Computing stuff tied to the physical world

Transformers!

In Hardware on Nov 20, 2011 at 00:01

(No, not comic strips or movies…)

With 230V experiments becoming more commonplace here at JeeLabs, I’m worrying about safety again. It doesn’t take much to get sloppy once you do things over and over. But sloppiness and 230V don’t mix well!

Given the recent trials, and some great comments, I’d like to get back to a safer voltage level for day-to-day testing. Using the isolation transformer only for incidental cases because no matter what, 230V is tricky.

According to Wikipedia, a “safe” voltage is 50 VAC or less. According to another page, it’s even lower: 30 VAC.

Until now I’ve been using my lab supply, which goes up to 30 VDC. But that’s limited, and more importantly it doesn’t let me see the effects of AC rectification and ripple. It’s time to come up with a better test setup:

JC s Doodles page 25

A couple of small well-insulated PCB transformers, one rated 2x 18V @ 45 ma and two with 2x 28V @ 20 mA secondaries, respectively. The 18V and 28V secondaries are completely safe (by themselves) – they might damage a circuit, but they won’t harm me.

When combined, these can produce from 18 to 148 VAC. Note that 148 VAC is still over 400 Volts peak to peak, so this is serious stuff. The best way to keep the risks down is still to use low voltages as much as possible, of course.

One reason why these voltages are less risky than AC mains, is that the output current is very limited. I picked the weakest transformers I could find, meaning that their secondary coils have a high internal resistance and small magnetic cores. That and magnetic saturation should keep maximum currents limited to 20..30 mA.

Tomorrow I’ll describe a convenient hookup for all these little power sources.

  1. About “safe” home grid – at home, I have two power grids: 220V AC and 12V DC. Second goes along the first one and terminates with standard coaxial plugs, – it’s perfect powering laptop and all kind of PC peripherals, DECT phone, ADSL modem, small NAS and others. Most of recipients don’t need any modifications for accepting this power source – same 12V DC coaxial plug is already there. Plus, one 10-amp 12V DC supply is way more efficient, that bunch of dispersed AC/DC adapters, and of course, it’s much easier to interface this with 12V sealed lead accu, which accepts current from the small photovoltaic panel on the roof.

    • I like the idea, but it’s definitely a trade-off: in existing houses you have to open up the walls and add (thick!) new cabling. Also, aren’t the losses substantial due to higher currents?

      My thinking was that if I can get AC current monitors to draw less than 0.1W, then a bunch of those won’t need much energy, and they let me at least monitor where the power consumers are at all times.

      But in a local way, I’m actually trying to do the same: a single efficient 12V brick to feed all the devices hooked up close to each other: around the server and around my workstation, in particular.

    • I have the same feeling towards the 12V (or 24V) DC bus in a house as JC. It adds complexity to the installation (and cost!, ~€1000 if done by a professional?) and the losses tend to be higher in nearly all scenarios. It is correct that a central 12V brick can be done with higher efficiency (but more than 92-93% is hard to achieve). But on the other hand all the small local power supplies can be optimized to deliver the exact required voltage. For charging a mobile for example, the 12V from the bus has to be converted to 3-4V. This is again producing losses. So no big positive effect over all.

      The UPS functionality when combined with a battery and PV is certainly a big plus. I was actually thinking of such a setup for powering my Homeserver (45W average 24/7).

  2. How about 2 solar panels in series, connected directly to the Ethernet cabling via a diode? With a 24V back-up battery connected somewhere along the cabling, charged via POE when possible and from AC mains otherwise.

    Then any POE-enabled ethernet device would work as is (POE switches, for example), and a small converter might be enough to power a server.

    Convenient (existing cables) and tidy (less power cables) ?

  3. POE really makes sense to me too. Uses ~ the highest possible voltage (low losses) and existing cabling. But the maximum power is quite restricted due to the low cross section of the cables. And you still have the conversion losses at all local consumers (because most of these do not work directly with 42V).

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