The Experiment: In this experiment were going build a power supply for the  legal limit HF 1.8-54 MHz LDMOS solid state power amplifier (SSPA). Will be using  (4) four HP DPS-1200FB Power Supply in series (4) four will produce 50V DC and (5) five with the 13v mod will produce 66V DC at 100amps  @240v AC.


Table of Contents
1. Part List
2. Prototype
3. Notes
4. Conclusion
5. Reference Links


Part List:

1. (5x) 441830-001 HP-Compaq 1200 Watt 12 Volt DC Out Put Redundant Power by HP
2. 10 / 12 Gauge Wire
6. 9/64 and 1/8 Drill bit




To produce 50v DC will need 4 (four) HP DSP-1200FB switching power supplies. I am going to set each supply for 12.55v out, this will give 50v. All the power supplies will be floated so that  12 volt is isolated from the power supply case.

Note: In this experiment, will be using 5 (five) HP DSP-1200FB switching power supplies giving the option to produce 66v DC out when each power supply is set to 13.5v.

Turning On the HP DSP-1200FB

Look at the back end of the supply, From left to right:

1. Six thin terminals labeled 33 through 38. The labels are actually only on 33 and 37, but you can fill in the blanks. Two wide terminals labeled 51 and 64…these are your return (-) and +12V DC output terminals.

2. To turn the power supply on bridge a 330-1000 Ohms resistor between terminals 33 and 36. Note: Terminal 36 is a little shorter the the other terminals.

Before moving on, plug the supply into a wall socket and check first that the green LED comes On, then using a digital multi meter measure the DC output left = GND / right = 12V.

Adjusting the DC output voltage

1. Remove the 4 (four) Philips head screws to remove the top cover.

2. Gently pull back  the black rubbery insulation cover.

3. Looking at the back of the power supply, the PCB on the right side you will see 3 (three) small photometers, the voltage adjust is the third one on the right.  Using a small screw driver adjust the voltage to 12.55 volts. Counter clockwise – 11.7  / clockwise + 12.7.

Once the voltage is set to 12.55 volts, if needed  float the DC output or re-assemble the case.

Floating the  DC output

Now, chaining the DSP-1200 together is easy. As long as they are NOT connected via DC Ground (which goes to the case, which goes to earth ground, which would lead to big problem). So to chain  DSP-1200 together the case has to be isolated from the DC Ground.

Warning: Do not disconnect the AC ground from the case and. Do not float the front PCB hold down screw next to the AC input.

To float the DC output you only need to isolate the two back PCB hold down posts/screws.

Note: If you’ve  floated a 441830-001 HP-Compaq 1200 Watt power  and only get 10.5 or 11.5v DC out. See notes section: Observation:

1. To float the DC output: (you don’t have to completely disassemble the power supply just remove the 4 (four) screws then the case cover) Looking at the back of the power supply assembly, you’ll  see two screw holding down the PCB board. Just remove the two screws then gently lift PCB up a little and slide a plastic washer over each each flange on top of the post.

Make 2 (two) 3/8″ plastic washer,  drill a 9/64 hole in the center of each washer, then put one washer on each flange on top of the post that goes into the PCB.

2. Now make 2 (two) 3/8″  plastic washers for the screw,  drill an 1/8″ hole in the center of each washers, then put one washers on each screw. Now put the screw back and screw down the PCB.

3. Now Check with the multi-meter to be sure that there is NO connection between the case and DC Ground. Check both the + (positive) and the – (negative) sides. If it’s not isolated from case re-check  both bottom plastic washers.

 If the above test passed then:
Plug-in the power supply and verify that there is 12.55v DC out.

Note: If the DC out is 10.5 or 11.5v .  See notes section: Observation

Output voltage might need a little tweaking to 12.55v, before re-assemble the power supply.

That’s it the DC output  has been floated from the case, with the AC ground is still connected to the case.

Note: The DC out tends to drop  under load to 12.51v,  that’s why the voltage was set to 12.55v

10/2/2017: Well one of the DSP1200 power supply went out with a big pop, was plugged in by itself, don’t know why it went out the others are fine. These are used so expect some  problems. Ordered a replacement.

Building The LDMOS Power Supply Case

Dimension: HWD-4.5×12.5×13.5






Observation: When floating the first  441830-001 HP DSP-1200FB A, Noticed that the DC voltage drops to 10.5 or 11.5v. When touching the housing the voltage would jump to 12.5v for a couple of seconds the back to 10.5 or 11.5 volts.

Did not want to re-adjusted the output voltage, wanted to know why the voltage dropped 1 to 2 volt when the DC was floated. To track down the problem removed the complete power supply assembly from the case, plug it back in and DC out was still 10.5 or 11.5 volt.

Thinking that it might have something to do with the AC ground, I jumped the AC ground lead to the DC ground and wow 12.5v DC. So there some inter action between the DC ground / AC ground. floating the DC output brakes the connection between the AC / DC ground, so I though want AC to pass and DC block.

My solution was to insert a .01uf disk capacitor between the case and the DC ground. That return 12.55v DC back  to the floated power supply.

13.5v Potentiometer Modification:

The current voltage adjust potentiometer will adjust the voltage between 11.7v to 12.7v.  To increase the DC output to 13.5v the voltage adjust potentiometer will have to be replaced  with a 2k potentiometer.

Some possible part number for SMD 2k potentiometer at mouser:
652-TC33X-2-202E, 279-3152W202P, 279-3142W202P, * 667-EVM-3YSX50B24, 1158221

Note: The Over Voltage Protection (OVP) will triggers around 13.6v-13.7v.

HP-1200FB Pin Assignment:

1-14-51-64 =main 12V feed
27-29 = PMBus
30 = Signal Gnd
31 & 32 = PMBus SDA/SCL lines
33 = Enable#
34 = Current Monitor
35 = PSU status
36 = Present
37 = 12V Standby Power
38 = PS alarm normally have an active low alarm output









Reference Links:

1. Reverse-engineering the HP DPS-1200FB power supply
2. Setting Up the HP DPS-1200FB Power Supply
3. Engineering Art From Science

Reference Info