Beware the Bear
Installing Multiple Power Supplies or Capacitors
Multiple supplies are an easy way to increase the available power or to make your track multi-voltage (12 VDC & 18 VDC). Adding a large capacitor downstream of the power supply will minimize noise (AC Ripple), help regulate voltage and massively increase surge current.
A diode auctioneering circuit is recommended to tie multiple power supplies to a common bus and to connect a power supply (or group of supplies) to a large filter capacitor. The following photo shows a typical diode auctioneering circuit. It requires one large diode (or diode bridge) for each power supply.
Simplified Schematic Showing Multiple Power Supplies, Diodes and Cap Installation
The positive output from each power supply is connected to the diode input. If four wire diode bridges are used then use one bridge per power supply and connect the power supply output to the two bridge AC inputs on its diode bridge. The negative from each power supply is connected to a common bus which is also connected to the negative terminal of the capacitor bank and is not connected to the diode bridge. The DC + (positive) output from each diode bridge is connected to a common bus and to the positive terminal of the capacitor bank. No wire is connected the diode bridge DC – (negative) output. In this way a four wire diode bridge will act like a single diode with twice the rating (i.e. a 15 Amp diode bridge will act as a single 30 Amp diode). In my case the 30 Amp rating of my diodes exceeds my power supply outputs and the diodes should never fail in the event of a fault as the power supplies are current limited and each supply will not put out more than approximately 120% of their rated output in the event of a fault.
The diode prevents current or voltage from one power supply flowing into and impacting any other power supply. The diodes isolates the power supplies from each other and from any capacitor mounted downstream of the diodes. This isolation protects the power supplies overcurrent circuitry in the event of a fault (short circuit) and allows the overcurrent protection circuit to function normally. Most power supplies have a foldback or crowbar circuit that senses output current and will reduce the voltage output in the event of a fault to maintain the output current at a safe level. If a larger supply was hooked in parallel without isolation the smaller power supply overcurrent circuit may not function properly and one or both power supplies could be damaged in the event of a fault. Such a failure did happen to one of my supplies at a race before I used diode isolation.
If multiple power supplies of the same voltage are used then the output from each power supply needs to be carefully adjusted to allow the power supplies to both contribute power to the track. Otherwise one supply will dominate and the other supply will not contribute power to the track until the first supply overloads and undergoes voltage rollback. This is not desirable. Most digital voltmeters are good to two significant digits (1/100 VDC) when set to measure 20 VDC. My track has three 18 VDC supplies and one 12 VDC supply. I adjust the output of each 18 VDC power supply under a 6 Amp load so that my three 18 VDC power supplies will share the load equally.
Heat sinking the diodes is important as each diode will dissipate approximately 1 Watt of heat for each amp of current passing through it. I used 15Amp diode bridges that are mounted to the metal enclosure of one of my power supplies. The enclosure is electrically insulated from the power supply and forms an excellent heat sink.
The capacitor bank is located downstream from and is electrically isolated from the power supplies by the diodes. The capacitor bank serves two functions. The first is that a capacitor acts as a filter and will dampen any AC ripple that may come from the power supplies. Its second function is to act as a current bank and provide surge current in the event of a sudden load increase (such as a car starting from a dead stop) or in the event of a fault or short circuit. My original capacitor bank is made from six 110,000mfd capacitors sourced from E-bay. The 110K caps are rated for 35VDC and are safe to use on 18-20VDC.
Some tracks use automotive car stereo stiffening capacitors. These are typically rated at 18 or 20VDC with 24VDC surge capacity. While some truly awesome current can be had from one of these I would be careful before using one. Operating a capacitor at more than its rated voltage can cause the capacitor to overheat and fail. Capacitor failure can be as benign as boiling out the electrolyte all the way up to sudden catastrophic failure. In either case a failed capacitor is good only for the trash. Personal injury is possible if you are in close proximity to the cap when it fails. I updated my cap bank and added four one (1) Farad stereo stiffing capacitors. They are wired in a series parallel arrangement. Two of the capacitors are connected in series (+ - + -) to form a string. The two strings are then connected in parallel. This arrangement gives a 2 Farad cap bank. Assuming that each capacitors is rated for 12VDC then the capacitor assembly is rated for 24 VDC. The 2 Farad cap bank is connected in parallel with the original bank to give me a 2.6 farad cap bank and truly awesome surge power.
Wiring from each power supply to the bridge can be relatively small as you are only transmitting the running current from each power supply to the bridge. On CRR I use short lengths of #14 AWG stranded wire from each power supply to the diode bridge. Then the big cable comes out. The wiring from the bridge to the capacitor bank and from the capacitor bank to the breaker panel is #8 AWG! Of course these wires are also kept as short as possible to minimize voltage drop. CRR can supply truly awesome power! CRRs power supplies are rated at 60 Amps continuous and can supply well over 100 Amps in the event of a short when the track is running at 18 VDC. At 12 VDC things are a bit softer as the 12V supply is only rated at 20+ Amps continuous. However the 2,660,000 mfd (2.66 Farad) capacitor bank still allows 100+ Amp output at 12 VDC in the event of a short.
The CRR installation allows changing power from 18 VDC to 12 VDC at the flick of a single switch! This approach has been added to several other tracks as well with equally good results. Obviously overcurrent protection for the track wiring is a must if such an installation is used. A six (6) amp fast acting fuse for each lane should provide adequate protection. Remember the function of the overcurrent protection system is to protect the track, its wiring and its power supplies. The overcurrent protection system may not provide adequate protection for controllers should they be hooked up incorrectly.
My next power supply upgrade (if it is ever done) will use new generation super capacitors. Super capacitors (ranging in size from 300 to 3,500 Farad) have been demonstrated to provide cold cranking amps equal to that of a automotive staring battery. Imagine 500 or 600 amps of surge power under your track.
Try it. You might like it!
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