Whether you are looking for a replacement power supply for your cable modem, or need a power supply for a new product and left it for last, there are ways to expand your possibilities.

For replacement purposes, most devices specify a range of input voltage it will accept. These can be shown as 9 - 12 Vdc, or 4 - 6 Vdc for example. It will usually specify a current requirement too. For example, 0.4 - 0.6 A, which may also be shown as 400 - 600 mA. This information means you have the flexibility to use a power supply with an output voltage from 4 - 6 Vdc with an output current from 0.4 - 0.6 Amp. This give you many options beyond perhaps the 5 Vdc 0.5 Amp power supply the unit came with.

As the majority of external power supplies used and produced today are regulated, this too yields extra options for that last minute prototype unit the power supply was never specified for.

The product you are working on has internal point of load devices that generate 5 Vdc and 3.3 Vdc from a 12 Vdc internal bus. You calculate that the nominal 12 Vdc current you require will be 1.7 Amps. So you want a 2.5 Amp unit to allow for a 50% cushion.

Now since we know that most external power supplies are regulated, your task is to find a 12 Vdc external power supply rated at 2.5 A, or greater. This gives you many other choices. Your minimum power rated unit would be 30 Watts (12 Vdc x 2.5 Amp), but you can also use a 40, 50, 60, or 100W unit which are all generally available units.

So a little fact finding up front can yield many options for a replacement power supply. Knowing your minimum power requirement for a prototype unit will add additional options for that last minute need.

In an effort to protect your products, switching power supply manufactures use many safety features. Sometimes these features can trip when it does not seem like they should have and then a perfectly good power supply can be uninstalled unnecessarily.

OVP - Over Voltage Protection is the most common culprit in these situations. OVP is a legacy from old linear power supplies that had a common failure mode of burning out the output regulator and then delivering much more than their rated voltage... This is an unlikely scenario in switching models however the circuit remains. There is more than one way to accomplish OVP but most circuits turn off the PWM chip when an over voltage event occurs. Lets say that you are working on a 5 volt power supply but to make up for line losses or to meet a special requirement you turn up the voltage to 5.4. The set OVP voltage may be as low as 5.6 volts. Some power supplies track the set voltage and raise the limit accordingly others do not. If the voltage is turned up to 5.6 during the installation or if there is a spike caused by loose connection, power surge, etc the unit could go into OVP. Now that the switching chip is turned off the capacitors have full charge but they are not being drained to the load since the power supply has essentially latched off. The OVP will not reset until the capacitors have drained below a set point. In most cases you will need to remove the AC power and wait ~2-5 minutes for the power supply drain the caps and reset. Meanwhile turn down the voltage potentiometer a bit and double check the connections on the power supply. Once the power supply has had a few minutes, it should operate as normal. A word of caution, if the fuse is blown in a switching power supply, the power supply has been damaged and you should not replace the fuse. This seems contrary to logic, however switching power supplies have over current limit to eliminate the conventional function of a fuse. Fuses are only used to prevent "catastrophic failure". (read fire). If you have an application that is causing you problems, we can help you work through your technical issues. 813-996-5230

Fuel Cells, Certain battery chemistries, and other electrolysis devices require a constant current instead of constant voltage. Usually this requires a costly Lab style supply or a custom power supply. However, we have cost effective solutions when it comes to this kind of application. Many of our supplies can be configured to meet your constant current needs, with a minimal, if any, external circuitry.

For AC/DC Constant Current solutions, the Multi-Stax models from POWERSTAX can be configured from 200-1200 watts in a 1U package, and they have a constant current option. The Mean Well RSP-1000 models also have constant current limiting.

For DC applications The POWERSTAX 500watt DC/DC can be configured for constant current. These are just a few of the options we have. If you would like to discuss a Constant Current requirement, call us at 813-996-5230 and ask for Al.

Determining which external power supply to use can be a difficult task.

In the past, there were AC/DC unregulated units with single ac input. The output voltage would swing up and down with change in input voltage.

The output voltage would swing up and down with change of load current. It was almost like hitting a moving target.

The new breed of external power supplies will vie with the older linear type for pricing. The new breed will also meet the new Green Power initiatives and CEC (California Energy Commission) requirements. So for many good reasons, the older linear type is being phased out.

As the majority of today's external power supplies are switch mode types, they are regulated, the task has gotten easier. They are mostly universal input covering from 90 VAC to 250 VAC at line frequencies from 47 - 63 hertz. That takes care of the output voltage swing problem due to change in AC input. They are also mostly regulated DC output. Meaning the output voltage stays constant regardless of output load current drawn providing you stay under the maximum output rating for the supply. That takes care of the output voltage swing due to changes in output load current.

→So, now your choices are few, in fact just two. What DC output voltage do I need? How much output power do I need?

Since your choice of output voltage is most likely fixed and determined by the input voltage needed by the device you are powering that takes care of one of your two choices right away. Now, you last, and really only choice is how much power do I need? This is going to be some minimum amount that your device will require in order to operate.

For example, your device requires 5 VDC in order to operate. So, you now know you need an external power supply with a 5 VDC output. Your device states that it requires 2 Amps in order to operate. This means that you need at least a 10 Watt (5 VDC x 2 Amps=10W) related supply. So now you search is narrowed down to 5 VDC power supplies rated at 10 W or higher output.

Like everything else, power supply pricing is based on the output power rating of the unit. So your search should focus on the closest power supply you can find with a 5 VDC output at am minimum of 10 Watts of power. However, you have many more choices since the output voltage is fixed at 5 VDC with the regulated supplies, you can use a 20 W or a 30 W unit if your search does not turn up a 10 W unit.

So what used to be a difficult task left to a component engineer has turned into a easier task not requiring an electrical engineering degree to complete.

Are you configuring you own power supplies to achieve redundancy or a custom mechanical configuration? Rack mount, Plug In redundant configurations and Ruggedized Assemblies are all available from AJ's Power Source. The AJ's Power Source guys are available to do professional and quick custom power supply assemblies. They have been building Mean Well assemblies since 1989.

View their website at: http://www.ajpower.com

Here are a few quick tips on specifying the right power supply for your application.

Is your power supply going to be exposed to the elements? Indoor or outdoor you need to consider both low and high temp ranges. Let’s start with the low temp side. Once started, most power supplies can handle very low temperatures since they are self-heating by nature. However, as you get below 0°C, lower effective capacitance begins to decrease the power supply's ability to start. When you get below -20°C the switching chip may refuse to work as well.

Fortunately we offer many models that start at lower temperatures. For those applications where we can't reach the low limit, we can suggest external heating circuits that will warm the supply until it comes up to temperature.

→Far more often it is the high temperature that is of concern. Every power supply has to be cooled either by air or conduction. For convection or forced air applications, an unrestricted path is the best way to make sure the power supply remains cool and performs to specification. In the real world applications often have space restraints, so unrestricted paths for cooling are sometimes impossible. When thermal considerations and space collide there is no substitute for heat testing. In most cases we can find the critical thermal points in a power supply from the manufacturer. After we get the component-specific maximum temperature recommendations, a test with the power supply installed in its designated home under the worst temperature and duty cycle conditions will demonstrate how close the supply is from its terminal temperature. This test is accomplished with thermocouples and/or infrared thermometers.

What’s more, you can now experiment with moving the power supply and other components around to try and direct the airflow properly. If you are unable to keep the power supply from thermal melt down you might consider increasing the power supply rating to one with a higher temperature range. If this is not readily available increasing to the next higher power range may accomplish the same thing. For instance, if you are unable to keep your 240 watt power supply cool enough in a 60C environment, try upgrading to a 320 watt. All of the components in the 320 watt are designed to handle 320 watts and will run considerably cooler at 240 watts.

What about extra fans? Extra fans are an option to increase the air flow for cooling; however you must always bear in mind the life expectancy of fans is considerably lower than solid state components. Therefore when using auxiliary cooling fans you should use conservative figures and only operate the fan when thermal conditions require it. For more information, give us a call. We will be glad to brainstorm your application.