the best thing about them, if you know somebody that works for a company that maintains industrial equipment, is you can often get them for nothing, or next to nothing. you can often find them being sold by companies that sell surplus electronics. in my experience, most open frame power supplies are very reliable, and the big failure item is the SCR that's used to crowbar the supply if there's an overvoltage condition. most of the time the SCR is shorted, and that's the only thing keeping the supply from working. removing the SCR will get the power supply running again (or replacing it, but make sure the supply is dialed back down below the crowbar's trigger voltage before reconnecting the crowbar circuit).
open frame supplies are usually analog power supplies, but many switching types are becoming common. i prefer the analog ones, since i do a lot of radio stuff. industrial switchers often don't have a lot of RFI suppression built in. on a standard open frame supply, you usually have two potentiometers in the regulator circuit, output voltage, and current limit. the output voltage control usually has a range of +/-10% of the nominal output voltage. the current limit operates from near zero, to the rated current of the supply. the current limiting is usually foldback limiting, so the voltage will start to drop when the current limit is reached.
three other useful features that open frame supplies often have are 1) sense inputs to compensate for wiring resistance, 2) the supply is not grounded to the chassis frame, and is floating, and 3) the raw DC voltage before the regulator is often about 20% higher than the nominal output voltage.
the sense inputs are usually found on the terminal strip next to the output terminals, so a terminal strip might be marked +S, +V, -V, -S. in operation, the +V and -V wires go to the load, and since they are carrying current, they usually are a heavier gauge wire. the sense wires can be directly connected to the output terminals, but in applications where the voltage at the load needs to be tightly regulated, the sense wires (often using #22 or #24 wire) are connected to the load end of the V+ and V- wires, to compensate for the voltage drop in the wiring. if the sense wires aren't connected at all, the supply output is disabled.
the fact that the supplies are floating, makes it easy to connect two supplies in series to provide a bipolar (+ and-) power supply, or use multiple supplies to get one much higher voltage.
the fact that the original unregulated DC is 20% higher than the nominal voltage, means that that voltage is available for use, either by modifying the regulator circuit (somewhat difficult), or by tricking it (much easier).
for the following examples, the supply will be a typical 12V open frame supply with a 6A output:
trick #1 more voltage... let's say we need 14.6V for a particular application. the voltage adjustment only goes to 13V (disconnect any crowbar circuit before trying this). connecting two silicon diodes in series between the +V and the +S, and two diodes between the -V and -S (all diodes forward biased) will give an extra 2.4V at the output terminals, because the sense wires are each seeing 1.2V less than is actually there at the output terminals.
trick #2 constant current charging of batteries... the current limit is turned all the way down, and a multimeter set for current measurement is connected between the +V and -V. turn the supply on, and adjust for the desired current on the meter. disconnect the meter (make sure you switch the meter leads back to measure voltage), and set the desired voltage for the batteries (13.6 for lead-acid). use the diode trick above if the voltage adjustment doesn't go that high. connect the batteries, and the voltage will fold back to some low value. the batteries are fully charged when they reach the set voltage.
trick #3 bipolar supply... using two open frame supplies, connect the +V of one supply to the -V of the other supply. connect the sense wires as needed (i.e. to the load, or directly to the output terminal). use the connection between the two supplies as ground.
i have a ham rig which is a 200 watt Kenwood transceiver. to actually run 200W, requires about 20A at 13.6V. somewhere i had come across two huge open frame supplies that were 5V@50A that were being discarded. the reason they were being discarded was the crowbar SCRs on both of them were shorted. i removed the SCRs and got the supplies working, but they only went up to about 5.8V. so i connected diodes between the sense and voltage terminals, and got both of them up to about 7V each. then i connected them in series, and adjusted for 13.6V, and got my radio operating at 200W. considering that the supplies together weigh about 75lb and take up as much space as a large PC, it looks kind of awkward, but it works, and the current limit on each supply is set to about 22A. the analog supplies might be big and bulky, but they do the job without all kinds of radio noise that i would get with switchers.
keep in mind that these are OPEN FRAME supplies, meaning the power transformer terminals are usually exposed. it's usually a good idea to find a box to enclose them in to keep stuff away from the line voltage terminals. there are usually exposed regulator transistors on the frame as well that should be protected.
open frame supplies are usually analog power supplies, but many switching types are becoming common. i prefer the analog ones, since i do a lot of radio stuff. industrial switchers often don't have a lot of RFI suppression built in. on a standard open frame supply, you usually have two potentiometers in the regulator circuit, output voltage, and current limit. the output voltage control usually has a range of +/-10% of the nominal output voltage. the current limit operates from near zero, to the rated current of the supply. the current limiting is usually foldback limiting, so the voltage will start to drop when the current limit is reached.
three other useful features that open frame supplies often have are 1) sense inputs to compensate for wiring resistance, 2) the supply is not grounded to the chassis frame, and is floating, and 3) the raw DC voltage before the regulator is often about 20% higher than the nominal output voltage.
the sense inputs are usually found on the terminal strip next to the output terminals, so a terminal strip might be marked +S, +V, -V, -S. in operation, the +V and -V wires go to the load, and since they are carrying current, they usually are a heavier gauge wire. the sense wires can be directly connected to the output terminals, but in applications where the voltage at the load needs to be tightly regulated, the sense wires (often using #22 or #24 wire) are connected to the load end of the V+ and V- wires, to compensate for the voltage drop in the wiring. if the sense wires aren't connected at all, the supply output is disabled.
the fact that the supplies are floating, makes it easy to connect two supplies in series to provide a bipolar (+ and-) power supply, or use multiple supplies to get one much higher voltage.
the fact that the original unregulated DC is 20% higher than the nominal voltage, means that that voltage is available for use, either by modifying the regulator circuit (somewhat difficult), or by tricking it (much easier).
for the following examples, the supply will be a typical 12V open frame supply with a 6A output:
trick #1 more voltage... let's say we need 14.6V for a particular application. the voltage adjustment only goes to 13V (disconnect any crowbar circuit before trying this). connecting two silicon diodes in series between the +V and the +S, and two diodes between the -V and -S (all diodes forward biased) will give an extra 2.4V at the output terminals, because the sense wires are each seeing 1.2V less than is actually there at the output terminals.
trick #2 constant current charging of batteries... the current limit is turned all the way down, and a multimeter set for current measurement is connected between the +V and -V. turn the supply on, and adjust for the desired current on the meter. disconnect the meter (make sure you switch the meter leads back to measure voltage), and set the desired voltage for the batteries (13.6 for lead-acid). use the diode trick above if the voltage adjustment doesn't go that high. connect the batteries, and the voltage will fold back to some low value. the batteries are fully charged when they reach the set voltage.
trick #3 bipolar supply... using two open frame supplies, connect the +V of one supply to the -V of the other supply. connect the sense wires as needed (i.e. to the load, or directly to the output terminal). use the connection between the two supplies as ground.
i have a ham rig which is a 200 watt Kenwood transceiver. to actually run 200W, requires about 20A at 13.6V. somewhere i had come across two huge open frame supplies that were 5V@50A that were being discarded. the reason they were being discarded was the crowbar SCRs on both of them were shorted. i removed the SCRs and got the supplies working, but they only went up to about 5.8V. so i connected diodes between the sense and voltage terminals, and got both of them up to about 7V each. then i connected them in series, and adjusted for 13.6V, and got my radio operating at 200W. considering that the supplies together weigh about 75lb and take up as much space as a large PC, it looks kind of awkward, but it works, and the current limit on each supply is set to about 22A. the analog supplies might be big and bulky, but they do the job without all kinds of radio noise that i would get with switchers.
keep in mind that these are OPEN FRAME supplies, meaning the power transformer terminals are usually exposed. it's usually a good idea to find a box to enclose them in to keep stuff away from the line voltage terminals. there are usually exposed regulator transistors on the frame as well that should be protected.
