[Repeater-Builder] Re: Help with power supply
FedEx delivers on the 27th and I will be able to size it up then. First I realize that 28V supplies are scarce but I have no need of one, I do need a good 25A rack supple for a GE MII 6 meter repeater that is going to be improved shortly. I like the idea of using the 220 input to lower the voltage. That was something I had not thought of. The conversion of the board and crowbar circuits are not a problem. If any one wants to swap an equivalent rack mount for the unit I am great with it. Charles, NM4V --- In Repeater-Builder@yahoogroups.com, Eric Lemmon wb6...@... wrote: Charles, You can't convert a 28 VDC Astron power supply to a lower voltage, unless you replace the power transformer and regulator board, and possibly the pass transistors. Since the Astron linear power supplies use a series-pass regulator design, the pass transistors could burn up if expected to regulate at full design current but at half the rated voltage. That said, you CAN install a 14 VDC regulator board (or modify the 28 VDC regulator board with components suitable for 14 VDC) and run the power supply at less than half its rated load. I would never do this, because the value of a good, high-capacity 28 VDC power supply is always much greater than one that has been hacked to operate at a lower value. Before you decide what to do, call Fred (the Tech Support guru at Astron) and ask his advice. Go to the Astron web site for the phone number. 73, Eric Lemmon WB6FLY --- In Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com , Charles Lowery clowery@ wrote: I purchased a Uniden 28 volt power supply today on ebay. It lookes like the Astron LSRM-25A or 35A both front and back (I assume Astron made it). Without seeing it, I assume it works OK (cost $2.00 + shipping). Has anyone converted one of these to 12 volts? I have no schematics for it and can not find one on the net. If converted what would be the available current? Charles, NM4V
[Repeater-Builder] Re: Help with power supply
--- In Repeater-Builder@yahoogroups.com, Charles Lowery clow...@... wrote: I purchased a Uniden 28 volt power supply today on ebay. It lookes like the Astron LSRM-25A or 35A both front and back (I assume Astron made it). With out seeing it I assume it works OK (cost $2.00 + shipping). Has anyone converted one of these to 12 volts? I have no schematics for it and can not find one on the net. If converted what would be the available current? Charles, NM4V Do you have no of model ? or open the rig and change the zenner diode for a lower value!
[Repeater-Builder] Re: Help with power supply
Charles Checkout the Astron page on the repeater builder website. There is a lot of info on general linear supplies using the LM723 regulator chip. You should be able to change the zener reference diode, and tune it down to 13.8v and deliver the same current. ***Danger, notice here *** You will NOT have the 13.8v overvoltage - crobar protection without changing that circuit also. You will have whatever it had at 28v. In my experience, regulator transistors seldom fail open, they often short, delivering the full transformer output into the crowbar circuit, Which if it fails to crowbar, frys your precious ham rigs. This supply will probably have 35 volts or more at the caps, even under load. If you dump 35v into your 13.8 rig, it will not survive!!! It might burst into flames!! If only the output voltage is lowered, the regulator transistors are going to dropping more voltage, and will get MUCH HOTTER!!! You will be dissipating 3 times more heat then it would have at 28v. Fans!! Several of them! My theory, ( without seeing this supply ) 120v input to transformer, transformer output 35v, bridge, Caps, 35VDC, regulated down to 28v. The transistors are dropping 7v. 7v @ 15a = 105 watts of heat. If you just tune the voltage reg down to 13.8, you will still have that 35vdc from the rectifier. The transistors are now dropping 21.2v 21.2 @15a = 318 watts of heat. That might be too much dissipation per transistor, and cause them to fail fast. You might gain an advantage by changing the 125 volt input to the 220v option. This will lower the voltage on the transformer secondary. You should still get around 17.5v or more at the secondary/rectifier-caps, which will reduce the heat generated in the regulator transistors. I use a junker automotive foglamp (free from junked truck) for load testing of unknown powersupplies. It pulls about 5 amps, and is tolerant of over voltage conditions and even runs on 12v AC. 73 Ed N3SDO
RE: [Repeater-Builder] Re: Help with power supply
Charles, You can't convert a 28 VDC Astron power supply to a lower voltage, unless you replace the power transformer and regulator board, and possibly the pass transistors. Since the Astron linear power supplies use a series-pass regulator design, the pass transistors could burn up if expected to regulate at full design current but at half the rated voltage. That said, you CAN install a 14 VDC regulator board (or modify the 28 VDC regulator board with components suitable for 14 VDC) and run the power supply at less than half its rated load. I would never do this, because the value of a good, high-capacity 28 VDC power supply is always much greater than one that has been hacked to operate at a lower value. Before you decide what to do, call Fred (the Tech Support guru at Astron) and ask his advice. Go to the Astron web site for the phone number. 73, Eric Lemmon WB6FLY --- In Repeater-Builder@yahoogroups.com mailto:Repeater-Builder%40yahoogroups.com , Charles Lowery clow...@... wrote: I purchased a Uniden 28 volt power supply today on ebay. It lookes like the Astron LSRM-25A or 35A both front and back (I assume Astron made it). Without seeing it, I assume it works OK (cost $2.00 + shipping). Has anyone converted one of these to 12 volts? I have no schematics for it and can not find one on the net. If converted what would be the available current? Charles, NM4V
