the shunt SCR circuit and the output capacitor. Otherwise, the SCR might not shut off. This would be fine if you use the SCR to protect the electronics against a failed zener, like in the Kill-A-Watt. This would blow the fuse or fusible resistor, alerting you to the problem, rather than wondering why your readings are off.

William K.
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Re: Cool Runnings
William K.   1/22/2014 11:31:43 AM
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OK, I remeber "Electronics" magazine, it was a bit different while indeed being sort of along the same lines. My recollection is also that the subscriptions were quite expensive, while the others were free to qualified subscribers. But it is probably 20 years since I have seen an issue of that publication and it may not have survived.

armorris
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Re: Cool Runnings
armorris   1/22/2014 10:26:58 AM
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WilliamK

The PTC idea is a good one.

The magazine was called "Electronics". It was not a hobby magazine,  but was similar to EDN or Design News. I don't know if it still exists. I had created another gadget which they had published, and they asked me if I had anything else. Back then, energy efficiency was not as big a deal as it is now, and the circuit costs a few cents more than just a zener diode.

Besides durability, the SCR voltage regulator does not waste power like a zener diode does. When the proper voltage is reached, the SCR fires, stopping charging of the filter capacitor and forcing the series coupling capacitor to store the excess energy for the following cycle. Of course, the output filter capacitor will have to be larger, because it will not be charged for as long during the cycle as with the zener.

The circuit is slightly temperature sensitive, because the gate to cathode threshold is the voltage reference. Of course, a small zener could be added to improve that or a small linear voltage regulator IC could be connected after the filter capacitor for a more precise output voltage.

Perhaps now that energy efficiency is a bigger deal, somebody might be interested in publishing it. I'll check it out. Thanks for mentioning it.

William K.
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Re: Cool Runnings
William K.   1/22/2014 9:49:33 AM
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Certainly an SCR and two resistors is a simple and yet ablle to be quite robust overvoltage protection scheme. Probably one of the real engineering publications would be quite happy to publish it. Either EDN or Electronic Design come to mind immediately. Youy would need to have some description and a bit of discussion, but it would be a valid contribution to the engineering community, the beauty being in the simplicity and the effectiveness.

Quite possibly one of those PTC circuit protector devices could be included to make the system resettabe without having to replace the resistor, which would make it more useful yet. You simply can not trust consumers to replace a fuse or resistor correctly.

armorris
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Re: Cool Runnings
armorris   1/21/2014 6:03:34 PM
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In all of my projects that use a capacitor coupled power supply, I use an SCR and two resistors in place of the zener diode. It's not as precise, but is a lot more energy efficient and much more robust. Both of my master-slave switches and all of my other such-equipped gadgets  survived a lightning strike near my house here in Florida, the "thunderstorm capital of the US" that destroyed half the electronics in my house.

I came up with that idea back in the late 70's and tried to publish it in a magazine. The magazine didn't want it. The only other time I've ever seen anybody else use it was another Gadget Freak contributor, who saw it in one of my Gadget Freak projects. If you need more precision, you can add a small linear voltage regulator IC.

You're right about the TVS diodes, but a sustained problem will not damage the SCR, but will just blow the fusible resistor. You can also just add the SCR circuit in parallel with the existing zener to protect the electronics in case the zener fails.

William K.
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Re: Cool Runnings
William K.   1/21/2014 4:17:07 PM
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You will need to determine if the TVS device you intend to install can safely handle the amount of power that would be delivered due to the harmonic content of the generators output, and also verify that the TVS would act at a voltage level below that which would damage the Kill-A-Watt power meter's electronics.It may be better to add a higher current Zener diode and a series fuse at the AC input.

armorris
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Re: Cool Runnings
armorris   1/21/2014 10:35:29 AM
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William K.

Thanks for your comment. I was not aware that a generator set would fry the zener diode. I thought a generator produced a true sinewave. I know that most inverters do not. I have a Kill-A-Watt power meter. I will add a TVS (transient voltage suppressor) diode across the zener. A TVS diode is designed to fail shorted, protecting the device, should the TVS diode be overstressed.

William K.
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Platinum
Re: Cool Runnings
William K.   1/21/2014 8:36:15 AM
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The comment about capacitor coupled power supplies is so very true. That applies especially to the nice little instrument, the "Kill A Watt", used for monitoring power consumption of appliances. If it is used for checking the power drawn from an inverter or generator set it will be destroyed, because the harmonics present in the power will result in excess current through the internal zener diode regulator. The zener fails open and so the excess voltage fries all of the internal electronics. The destruction is nearly total. Unfortunately the device does not carry an adequate warning about only using it on pure power systems.

A good engineer could redesign the power supply system so that it could survive, those harmonics, but it would raise the cost and probably make the package a bit larger. Adding a series resistor and a higher current zener diode might be sufficient, but they would increase the heat load a bit.

armorris
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Platinum
Re: Cool Runnings
armorris   1/21/2014 1:23:03 AM
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I used the 10 ohm carbon composition resistor as a peak current limiter and a sort of fuse. I would have preferred a fusible flameproof resistor, but I could not find what I needed at Allied Electronics, the sponsor of the Gadget Freak blog. As long as the resistor is kept away from other components, it will just burst and not do other damage. It can take a pretty good jolt before blowing. I had one blow one time and it just split open harmlessly. I soldered the current limiting resistors on the circuit boards of my AC powered Gadget Freak projects such that they are easily replaced. Alternatively, you could use a higher wattage resistor and a slow blow fuse.

The only one of my projects that had the current limiting resistor blow was my master-slave switch (Gadget Freak case #195), which was hooked to a UPS. When the power went out and the inverter kicked on with its "modified sine wave" the resistor blew. Capacitor coupled power supplies require a true sine wave. The resistor is just to prevent a fire if the coupling capacitor shorts.

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