Odd power supply question

Damien Allen

I was wondering if anybody knows a way of obtaining a DC supply from the phase conductor ONLY of an AC supply? or obtaining dual rails from just the positive conductor of a DC supply?

It would be extremely useful if it is possible.

stamptrol

Damien,

I assume you mean without access to ground or neutral?

On the AC side, you might be able to do what you want with a current transformer, BUT the current in the phase conductor must be present at all times. As well, there are some technical risks associated with current transformer work. Learn before experimenting!

The DC effort may be as simple as using a negative voltage chopper circuit.

What is it, exactly, you're trying to do?

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Tom Sisk

http://www.siskconsult.com
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Damien Allen

Where i work we have a long line of concrete moulds for railway sleepers that are on wheels which run on rails. I want to run power on the mould tracks to power temperature sensors. i cannot run a positive and negative supply because the supply could be easily short circuited. Doing this would negate the need for batteries and drastically reduce maintenance.

I hope that sounds clear. If not i'll draw a pic to illustrate what i'm trying to achieve

Mike Green

stamptrol had the right idea. Is there some particular reason why you need both a positive and a negative supply? There are plenty of temperature sensors and op-amps that can run off a positive supply without a negative supply. There are also plenty of dc to dc converter circuits that can easily provide a low current (10-50ma) negative supply from a positive supply. The MAX232 type RS232 interfaces are perfect examples.

Another possibility is a 1-Wire like setup (look at Maxim/Dallas' website). These multiplex power and a signal along a single conductor (with a ground wire for reference) and there are some temperature sensors included in the device series. The devices are addressable and are commonly used in building control systems where ambient temperature sensing at multiple specific sites is needed with minimal wiring.

Here's one example of a 1-wire temperature sensor rated from -55C to 100C with an accuracy of 0.5C (from -10C to 85C): www.maxim-ic.com/quick_view2.cfm/qv_pk/2816.

The 1-Wire bus is tolerant of short circuits. If the bus is shorted, it just stops working until the short goes away long enough for the necessary command sequences to be passed around.

Post Edited (Mike Green) : 5/22/2007 5:50:56 PM GMT

Damien Allen

I'm sorry, i don't think i was very clear and my technical description was a little inaccurate. I 've created a drawing to illustrate my point. I actually think that it's not possible to achieve.

Please give me your thoughts and thanks for the replies so far...


Damo

Mike Green

Basically, you absolutely must have two conductors, one to carry the electrons "up" and the other to carry the electrons "back". If the rails are insulated from the ground, you could use the ground possibly as another conductor. If the sensors are attached to the molds, perhaps you could have a series of inductors spaced along the track that could produce a low frequency magnetic field that could be picked up by a coil on the sensors, rectified, and filtered. Perhaps you could use solar panels if the light level is high enough on average. You could also use lamps along the track that could illuminate small solar panels (a few inches square) as they move by. Perhaps you could run a 3rd rail (conductor) with a "shoe" to contact it. Since the voltage would be low (even as low as 2-3V could be boosted to charge a 6V or 12V battery) and the current would be small (certainly less than 100ma), there may be exceptions from the usual regulations for this sort of exposed low-power source.

As with any kind of battery operated equipment, reducing power requirements is essential with the controller waking up only long enough to send a reading, then sleeping again until another measurement is needed. The peripherals also need to be power switched.

Post Edited (Mike Green) : 5/22/2007 8:33:49 PM GMT

stamptrol

Damo,

I think Mike has summarized your options. Inductive may be the way to go, but may be expensive if many moulds have to be measured.

I take it the temperature is monitored to keep the curing process under control. Have you tried an infrared gun-type thermometer? If the outside temp of the mould is a useful measurement, you could have the infrared unit mounted on a swivel and servo controlled to swing from one mould to the other. Many can give accurate readings from 100' away.

Always nice to take a stab at a real-world problem!

Cheers,

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Tom Sisk

http://www.siskconsult.com
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Damien Allen

There are only three probes per line required. Infra-red is not an option as accurate concrete temperatures are required. The measurements are used to determine curing and are also used for quality records. As the concrete is left in the moulds overnight i am worried that a battery may 'run out' and leave me with no measurements.

Currently there is a really crude method in use where simple compensating cable is run in conduit and sprouts out just roughly where we require the measurement and is strewn across the mould. As moulds are moved about during the de-moulding process these extension cables are frequently destroyed. Repairing these 'probes' occupies an unbelievely large amount of the maintenance teams time. Also we get through about 200 metres of compensating cable a week. We have tried various methods of protecting the cable but it is not very practical considering our process.

Are there any other 'remote' power source options i could consider??

Damien Allen

I've just had a thought! has anybody had any experience with geothermal power sources? Concrete gives off a large amount of heat during the curing process, i could use this energy to trickle charge a battery. The concrete gives off heat for about 4-6 hours.

I'm off to investigate.

BTW thanks Tom and Mike for your input. Alas i have many of these real world problems to solve...wait for my next one!

Damo

stamptrol

Damo,

So, are the probes left in the concrete or are they on the mould surface? What I'm getting at is, if a good remote power source were found, would all the probes, etc be re-usable without a lot of repair? Are the electronics on the mould box reasonably well protected?

One of Mike's suggestions reminded me of the "space elevator" project where teams build an "ascender" which climbs a metal tape, eventually to near space. One of the power sources is a set of photovoltaics on the moving ascender and the power source is a bright spotlight on the ground.

In your case, if the electronics were well protected and the control box had some photovoltaic cells on it (maybe embedded in clear resin or silicone), you could energize the system with a bright light from several feet away. That way, you could continue to power the electronics after hours and preserve the data.

Thermocouples could, in theory, be used for power (there are several fans systems for wood stoves which work on that principle from the heat given off by the stove) but they would certainly have to be in close contact with the concrete.

Maybe little wind turbines on each mould powered by a big honking fan?

Keep at it!

Cheers

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Tom Sisk

http://www.siskconsult.com
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Damien Allen

Hi Tom, due to the high quality constraints placed upon the manufacture of the pre-stressed concrete sleeper along with with strict railway regulation the probes must be at least 25mm below the surface of the concrete in the mould. Solar power is out of the question as the factory utilises high pressure mercury vapour lighting which has bad colour rendering and daylighting properties. Also the factory line is approx 120m long and there has to be a probe at the beginning, middle and end. If i could run power to a lamp i would just power the probe units directly instead so that rules out the lamp idea although i quite like it.

The factory is quite rough and ready and whatever the solution may be, it has to be virtually indestructable.

I've attached a drawing of a mould and a very poor (sorry) picture of the factory.

Edit: Nothing can be place on the top of or side of the moulds. The electronics are contained within the chassis of the mould. We have mobile plant and equipment that move over the moulds. The probe embedded within the mould as depicted is totally re-useable. I've done some simple trials and i've had no problems.

Post Edited (Damien Allen) : 5/23/2007 8:18:21 PM GMT

Dave Paton

The best I can offer is to put a connector at each probe drop, and hav someone manufacture a large number of connectorized temp sensors for you. Put the sensor in, plug it in, and then unplug it and let it go with the sleeper when it's de-molded and moved along. The cost for the sensors would have to be added to the mfg cost of the sleepers, but that's about the only thing I can come up with. Durable connectors are easy, and in 1000 or more qty, it'd be easy to have someone make the probes for you.

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This is not a sig. This is a duck. Quack.

Damien Allen

That is currently the system in place Dave, it is not just the case of the probes becoming damaged during demoulding, its the processes before which require machines to pass over the moulds. Each probe could be damaged 2-3 times a day sometimes more. It takes up valuable time repairing them but it has to be done because how critical they are to us and our records. Another problem we have is operatives forgetting to place the probes into the concrete, which is found out later and the probes are just strewn across the hardened concrete which voids our quality system.

Time and reliability are the major factors not cost

Post Edited (Damien Allen) : 5/23/2007 8:27:17 PM GMT

Damien Allen

Tom you mention about thermocouples, i know they generate voltage in the order of millivolts but i'm unsure of the current produced. This concept is very interesting. Are there a different type of thermocouple that could generate a larger voltage that could be amplified or converted by other means such as a charge pump?

Cheers,

Damo

Robert Kubichek

Couldn't you use rf induction for the power source??
You have a series of fixed coils stationed along the track that are energized when a mold come past..
When each mold goes past a data reading station, the coil on the mold passes in close proximity to the fixed coil...
The resulting energy induced to the mold coil is then stored, and used to transmit data.

Bob

Damien Allen

Robert
running power is difficult down the lines is difficult also the moulds might only move 2-3 metres in either direction whilst being demoulded before being set up for cast again.


Damo

Mike Green

If you can't run power down the lines then it's not doable that way. Maybe you need to focus on how to make the battery system work better:

1) Try to reduce the power consumption, both peak and average.

2) If you're not already doing it, monitor the battery voltage and include that with your other readings so the central system can warn when the battery needs charging or replacement.

3) Since many types of rechargable batteries tend to have a rapid fall-off in their output voltage close to full discharge, perhaps have a backup primary battery. You could have a 12V rechargable battery pack connected through a diode and a 9V alkaline battery pack connected through a diode to the main power bus, then use a switching regulator to drop this to whatever your circuit needs. When the rechargable pack drops below 9V, the backup battery could run the unit for hours to days with some kind of obvious alarm going off periodically.

Robert Kubichek

You know, the gas and utility companies have a system using passive transmitters that have a range of around 50'
that transmits meter data when the device recieves rf energy.
It is accessable using mobile, and hand held instruments..
The same technology should be adaptable to your needs...
It is the same technology used in rf tags...

stamptrol

Damo,

The thermocouple system uses a whole bunch of t/c's in series to get up to few volts. Once there, its easy enough to step up the voltage to run the electronics.

The woodstove fan I told you about runs a 6" diameter fan. The t/c block is about the size of a deck of cards and has fins on it. In your application, the fins could be gotten rid of, I bet. At room temperature, just putting your hand on the t/c block will get the fan turning slowly.
At retail, they're about $100....but thats only a few hours of maintenance guys' time!

PM me if you want to chase this a bit further.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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