17-04-2011, 08:24 PM
Always gladdens my heart to see well constructed home-brew gear, rather than stuff slung together on the basis that 'if it works, I can re-build it later'. Why would it not work if designed and built properly? Your PSU looks handsome Rob - I love those neat rotary switches with their positive click at each turn. People say you can't make stuff as good as commercial gear, but often, you can. A lot of commercial equipment is built down to a price - not up to a standard - it may look nice on the outside, but it's a different story when you take the lid off.
The popular twin meter PSUs sold under the Manson EP925 and other brand names (Palstar?) have been around for years - there must be thousands of those PSUs throughout the world that have given, and continue to give, giving trouble free service. That said, from personal experience having repaired several over the years, they have certain design shortcomings which will be apparent on taking the lid off. They're clearly built down to a price but represent good value for money, so long as too much isn't expected of them.
The most important shortcoming is the lack of over-voltage protection, which isn't uncommon in "budget" PSU's. This means that if a collector/emitter short arises in one or more of the power transistors, the full rectified DC output of the mains transformer (>25V) would be plonked
across any equipment connected to the PSU. The adverse effects of this would depend on the ability of that equipment, (multi-band amateur radio transceiver or whatever), to withstand that sort of over-voltage.
An over-voltage crowbar circuit could have been incorporated into the PSU with just a few components, but for some reason the maker’s left that feature off, yet included a fan control circuit using 13 components, including one IC, two transistors, two diodes, and a thermostat. This fan control circuitry is needless sophistication, and the wisest thing to do is to disconnect the wires from the thermostat and insulate them, which will leave the fan on 100% of the time.
If you take the lid off you'll note the following:
The fan:
The fan doesn't shift much air - the term "forced air cooling" being overly optimistic. The flow of air needs to pass over the heat sink onto which five 2N3055 power transistors and the bridge rectifier (rectum frier?) is mounted. However, the airflow doesn't do that. The mains transformer blocks the path, so the air just blows at the transformer and out through the vent holes in the lid. Any air that passes over the heat sink does so more by luck than design.
The heatsink:
In the ones I've repaired the heat sinks have been dull silver alloy. Elementary physics ( black body/non-black body radiation etc), dictates that the ideal radiator is smooth, black, and shiny. In the EP925's I repaired, the heat-sinks had the opposite of these qualities. If heat-sinks aren't fan-cooled, (and that must include the EP925 as the fan does very little), ideally, they should be large, black, and mounted externally and upright to enable convection currents to rise past the heat sink causing a flow of cooler air to pass over it. The heat-sinking on the EP925 is internal, horizontal and compact. It consists of two stacked above one another, carrying the five 2N3055's and the bridge rectifier between them. Thus, the upper heat sink shields the lower one and little heat can be dissipated from the upperside of the lower heat-sink, and the underside of the upper one.
Of those that I repaired, in each case, for whatever reason, all five power transistors had failed - happily, open circuit. Quite beyond economic repair by a dealer - a labour of love as a favour to a friend.
There's lots more I could say about the design shortcomings of that admittedly neat looking budget PSU, but the homebrew section of the forum isn't the time and place to say it. I merely cite it to illustrate that 'commercial' doesn't mean 'better' and home-brew can incorporate whatever features you feel are essential or desireable.
Keep the soldering iron hot!
David
The popular twin meter PSUs sold under the Manson EP925 and other brand names (Palstar?) have been around for years - there must be thousands of those PSUs throughout the world that have given, and continue to give, giving trouble free service. That said, from personal experience having repaired several over the years, they have certain design shortcomings which will be apparent on taking the lid off. They're clearly built down to a price but represent good value for money, so long as too much isn't expected of them.
The most important shortcoming is the lack of over-voltage protection, which isn't uncommon in "budget" PSU's. This means that if a collector/emitter short arises in one or more of the power transistors, the full rectified DC output of the mains transformer (>25V) would be plonked
across any equipment connected to the PSU. The adverse effects of this would depend on the ability of that equipment, (multi-band amateur radio transceiver or whatever), to withstand that sort of over-voltage.
An over-voltage crowbar circuit could have been incorporated into the PSU with just a few components, but for some reason the maker’s left that feature off, yet included a fan control circuit using 13 components, including one IC, two transistors, two diodes, and a thermostat. This fan control circuitry is needless sophistication, and the wisest thing to do is to disconnect the wires from the thermostat and insulate them, which will leave the fan on 100% of the time.
If you take the lid off you'll note the following:
The fan:
The fan doesn't shift much air - the term "forced air cooling" being overly optimistic. The flow of air needs to pass over the heat sink onto which five 2N3055 power transistors and the bridge rectifier (rectum frier?) is mounted. However, the airflow doesn't do that. The mains transformer blocks the path, so the air just blows at the transformer and out through the vent holes in the lid. Any air that passes over the heat sink does so more by luck than design.
The heatsink:
In the ones I've repaired the heat sinks have been dull silver alloy. Elementary physics ( black body/non-black body radiation etc), dictates that the ideal radiator is smooth, black, and shiny. In the EP925's I repaired, the heat-sinks had the opposite of these qualities. If heat-sinks aren't fan-cooled, (and that must include the EP925 as the fan does very little), ideally, they should be large, black, and mounted externally and upright to enable convection currents to rise past the heat sink causing a flow of cooler air to pass over it. The heat-sinking on the EP925 is internal, horizontal and compact. It consists of two stacked above one another, carrying the five 2N3055's and the bridge rectifier between them. Thus, the upper heat sink shields the lower one and little heat can be dissipated from the upperside of the lower heat-sink, and the underside of the upper one.
Of those that I repaired, in each case, for whatever reason, all five power transistors had failed - happily, open circuit. Quite beyond economic repair by a dealer - a labour of love as a favour to a friend.
There's lots more I could say about the design shortcomings of that admittedly neat looking budget PSU, but the homebrew section of the forum isn't the time and place to say it. I merely cite it to illustrate that 'commercial' doesn't mean 'better' and home-brew can incorporate whatever features you feel are essential or desireable.
Keep the soldering iron hot!
David
