External power supply for 16.5V portable computer (Zenith SupersPort 286e)

Question

I have acquired a Zenith SupersPort 286e portable computer without its battery and the external power supply unit seems dead (corrosion and/or leaking capacitors).

The problem is that the power suppy is 16.5 Volts (DC) and 2 Amps.

I couldn't find any new 16.5 Volts external power supply as it isn't a standard anymore.

I went to a specialist (the owner of a shop specialized in batteries and power supplies) and he told me that I could try a 16 Volts power supply with 2.2 Amps to compensate for the missing 0.5 Volts.

I couldn't find any precise documentation on the power supply for this computer. The only other thing I found out is that the battery was 12 Volts (I couldn't find how many Amps it had).

So how can I choose the right power supply? Should I listen to the specialist (a 33€ power supply that goes from 9 to 24 Volts with 2.2 Amps), should I go for a 12 Volts power supply (like the battery was, but still, I don't know how many Amps it was)? Or am I completly wrong and I should go for something else completly?

Answer 1

The first thing to do is check the label on the computer to find out what its power input requirements are (it might be different from what the label on the power supply itself says). Many devices will operate on a range of voltages and currents, and any power supply that falls in that range will work.

For example, a Soekris net6501 can be purchased from the manufacturer with a 12V power supply, but if the power supply goes bad I don't actually need an exact replacement. In fact, that system itself just requires 30 watts of power at any DC voltage between 6V and 25V. I happened to have a Lenovo T42/T43 laptop power supply that produces 72 watts at 16V and it works just fine.

Even if the computer doesn't list a range of input voltages, it is likely that a reasonably close voltage will be good enough. 16V is only about 3% away from being correct so I think it's worth giving it a try.

The battery being rated at 12V is not by itself a good reason to assume that a 12V power supply will work (but it is a good reason to suspect that a 12V power supply may work, although I'd try the 16V one first). Whether it would work or not depends on the system design. I have no idea what the actual design is, but it is entirely plausible that the computer will actually run on any voltage from 10V through 24V, but any voltage 12V or lower won't allow the battery to charge, while any voltage higher than 18V will damage the battery (assuming you have one).

It's also entirely plausible that any voltage higher than 14V would damage the battery. This is of course unlikely if the original power supply was 16.5V, but are you sure the power supply you have is the one this computer originally came with? Maybe the original power supply stopped working and a previous owner decided to use a 16.5V one as a replacement? Remember, just because the power supply says Zenith doesn't mean it's the correct one for a Zenith SupersPort 286e.

Answer 2

Any power supply within about 5% of the rated voltage should work, so a 16V supply (or a 17V one) should work.

If you're getting a modern switch-mode supply, you want the amperage rating to be at least as large as the one you're replacing, if not larger: I'd go for 2.5A or higher. If you're getting an older transformer+rectifier supply, on the other hand, you want to match the rating as closely as possible without going under.

The adjustable power supply is probably your best bet, but make sure it can deliver the full 2.2 amps at 16 volts: some adjustable supplies give their amperage rating for a certain output voltage, and deliver more amps at lower voltages, and fewer at higher voltages.