A Multiple Power Supply For The Workbench


When working with a new prototype on my bench-top, I tend to use one single power supply to power it up. However, sometimes, I need to power up more circuits at once, possibly using different voltages. And it is in those cases that I am forced to use more than one power supply. There are also some cases where I need a dual power supply and, besides the case where I need a +/- 12V, I don’t have a power supply unit capable of giving me other dual voltage values. I even built a simple device, capable of converting a single dual power into a dual one, for those cases where I need it. But that converter is able to provide only a very small amount of power and that is not good when the powers involved are high.


At the end, I need a power supply unit capable of providing me with different voltages at the same time, possibly also current generators and, certainly, the possibility of having both single and dual power supply.
Units on the market capable of providing such features are very expensive, and they don’t even provide all the possible capabilities that I need while, instead, they provide other capabilities that I normally do not care about
To solve this problem, I decided to make my own power supply unit, capable of doing all the things that I find useful in my lab, and at a fraction of the cost of a professional power supply unit.
Let’s take a look at the schematic I designed.

schematic_v2Download


First, in order to be able to connect together a positive and a negative polarity, without having a short circuit, I needed to have totally separated circuits, with independent transformers, so that I could establish any point of reference on each transformer and connect together such points to obtain dual power supplies.
Because of that, I have a total of four transformers in this schematic.
The first on the top, though a single transformer, it provides two independent outputs on two independent secondaries. I used that transformer as the base for building two independent variable power supplies that could be connected together through a switch to obtain a single variable dual power supply, with the two branches independently regulated, useful in certain situations.
The next two transformers, which have exactly the same characteristics, are used to power up the two sides of a fixed dual power supply. One transformer is used for the positive output with respect to the reference point, and the other transformer is used for the negative output with respect to the reference point.
The fourth and last transformer, on the bottom left, is a very small one used solely to provide the power to move a cooling fan for the power supply case.
The two variable power supplies are each made around a buck/boost converter manually controllable, which works both as voltage and current generator and also provides a display to visualize various parameters.


Here is the user manual of this little device.

ZK-5KX_voltage-converterDownload

It has several types of protection, it can be powered with a voltage anywhere in between 6 and 36V, and it can provide a voltage output between 0.6 and 36V, with a maximum current of 5A which can also be regulated, therefore offering the capability of working as a current generator.

It is also very easy to setup and use. It has a 4 wires connector. Two wires provide the input DC voltage and two wires provide the regulated output, either as voltage or current generator.

The manual goes on explaining how to use the rotary encoder and the two auxiliary buttons on the front panel, as well as the dimensions necessary to accommodate the devices on a larger panel of a bigger device.

Going back to our schematic, you can see that I used two of those regulators, represented here with the two blocks. So, each secondary of the large transformer is independently rectified and filtered by this section, to obtain a DC voltage of 34V with up to 2.5A. This two DC voltages go to the input of the power regulators and the outputs are available at the corresponding binding posts on the power supply front panel. Note also the switch that connects together the negative pole of the upper regulator with the positive pole of the lower regulator, to eventually provide a dual power supply where each branch can have and independent value of voltage or current.

The second section, with the two smaller transformers, is the one that provides the fixed voltage power supplies, very useful when working with devices like op-amps, for example, or any other device that needs to be fed with a dual power supply.

The secondary of both transformers T2 and T3 go each through a bridge rectifier and a filtering electrolytic capacitor.

From there, the positive voltage section is made of 3 separate regulators:

– an LM317, which brings down the DC voltage to a more comfortable value for the next regulators

– an LM7812, that provides a +12V to a corresponding output binding post.

– an LM7805, that provides a +5V to yet another output binding post.

Each of the 3 regulators has its own filtering capacitor, to minimize the residual ripple present to the voltage because of the rectification of the AC power provided by the transformer.

You can also see a number of diodes 1N4007, used as a protection for the solid state regulators during the power down of the whole power supply device.

The negative voltage section works exactly the same way, but uses the negative voltage regulators LM337, LM7912, and LM7905.

Another useful feature of this power supply system is the availability of two USB connectors on the front panel, used essentially to provide a 5V power supply, to power USB based devices, or to simply charge the battery of a device that has a USB power connector.

These two USB outlets are visible at the bottom left of the schematic, and it is perfectly visible that only pins 1 and 4 are used, which are those providing the 5V. The data pins 2 and 3 are not even connected.

Finally, the schematic shows also the last transformer, with its own half-wave rectifier and filtering capacitor that powers the cooling fan for the entire case. Why not, since I had laying around that little transformer for which I don’t have any other use.

And now, if you are interested in making your own power supply system, I suggest you to take a look at the video presented below, where you can found details on how to make also your own case for the device, as well as a number of useful information for its assembly.

Lights On! (part 2)


Hi there!
It is summer, and I have a number of upgrades in mind for my outdoor living areas: some involving electric upgrades, some involving gardening, some involving landscaping.
One thing I have in mind involves the night illumination of the gazebo and its surroundings. The current illumination is based on low power LED strings, powered by batteries recharged during the day by small solar panels.

However, I found that this kind of illumination has a lot of drawbacks. First of all, the lights are very dim. They are just fine when entertaining guests having conversations while enjoining the freshness of the evening air. But when it comes the time to do some table game that requires being able to look at pictures or reading stuff, often we end up going inside where we can have more light.
In addition, I need to turn on and off the lights manually, whenever I need them. If I leave them always on and let their sensors take care of the switching, they end up using the whole battery by the middle of the night and, sometimes, they take so long to recharge that by the time it is evening they don’t last that much anymore.
The solution would be to use bigger solar panes, but the batteries are still small and they also need their time to recharge. If I let them recharge too fast, they will start loosing their capability of retaining the charge too soon, and I would have to replace them relatively often. So I would trade off the usage of solar panels with more waste in exhausted batteries.

The right thigh to do, in my view, is therefore to use regular 120V lamps, obviously low power LED, but brighter than the ones I currently have. The thing is that I do like the automatism of having them turn on and off automatically from dusk to dawn. So, I thought I could design and build my own dusk to down automatic switch, which actually require just very few component and it is very cheap to make.

Schematic


Above is the schematic of my dusk-dawn automatic switch. It is a very standard design, and it requires just a few components.
The load is basically made of the LED lamps we want to control.
The 120Vac input goes directly through a protection fuse and to the series of a 50K resistor and a photo-resistor which is the sensor that will detect the daylight condition to turn the light bulbs on and off.

When there is enough daylight, the resistance of the photo-resistor is very low, in the range between 1K and 4K. When we are getting closer to dusk, the daylight starts dimming and the resistance instead increases. Once the resistance hits the 16K threshold, the voltage at its leads goes above 30Vac.

It is at this point that the DIAC starts conducting and it triggers the TRIAC. The TRIAC, in turn, will let the current flow through the load, thus turning on the lights.
Very simple right?

Lights On!

Hi there!

It sometimes happens that we build something out of necessity, to help us with little day-to-day tasks, and this is the case for today’s project.

The boiler’s room in my basement is a very cramped place that I need to access frequently because I keep in there a freezer for groceries. It turns out that I often have my hands full when I come back from taking something from the freezer, and it is difficult to reach for the light switch.

To obviate to this problem, I decided to build an automatic light switch, so I don’t have to maneuver it manually anymore. And, since I was at that, I decided to make one that not just turns off when I leave the room, but also turns on the light automatically when I enter the room.


The core of this device, technically called an Occupancy Sensor, is a PIR, or Passive Infrared sensor. I have a version of it called HC-SR501 that puts together a Pyroelectric Infrared Detector, or PID, with a bunch of other electronic components that make the sensor useable with very few external components. The PID is concealed underneath that white little dome, which is nothing more than a Fresnel lens that concentrates the light on the actual sensor, thus increasing its sensitivity.


The PID used in this device is called LHI 778, and is capable of detecting infrared emissions over a background noise of up to 85 degrees Celsius. You can see from its data sheet that it is like a small metallic cylinder with 4 pins coming out of it. This PID actually contains two infrared sensors connected in series, to increase its sensitivity.

This one is the schematic I made to use the PIR motion detector. The Detector is connected through the pin header J1 on the left. The header provides the power supply for the detector on pins 1 and 3, and captures the output signal on pin 2.

The signal from the detector goes to the base of transistor Q1 which pilots a relay that is used to control the lights of the room where the device is located.

The 5V power supply for the transistor and the relay comes from an old USB charger, so I didn’t have to build a power supply just for this application. LED1 and R1 provide a visual indication that tells us when the gadget is turned on.

The actual power supply for the whole thing comes directly from a 120V socket, goes through a 1A fuse, and through a power switch.

When the switch is set to on, both the USB charger and the common terminal of the relay receive the 120V. Power socket J3 receives the 120V only when the switch is on and, simultaneously, the PIR detects the presence of a person.

The following archive contains all the files you will need if you decided to build this device for yourself, and more.

lights_onDownload

You can also watch this video for further details on the construction of this device.

Important note: this device involves the use of potentially deadly voltages and you should not try to replicate it if you have no experience with high voltages. Build it at your own risk.