How can I properly tell if a wall wart is OK to use for old hardware?

Question

I bought a Sega Genesis model 1 from a local thrift store recently for $10. It was sold as "broken, for parts only". It didn't come with any cables.

I've repaired several vintage gaming systems in the past so I took a chance on seeing if I could repair it.

When I got home I realized that I didn't have a proper power supply for it. I have several Genesis model 1's and 2's but the only power supplies I had for them were for the model 2's. Which takes a smaller barrel plug.

Anyway, first thing I do is Google the power requirements for a model 1 (couldn't find it listed on the actual unit) and after some investigation, it appears the model 1 needs 1.2A/9V DC. I'm going from memory but that should be pretty close.

So I dig in my parts bin and I cannot find anything that fits and has those specs (several 1A but no 1.2A).

Next I get on eBay and find a vendor that claims to have sold thousands of power supplies for Sega and claims he has exactly the one I need. His ratings were good but when I asked him the specs, he said it was 350mA and 10V. Well, I figured the 10V would probably be OK but the 350mA seemed too weak. He assured me it would work (and it was cheap) so I bought it.

It arrives and when I plug it in, it worked like a champ. Total investment $19.95 for a working Sega Genesis.

My question is, how is this possible? Are the 1.2A specs online just simply too conservative? I have tons of 500mW supplies so they would probably work too. How can I "properly" tell what kind of power a unit like that REALLY needs?

EDIT

Sorry, I meant 350mA NOT 350mW.

Answer 1

It's possible to construct an unregulated wall power supply with a big transformer, a few diodes, and a capacitor, or with a small transformers and some fancier electronics. Historically, the former approach was cheaper, but nowadays the electronics required to use a small transformer cost less than the copper, iron, and shipping weight of a large one.

With regard to the reason a unit whose nameplate specifies 1.2A would work with a 350mA supply, there are a few reasons:

1. The Genesis may be using a linear regulator inside, which will convert a 7-10 volt input into 9 volts for the electronics, and may if anything be slightly happier with 7 than with 10, especially when it's drawing a lot of current. Note that an unregulated supply which is labeled for 10V 1.2A may output 11V when no current is drawn from it, and 7.5V when 1.2A is drawn from it, and that such a supply might be cheaper than one which outputs 8 volts all the time. When the Genesis specifies 10 volts, that may not mean it actually needs 10, but rather that it can handle 10 volts without trouble.

2. Many supplies today, when used beyond their rated current capacity, will output a voltage which is reduced proportional to the amount of overcurrent; a 10V 350mA supply may output 3.5W at 350mA (meaning 10 volts) and also at 466mA (by outputting 7.5 volts).

3. Perhaps most importantly, the Genesis is a cartridge-based machine, and power from a plugged-in cartridge will need to come from the supply. Sega may have wanted to allow for the possibility that some sophisticated cartridges might need a lot of power, and thus engineered their supply to accommodate that. I don't know if any particularly power-hungry cartridges were ever released, but since large-ROM technology was in flux it would not be inconceivable that some ROM supplier might start offering chips which were unusually cheap but power-hungry. Having reserve power capacity in the supply would make it possible for the Genesis to accept cartridges using such chips.

I would expect that if you were to plug into your Genesis a cartridge which drew Sega's specified maximum allowable current, a 350mA supply would not be able to power it usefully. If, however, you plug in cartridges which consume a much smaller amount of current, then the machine as a whole would require a lot less than the 1.2A of current specified on the nameplate.

Answer 2

The rating on the wall wart for current is the maximum it can supply at the rated voltage output usually. What your Sega takes from that wall wart is a totally different matter so, when you concluded that: -

it appears the model 1 needs 1.2A/9V DC

You may have been misguided.

The best way to determine what current is needed by your equipment is to contact the original supplier but this may not be appropriate due to the data being unavailable on olde products like the Sega thing. Measuring it yourself can appear easy but, are you measuring current taken when device is working the hardest?

Answer 3

IIRC, there should be a label on the bottom of the Genesis console showing its input power rating.

For example: MK-1631

And: MK-1601 (it appears that your 9V 1.2A recollection is correct)

There may also be a hint embossed in the plastic near where the adapter plugs in (your mileage may vary).

Using an adapter that provides a different voltage or insufficient current is at your own risk - the Genesis or the adapter may fail.

Answer 4

The specs on the Genesis is what the Genesis engineers and Genesis procurement worked out to be the ideal specs for the system. It covers the needed voltage and the possible amperage plus reserve. It may be higher than expected max draw plus a 10% buffer because maybe it was cheaper for Sega to get branded 9V 1.2A supplies than it was 9V 1A supplies.

The Genesis has expansion ports for add ons, both self powered and console powered. Newer models had slight different board revisions and features removed. Different cartridges have different needs. The amperage required for a single console without expansion will be less than the maximum.

The internal electronics are also not super picky about voltage. Any electronic device/ic has a range of acceptable voltages and often a 12% increase in voltage would be ok. 10 is 112% of 9V.

In this particular case, the Genesis uses 7805 linear regulators to its internal 5V rails. The 9V or 10V is used exclusively for that internally, only exposing the 9/10V input rail to the expansion port otherwise. You could power the Genesis with a wide range of voltages if you meet the drop-out voltage on the low end and heat management at the high end.. So a little increase in voltage is fine. The system wastes up to 6 Watts at maximum in heat through the regulator anyway.

That said, providing 30% of the required amperage is likely to greatly underfeed the system. This could lead to save state failure, brown outs, slowness, or any variety of issues. It's better to get a properly rated quality adapter.

Answer 5

In the first place, those adapters sold online are China-originated, it's ok for Chinese manufacturers to put wrong rating on their PSUs -- both ways. So, what you actually need to do is measure the maximum current that this PSU supplies. Just set your multimeter to 10A ammeter and attach it to the power supply for a short amount of time. It won't do any harm to it since it's a flyback converter and they can take short circuit alright.

Then, you can also measure the actual current that your console consumes. That should answer all your questions. :)