# fun with open frame power supplies

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#### unclejed613

##### Well-Known Member
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.

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.

#### tcmtech

##### Banned
Yea, I collect them too. Probably have 50-100 of them of every possible shape, size and design now from a few watts to 1- 2 KW.

Many 20 - 30+ years old with 10's to 100+K running hours and still fully functional.

Nice thanks!

#### unclejed613

##### Well-Known Member
Power One Forever..... lol they are well made and very reliable, and except for one +150V one that had a couple of bad diodes, i don't think i've ever seen another one where the failure wasn't the crowbar circuit.... well there was a +5V 25A one that died because of multiple bad electrolytics in a piece of banking equipment, but it was a switcher, not a linear. we replaced it with a $30 PC supply because the replacement switcher was$450. the PC supply worked just fine in that machine.

Delta and Sola are two other manufacturers that make them.

quite often, the error amp is some variation of the LM105/LM305. some even use a general purpose op amp like the LM324.

i have reference design schematics for open frame supplies dating back to the 1960's (before op amps (well, technically the circuits are op amps, but made with discrete transistors)).

it looks like Power One, and others just lifted the reference designs and began building power supplies, as there's very little changed between the reference designs and the finished product. nowadays semiconductor companies only let you see the reference designs if you have an NDA, instead or publishing them in the data sheet. i think i might actually post one of the reference designs for a discrete transistor type supply...

#### JimB

##### Super Moderator
Power One Forever..... lol they are well made and very reliable
I will agree with that.

I love the mechanical design, the reservoir capacitor screws directly on to the circuit board and can be easily removed, there are holes in the chassis for access to the heads of the Allen screws in its terminals.
The T03 transistors and big diodes can each be removed for replacement by undoing two screws, there is a bespoke spacer which insulates the transistor pins from the chassis and holds the circuit board clear of the chassis.
Remove all the transistors and you have released the circuit board from the chassis.
Simply brilliant.

JimB

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