Eric's Power Distribution Center
The 'Oscar' Power Conditioner - it may not be neat, but it gets the job done...
This project is the power distribution, sequencing, filtering, and surge protection for my basement home theater system. As my theater system neared completion, two observations became clear: 1) The amplifiers throw off much more heat than I expected which required that the equipment be moved to another room - thus necessitating some type of remote power triggering, and 2) the collective heavy current draw of the equipment means that it cannot all be powered up simultaneously. As a result, I started looking into power sequencers by Furman, Panamax, and others, but none of them offered anything that really fit my needs, so like all other endeavours, I started doing some research. One interesting piece of equipment that I came across was a Furman MP-20 Power Relay Accessory. It is essentially a 120v relay that is powered by a 12v trigger signal with a flexible timer circuit for start-up/shut-down, but I was turned off by the price: $150 retail for a duplex outlet, a relay, a box, and a small timer circuit.
So I started thinking (I know, sometimes a dangerous activity...). Like all DIY efforts, the discovery of a problem is frequently followed by the opportunity to realize something that exactly fits your needs at substantially reduced cost. I know, it takes time, but that's the fun and rewarding part for the DIY crowd! If you don't agree, don't bother reading any further...
Design Criteria and Parts Selection:
Good, you're still here...on with the details!
My power distribution center needs to fulfill several specific requirements: 1) have enough outlets for all of my equipment (preamp, CD player, DVD player, Projector, left channel amp, center channel amp, right channel amp, 4-channel surround sound amp, subwoofer equalizer, and a subwoofer amplifier), 2) offer power filtration (EMI/RFI) so each device is isolated from the others, 3) provide sequenced power-up capability via the 12vDC remote triggers on my preamp, and 4) offer surge/spike protection to the connected equipment. The last criteria is that all of this all fit into a reasonably sized box so it doesn't take up huge amounts of space. So lets take tackle these one at a time:
A quick equipment count reveals the need to power 10 devices (plus maybe one or two more for room to grow). Easy enough: 6 duplex outlets are sufficient to handle my needs. Outlet #1 (for the preamplifier) will always be on, outlets 2 through 6 will be remotely switched on and off as needed by the preamp.
Here is where things get fun. I've had a high capacity 20A Corcom 20EQ1 hospital grade power filter laying around in my parts box for some time now. I figured this would be a great time to use it. I also wanted each outlet to have its own power filter to provide some isolation from device to device. This prevents electrical noise from digital equipment from infecting power amps and other equipment. I found a huge stash of inexpensive power filters at Excess Solutions. I selected a Delta 10DKAS5 - a general purpose 10A filter that provides line-to-line and line-to-ground filtering and has screw terminals for wiring. I'm also a big fan of using isolation transformers to provide balanced power for audio equipment. The Equi=Tech website has some interesting reading on the advantages of balanced power isolation transformers in the field of audio. For this I chose an Avel Lindberg 800VA 60-0-60 transformer from Parts Express. The balanced power transformer will power the more "sensitive" elements of the audio chain: the pre-amplifier and the source components like the CD and Blu-Ray players.
One thing is clear: The total initial power draw of powering up all of my home theater equipment is rather high. Fuses and circuit breakers will pop, so some sort of delay-based sequencing is necessary. The preamp that I use has two 12vDC triggers that are fully assignable to the various audio inputs, so this provides a great deal of flexibility. I decided that DC Trigger #1 would activate whenever the preamp is powered on. This provides power to my CD player, left and right channel amps, the subwoofer equalizer, and the subwoofer power amp. Thus, preamp DC Trigger #1 activates outlets 2, 3, and 4 - everything necessary for 2-channel audio reproduction. Preamp DC Trigger #2 activates outlets 5 and 6 and is active only when the preamp's input selector is set to DVD (for watching movies). This powers the surround sound amplifier, the projector, the DVD player, and the center channel amplifier. Having two 12v triggers from the preamp is nice, but I still need a more fine-grained method of controlling power up for the equipment. For this, I chose heavy duty 30A/240V DPST power relays (Tyco part number T92P7D22-12) from Digikey and a great little relay timer kit (K8015) sold by Apogee.
This is the final element for my power distribution center. It is also the element that I knew the least about, so I did a great deal of research. What I learned is that surge/spike protection is not necessarily that complicated. I found a treasure trove of information on the Google Patents web site. In particular, I found two patents (4,677,518 and 4,901,183 - both of which were already expired when I found them) to be very useful in my learning process. Essentially, there are two forms of protection available: clamping devices and crowbar devices. Clamping devices are things like Metal Oxide Varistors (MOVs) and Transient Voltage Suppression Diodes (TVSs) which act to suppress (absorb internally) any temporary voltages that exceed their activation threshold. Both MOV and TVS devices react very quickly to over voltage conditions - on the order of nanoseconds. Crowbar devices (such as a Gas Discharge Tube) act as a surge arrestor by forming a short circuit whenever activated. GDTs are more slow to react (typically in the microsecond range), but can handle much larger events than MOVs and TVSs. The key is to arrange things such that various "protection stages" (TVSs, MOVs, GTDs) are separated by having "delay stages" between them. Conveniently, power filter (EMI/RFI) circuits make excellent delay stages and were already a part of my design. I chose Gas Discharge Tubes from LittleFuse's AC120 series, Metal Oxide Varistors from LittleFuse's iTMOV series, and Transient Voltage Suppressions diodes from TS's 1.5KE line of products.
Unexpected Challenges (aren't there always some?)
Construction involves two steps: laying out the logic of the circuit and physically wiring everything together. I started with the logic of the circuit and ran into a few small challenges that needed to be overcome.
Balanced Power Transformer and 12v Power Supply:
The first challenge involved the 12v trigger signal from my preamp. The manual indicates that the current delivery capability of the DC trigger is very limited - just under 50mA for each one. I was originally planning to use the DC trigger to directly control the relays and the timer circuits, but their collective current consumption total just over 1200mA - ooops a bit too high. So a new 12vDC power supply was needed. The easiest solution was to modify the 800VA transformer and add an additional secondary winding to provide the necessary voltage. This worked out pretty well because I had to modify the transformer anyhow. It seems that this transformer is designed to provide 60v-0v-60v under full load. This is a problem, because without a full 6.5A load on the transformer, it delivers 66v-0-66v or 133vAC rather than the 120vAC that I was seeking. After speaking with the nice people at Avel Lindberg, I learned that the two secondaries are wound together and at the same time, so reducing the voltage output of the transformer was just a matter of unwrapping it, unwinding a few turns, and re-wrapping it (an activity they don't condone or endorse for some strange reason...). So, here it is, all unwrapped.
The next issue was determining the proper number of turns to unwind from the secondaries. For this, I used five 100w light bulbs that I wired in parallel with one another and my voltmeter. The rest is easy. Plug in the transformer, connect the secondaries to the light bulbs and the meter, turn the bulbs on one by one and record the resulting voltage drop each time. The intended load on the transformer is just over 2A for the preamp, CD player, and DVD player. So it became an iterative process: unwind a turn or two, remeasure the output voltage with the various loads, and keep going until I hit the target of 120vAV with a 2A load. In the end, this meant removing 9 turns from both secondaries. The picture above is after removing these turns, you can see the bare spot on the toroid where the stranded leads are attached. So, I loosened the nearby windings and spread them out a little to fill in the "hole."
Then, since the transformer was open, this was the time to add a new secondary that would power the 12vDC needed by the relays and the delay circuits. I used 18ga insulated solid core wire and began another iterative process: add a few turn, stop and measure the voltage, add a few more, stop and measure again, add another few turns... I used a spare bridge rectifier and a set of 35v capacitors I had in my parts box to create the power supply. I think it took a total of about 18 turns to provide a 12vDC power supply. Once this was done, it was time to re-wrap the transformer and bolt it to the chassis with rubber pads on top and bottom. In the images below, you can see the resulting transformer and 12v power supply already mounted in the completed enclosure. The transformer has its own fast-blow 4A fuse to make sure the devices connected to it don't exceed the 800VA rating and I added a thermistor to keep the fuse from popping when I powered it up those 1,000 times during construction and testing. Once completed and tested, the thermistor can probably be omitted. If you are looking to duplicate what I have done, save yourself the trouble and start with a 55-0-55v balanced power transformer. It seems that these transformers are rated to sag about 10% under full load, so under minimal loads, their voltage runs about 10% high. Thus, a 55v secondary will provide 55 * 1.10, or about 60.5v. Using two seconaries in parallel thus provide 121VAC, which is exactly what you need for use in the United States.
Chassis Layout for the 120V AC Power Supply Circuit:
There are number of considerations to balance here: the circuit layout and the physical layout. Since the working space is limited, it was important to arrange the physical components in a manner that would lead to the most straight forward wiring scenario. This part is important since we're dealing with high current voltages here. All internal wire is 14g stranded wire. Below is an image of the 120V AC circuit - click for a larger view.
After all of the internal parts arrived (and were properly modified to suit my needs), I needed to see how much room all of this would consume so I could choose an appropriately sized chassis. I ended up with a prefabricated Hammond chassis that measures 17in wide, 10in deep, and just about 4in tall. I started by taping all of the sides with masking tape so it wouldn't get too scuffed up while I worked on it. Then, I spent a little time placing components into the chassis to see how it would all be arranged.
Next up was drilling and cutting. The most challenging part of the task was cutting all of holes for the outlets (do this before you mount anything inside the chassis). I used my dremel tool and a heavy duty cut-off disk to make the rectangular cutouts. I placed the outlet upside down on the taped outer surface of the chassis and traced an outline with a pencil. Be sure to wear ear and eye protection while cutting the chassis as debris and sparks fly everywhere while cutting! Once the rectangular cutouts were made, I placed the outlets into their holes and marked the places where the screw mounting holes would go. I also drilled the holes for the power relays, the EMI filters, and the DC Trigger jacks.
It seems that no matter how far ahead you plan, there is always one more hole that you forgot to drill. So, out comes the drill again. Just be sure to dodge the already installed parts and use an air compressor to remove the metal debris from the chassis. You really don't want electrically conductive metal shavings rolling around the inside of a chassis like this...
There are two primary choices for power relays: mechanical relays (that I have chosen for this project) and solid state relays (SSR). The SSRs are attractive primarily because of their very low trigger power requirements (typically 5-10mA), but they require significant heatsinking if they are going to provide any significant level of current on a continuous basis. SSRs are essentially a high-current mosfet and the more power it passes, the hotter it gets requiring a sink for the heat. That was a deal breaker for me, so I went with mechanical relays. The penalty here (for me) is that the coil for each relays consumes 150mA which exceeded the trigger rating of my preamp and necessitated an additional 12VDC power supply. Another positive attribute of the mechanical relays (for me) is that they make a satifying "click" when they open and close - very handy when you are building and testing your project! The relays I chose are heavy duty 30A/240V DPST Normally Open power relays (Tyco part number T92P7D22-12) from Digikey. Any similarly rated relay will do, I chose these for their high current handling capability and their long life (100,000 cycles).
The relays are rather rectangular in shape, but not very tall - thus it made the most sense to mount these on the side wall of the chassis. These were pretty easy to attach, but there are a few details to attend to. Despite being "chassis mount" relays, they need a little bit of work in order to achieve a proper fit to the chassis. The mounting tabs at each end actually protrude upward a bit from the body of the relay, thus there is an airgap between the chassis and the relay itself when mounted. If you wish to use the chassis as a bit of heatsinking for the relays, the tabs need to be filed down. This is easy enough to do with a flat metal file - just lay it down on a flat surface and rub the relay back and forth on top of the file. A few minutes is all it takes to completely flatten the mounting tabs so the relay body actually rests on the chassis. Then I added a drop of thermal grease to each relay before it was fastened to the side wall of the chassis. Finally, although these are specified as 12v relays, the specs indicate a maximum turn on voltage of 9v for the coils. I'm not quite sure that I understand why they aren't called 9v relays, though... Anyhow, my custom 12v power supply is a bit too high for these relays (since they get warmer than I would like), so I added a 16-20ohm resistor in series with the coil for each relay to bring the operational voltage down to about 10v (reducing current draw of the relay from about 140mA to closer to 100mA). This will drop further as the 12v power supply is used to drive two timer circuits and four additional relays.
Since the relays are dual pole, single throw relays (DPST), I used them to switch both the hot and neutral lines to the outlets. When the relays are open (off) there is no connectivity to the AC mains except for the Ground connection. The tabs across the top are for the low voltage coil while the contacts at the bottom actually switch the 120vAC power.
You can see the EMI/RFI filters in the image below. The big one is rated at 20A and the smaller, stacked filters are each rated at 10A. Each outlet has its own dedicated filter and fuse. The power provided by the balanced power transformer is filtered twice - one filter before the transformer and an additional filter for each of the two outlets that are driven by the transformer These two balanced power outlets are for the most critical components: the preamp, the subwoofer equalizer, the CD Player, and the DVD player. The 10A filters are stacked one on top of the other to help preserve some space inside the chassis. All of those parts plus the necessary wiring eat up the available real estate pretty quickly. It worked out just right when mounting these that the spacing of the mounting tabs are just big enough to hold the nut for the mounting screw. This means that there is no danger of this nut working its way loose, since the nut is unable to rotate.
The image below shows the almost finished chassis - only the 12v control circuitry is missing at this point.. The outlets, transformer, relays, and power filters are all installed and wired. The two white outlets on the left provide twice filtered balanced power (60v-0-v60v). The first outlet is always on and the second one is switched (immediately) via DC Trigger #1. The next two tan outlets (#3 and #4) are also switched by DC Trigger #1. Outlet #3 turns on immediately, while outlet #4 is delayed by about 10 seconds. Outlets #5 and #6 (the right most two tan outlets) are switched by DC Trigger #2. Outlet #5 turns on immediately and outlet #6 is delayed by about 15 seconds.
There are two rows of holes between outlets #2 and #3 in the image below. The top row are for indicator LEDs. One indicates that power is turned on and the other two indicate that the first stage of surge protection (MOVs) is functioning properly. The second row is for the two DC Trigger jacks and will be connected to the preamp. The small holes next to the top left corner of each outlet is for an indicator LED. I wired this directly to the back of the outlet so that when the outlet is powered up, the LED next to it lights up. Nothing fancy was used here, just your typical 2-cent diode, a 100kohm resistor to bring the voltage down, and the LED.
12V DC Control Circuit:
Since the DC power supply from the preamp's triggers is not sufficient (it provides only 50mA at 12v) to drive all of the relays and the timer circuits (which require about 1200mA in total, only about 24x too much current draw...), the DC Trigger outputs from the preamp directly drive ONLY the two black relays you see in the bottom left of the image below. These two relays connect only to the 1/8" chassis mounted jacks and directly switch the internal 12vDC power supply for the relays and time-delay circuits. Creating this arrangement was a little more complicated than driving things directly from the preamps DC triggers as I had originally planned, but it prevents damage to the preamp as a result of the excess current draw that would otherwise occur. The entire circuit diagram for the 12v control circuit is below - click for a larger image.
The image below shows the delay timer circuit after being assembled. Its a great kit that, for about $20, has 15 or so different functions depending on the setting of the DIP switches in the middle of the board. It took less than an hour to build and test the circuit, and was a fun little side project all by itself. The blue terminal block on the left is the control input for the circuit (driven by my new 12VDC power supply) and the tan terminal block on the right connects directly to the encased relay you see on the board. This is the output of the circuit and is used to control the heavy duty black relays that provide AC power to the outlets mounted to the chassis. These timer circuits only switch the low-voltage DC power needed to drive the heavy duty power relays, they do not carry any AC power. I used two of these time delay circuits - one for preamp DC Trigger #1 and the second for preamp DC Trigger #2. In the completed chassis, they are stacked one on top of the other to conserve some space.
The last element of this design is the surge protection. All of the patents that I read and all of the research that I performed indicated that the best method for surge protection is to have multiple stages of different types of surge protection, each separated by a filtering mechanism of some sort. The first protection element is a set of three Metal Oxide Varsistor (MOV) devices shown in red in the image below. One spans Line-to-Ground, a second spans Line-to-Neutral, and the third spans Neutral-to-Ground. MOVs are "clamping" devices and absorb the extra voltage above their rated limits and dissipate it as heat or shunt it to ground. If this happens repeatedly, the device eventually breaks down and doesn't function as intended, so they tend to need replacement from time to time. When they fail, they fail as a short circuit, which will cause the upstream circuit breaker to open, thus protecting the downstream AV equipment. These particular MOVs feature a built in thermal fuse and a third leg that can be used to drive an indicator LED. Thus, as long as the LED remains lit, the thermal fuse incorporated in the MOV is still good and the MOV is still providing protection. Following the MOV devices is a bank of five fuses: one for the balanced power transformer that powers outlets #1 and #2, and one fuse for each of outlets #3 through #6.
The next important stage is the set of EMI/RFI filters in the chassis. Again, each outlet has its own filter. The EMI filter that supplies the balanced power toroid then has a set of three Gas Discharge Tubes (GDTs) arranged in a similar manner to the MOVs: L-N, L-G, and N-G. These are "crowbar" devices that form a permanent short circuit when their operating voltage is exceeded. They don't react terribly quickly, though, response time is usually measured in the milisecond scale, so they won't offer total protection on their own and are best used in combination with other forms of surge protection. I don't have an image of these as installed (see below for what they look like), but they are connected across the primary legs of the transformer. Using three devices like this provides the ability to stop surges/spikes no matter which wire (Line or Neutral) they ride in on from the outside.
The final element of surge protection is the Transient Voltage Suppression (TVS) diodes that are connected across the back of each outlet. You can see these connected across the back of the outlets several pictures up. They, too, are connected L-N, L-G, and N-G. Interestingly, they turn into permanent short circuits when they fail (yes, I tested a few before I installed them), hence the necessity of the fusing where the power enters the box. These devices are a nice compliment to the gas discharge tubes above because they react so much more quickly (nanosecond scale as opposed to the milisecond scale of GDTs). All outlets feature two methods of surge/spike protection (MOVs and TVS diodes) and the balanced power outlets feature three forms of protection (MOVs, GDTs, and TVS diodes).
The AC pathway for the balanced power outlets (#1 and #2) follows the sequence: MOVs, fuse, EMI filter, GDTs, balanced power isolation transformer, power relay, EMI filter, TVSs, and then outlets.
The AC pathway for the remaining outlets (#3 through #6) follows the sequence: MOVs, fuses, power relay, EMI filter, TVSs, outlets. Each amp that is plugged into these outlets also has its own internal EIC power filter.
An "under the hood" image of the completed power distribution center is at the very top of this page.
Here is a link to a few "project kits" for interesting little control circuits: KitsRUs.com, Apogee Electronic Kits, Arcade Electronics,
As I alluded to earlier, I did have a small problem with one of the relays in my box after about 18 months of use. The side panel where the relays are mounted gets a bit warm during operation and this caused one of my relays to fail. The voltage from my 12v power supply must have just been on the cusp of too much power for them to handle. The relays turn on in two configurations. For listening to music, only the first three are used along with one timer circuit. For movies, all five relays come on along with two timer circuits. It was the center relay for the three that come on for music listening that failed and this makes logical sense. When only three relays are used, the power draw on the 12v supply is not as great as when all relays are operating, resulting in a higher voltage being delivered to the first three relays. This extra power caused them to overheat and it is the center relay that cannot dissipate any heat since it is sandwiched between the other two relays that are already warm. After about a year and a half, I started to notice that this relay wasn't closing right away when the system was turned on. So I replaced that relay, filed the relay bodies down to get better contact with the chassis, added some thermal grease between the relays and the chassis, and reduced the voltage to the coil with a 20ohm resistor. Now things are a bit cooler for the relays. It has been a few years since this mod was made and everything still works very well.
So, after living with my power distribution/sequencer/filter for several years (and attending to a design flaw), I have to say that it functions very reliably and does its job quite well! It flawlessly sequences my theater equipment - thus simplifying the overall operation of my theater by powering up only the devices that are needed for listening to music or for watching movies. For just about $400 in parts and some time, I have a power filter and sequencer that suits my needs perfectly and can easily manage more than 20A of current for my theater system.
I have not yet had occasion to verify its surge/spike protection in situ, and honestly, I hope that I never get this opportunity... The last item to complete is to mount an LED in the theater room and run the wires back to the jack I mounted on the chassis as a "power on" indicator. Fortunately, this is an easy and straightforward item to add.
All in all, this project was another fun and interesting DIY learning process. The rewarding part is that the price tag of my completed power distribution center is significantly less than any comparably equipped, commercially available product from any of the big names.