by Josh Barfield N4NZ
Everyone needs a good power supply for today’s solid state HF and VHF radios. 20 years ago, your only option was a bulky, hot and heavy linear power supply from Astron or similar (which are still great supplies), but 35 amps of 12 volt DC weighed about 15 pounds and put out a bunch of heat as well. Thanks in part to the advances in computer servers, switching power supply technology has improved greatly from even several years ago. Today’s modern computer servers need lots of clean 12V DC power that can ramp up or down in current instantly – sound familiar?
The good news about these supplies is that they can supply an incredible amount of current from an incredibly small package. The 1200 watt supplies can deliver 60 amps of clean power – that’s 30+ pounds worth of Astron supplies – from a small 2 pound module.
There’s only two problems: 1) We are used to having a 13.8V supply, not a 12V supply, and 2) the raw HP Server supply leaves something to be desired in packaging.
Modding these widely-available HP Power Supplies to obtain 13.8V DC has been around for several years. Problem is, most of these supplies are just different enough inside as to make a universal modification impossible – every specific version is just a bit different. In addition to increasing the voltage, these supplies must also be modified to raise the self-protection “crowbar” circuit that shuts the supply down if an overvoltage condition is detected (which the 13.8V output will usually trigger).
Since these are built specifically for the server manufacturers (usually by third party companies like Delta Electronics), there are no schematics or circuit descriptions available. All these mods typically are reverse engineered. There are probably a couple of hundred different models and part numbers of these supplies, so it’s nearly impossible to keep track of a simple set of mods.
Having said that, Chip KD4C recently ran across a simple 2 resistor mod for a specific HP supply part number that’s fairly easy to do and seems to work great.
This is the original video from YouTube user “Mr. BBI” explaining why this particular model is easy to hack. The video is overly long (thanks YouTube monetization model!), but at the 21 minute mark he starts talking about resistor values and the potentiometer positions for both the voltage and crowbar mods. He then goes on to show just how far he can push the supply in both voltage and current! While I don’t think may of us *need* a 15.5V supply that can deliver 55 amps, it’s certainly possible with these!
This video by YouTuber “767 CB” explains how he traced the original solder point to a much easier location (so you don’t have to solder to a tiny pot). Around 2 minutes in he explains where to connect the resistor. He didn’t use shrink tubing on his resistor, but I think it’s wise to wrap it up. But his mod certainly works.
Now that the mod is done, how do we package it for use in the shack? 3D Printing to the rescue! This is the 3D printed cover (designed by Bill K5FZBL, president of DARC) with a parts list for the USB step down converter, power switch, and voltmeter.
https://www.thingiverse.com/thing:3781332
Bill’s 3D Printed cover contains 2 Power Pole connectors, as well as a power on-off switch and a voltmeter module (available from Amazon or AliExpress), resulting in a decent, safe package with no exposed voltages. There’s also space for an optional USB supply on the side so you can power any USB devices. Hooking it all up requires just a bit of soldering to the exposed “blade” connector on the HP Supply. It’s a bit tricky to make sure that you get a decent connection that will withstand 30 or so amps, so I recommend 12 gauge wire for the power pole connections. The space is tight but you can make it work and do it safely.
You need this *specific* HP Supply for these mods to work: DPS-1200SB A and HSTNS-PD30
Luckily there appear to be hundreds (if not thousands) of these available for sale on eBay, in the $20-$30 price range (and another $5 for parts). The original seller that is mentioned in the Mr BBI video no longer offers these, but they are listed by other sellers. Please confirm with the seller the *exact* part number listed in the picture to ensure the mod will work.
I used a 51k resistor for the crowbar hack and a 15k resistor for the voltage side. That makes the output 14.5 volts. You can play with the resistor values for desired voltage. I believe a 20k on the voltage side gives you 13.7v. I had to adjust the pots as mentioned in Mr. BBI’s video at the 21 minute mark or crowbar protection would trip when I tried to put a load on the power supply. The pots are fully CCW for the first pot (voltage) and half way (horizontal) for the right pot (crowbar). Chip had some problems with the crowbar at 14.4V output, but changing the 15k resistor to 18k yielded 14.1V and no crowbar issues.
As I mentioned, everyone needs a good power supply for their radios, and this seems to be an excellent solution for a very reasonable price!
