Low-ripple HP power supply
Jun 21, 2017

I got a power supply from eBay a while ago:

That’s an HP 6255A: 2x 0..40V @ 1.5A, max 80V in series. But the interesting spec is its very low noise: <0.5 mV and <0.2 mA.

These supplies are very clean - and it’s all done with old-school linear regulation. So there can be a lot of heat to get rid of:

That back section is painted in RED for a reason: it contains some resistors which act as energy radiators. The other section is a heat sink with fins for the trusty 2N3055’s.

This was sold as US unit (with substantial shipping costs, alas), so it came set for 110 VAC, and initially I didn’t even realise it could be adjusted. Luckily, these units can in fact be modified to operate at 220V and will support a line frequency of 50..400 Hz (the latter is common aboard airplanes).

As it so happens, the manual can be found online and describes every detail, including schematics and adjustment info. Turns out that you have to cut some traces and solder a jumper inside to convert it to 220V:

Next problem was badly dislodged meters:

For one of the meters, the plastic mounting bracket was in fact missing. And as you can see, this is all really old-school electronics. Discrete components, and it’s basically just a doubled-up unit, with all the front panel elements mirrored for more natural use.

Here’s another close-up - neat and tidy:

I fixed some bad connections on the pots and after that, everything started working!

So now I have a shiny, as-new, heavy, hot, eh… brick to power circuits with very good regulation and extremely low noise:

I’m considering replacing the meters with V+A LED displays … some other day.

There’s also a connector strip on the back, which can be re-wired to support a form of “remote control”. Analog remote control that is, i.e. external voltages can be used to drive the settings of this supply. Whatever.

I have more old gear here, all surprisingly operational and useful, which I’d like to mount in some sort of scope trolley cage:

That way it could be carted under the desk when not in use. Because, let’s face it, how often do you really need to use old & bulky electronic gear, when a small LiPo-based regulator is often more than enough for (digital) projects, many hours on end?

Still, I’m happy with it. Quality equipment deserves a second chance, even if it’s old.

For comments, visit the forum.

Quad 18650 power supply
Jun 14, 2017

Might not have been such a great idea to 3D-print this DIY supply I made in red:

(it does look slightly alarming, I’m afraid)

This is a little portable power supply I made a while back, with four very common type-18650 cells (widely available from eBay, despite today’s air shipment regulations).

It has the following properties (“specs”):

  • adjustable voltage from zero to ≈ 13V
  • maximum current 3A’ish (see below)
  • with (slow) USB charger built in
  • built with low-cost parts from eBay
  • tape + hot glue to keep it together

I’m tired of wires running all over my desk - this is one way to try and cut down on some of that while tinkering with electronics.

There is one bit of cleverness (with thanks to Martyn for suggesting this approach):

The 4 LiPo cells are placed in series while the supply is switched on, for a maximum feed of ≈ 14.4 to 16.8 V into the switching regulator. This translates as over 12V of regulated voltage coming out this supply.

For charging, with the switch in the “off” position (flush is “on”, extended is “off”), the cells are placed in parallel, tied to a tiny USB LiPo charger. A full charge probably needs the entire night.

The components used were as follows:

  • 4 unprotected (!) battery cells - eBay
  • adjustable regulator - like this one
  • USB LiPo battery charger - eBay
  • 6-pole double throw switch - eBay
  • 3D-printed holder - Thingiverse
  • four 1 Ω @ 1 W resistors - optional

Here is the setup while it’s being assembled:

It’s not easy to find an eight-pole double-throw switch, but fortunately, six poles is enough: one cell is permanently connected to the charge board (with a 1Ω resistor in series), while the other three are each connected to the switching side of the multi-pole push-on/push-off switch.

It’s a very neat puzzle to figure out how to connect things up. Since both sides of each cell are switched, it’s quite straightforward and will be left as exercise for the reader …

When charging, all cells are connected in parallel. To limit the equalisation currents which flow when cells are in varying states of discharge, each cell has 1 Ω in series - also reducing switch contact wear & arcing.

One caution though - and something I did not take into account at the time: with unprotected LiPo cells, especially those fat 18650 models, shorting out “+” and “–” can cause large currents to flow (over 25A).

It’s surprisingly tricky to assemble such a circuit while making sure such shorts never happen. All the loose wires are at risk until they have been properly connected, and of course any mistake could cause trouble.

Anyway, I did get through the assembly without such problems. The final test was to measure the short-circuit current on the output of the regulator, since it has no adjustable current limit. It turns out that the maximum current is around 5 Amps (using a standard multimeter in 10A range) - well above the circuit’s specified 3A max!

Note that though the output current draw is measured and displayed, it’s not limited. Which makes this supply less generally useful for breadboard tinkering, and more something of a “pre-regulator”. It does have a lot of energy and the 12V range means it can be used when 5V USB is not enough.

One constraint to be aware of with cheap current-sensing supplies, is that the shunt resistor is “low-side”, i.e. the output “–” is not exactly the same as the GND level from USB. This is not an issue as long as you don’t plug in the charger during use.

As a simple adjustable voltage source, this unit works great. For taking along on a trip and through airport security: maybe not …

For comments, visit the forum.

Hello Pi-hole, goodbye ads!
Jun 7, 2017

It’s that time of year again: this is the start of “summer mode” for the JeeLabs weblog.

Instead of weekly article series, there will be just one post per week, covering a somewhat wider range of topics than usual.

This week, I’d like to report on a very nice little gadget to block most adware and even entire sites from all machines on the local LAN and WLAN network here at JeeLabs:

All the magic comes from an OSS package called Pi-hole, which is a word-play on this:

It’s delightfully easy to install, and a great match for dedicated Linux boards like the Raspberry Pi, Odroid, or CHIP. I used the latter, since it’s the cheapest of them all (when they’re in stock…) and has both flash storage and WiFi. It even does LiPo backup.

I also 3D-printed a little cover (in a hurry) to dim its annoying LED, using this design.

It’s brilliantly simple: Pi-hole takes over as DNS server. This is done with a single configuration change on my home router: I just tell it to announce the Pi-hole as local network DNS server, instead of the router itself (which delegates lookups to my ISP).

Pi-hole itself in turn, maintains a list of over 100,000 domains which it hijacks, and redirects to its own IP address. The result is that when a browser (or any other app!) requests info from one of these domains, it’ll see Pi-hole’s error reply instead, like this:

This disables images and tracker requests. Also on all your mobile devices using WiFi!

Furthermore, Pi-hole includes an excellent web-based admin / configuration panel:

Note that there’s one pesky little detail: you must get the DNS routing absolutely right. This means you have to make sure Pi-hole will use the router, not itself, for forwarding DNS requests it doesn’t know about.

Any mis-configuration will cause all DNS lookups on your network to fail (!). Note also that the Pi-hole must remain powered up and working properly at all times.

But once set up, it’s absolutely brilliant. I’m so delighted with this, that I’ve also black-listed “facebook.com” - good riddance…

There’s no performance penalty since only the DNS requests are redirected, not the actual page transfers and downloads. In fact, some sites may become faster as their adware payloads are never seen or fetched.

For comments, visit the forum.

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