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

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.

• You simply cannot. A power requirement is written by an engineer doing a range of tests and verifications. They often exaggerate, but not by 1.2A versus 35mA (350mW at 10V = 35mA!), there most certainly is something else going on. – Asmyldof Jul 14 '15 at 13:25
• I presume you mean mA not mW. Properly you could simply measure it and compare with the ratings of the supply, but if you miss something and it draws more current under some unanticipated conditions something (supply or device) could suffer. Supplies (properly made ones, not some of the eBay junk) are designed to fail in a safe manner but it's bad karma to be testing it all the time. – Spehro Pefhany Jul 14 '15 at 13:25
• 350mW is extremely low, are you sure about those numbers? In the end, without having some manufacturer spec that you trust in, the only way to figure it out is to measure it under all conditions – PlasmaHH Jul 14 '15 at 13:25
• Probably the Genesis uses much less current than that 1.2 A. To tell "properly" what current is needed you would need to measure the current the device takes and find under what conditions (loading a CD for example) it consumes the highest current. – Bimpelrekkie Jul 14 '15 at 13:25
• Most power supplies have a data plate that shows the ratings. Could you take a photo of the one you bought and post it in your question? Maybe someone can then tell you what you've really got. – JRE Jul 14 '15 at 13:36

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?

• That's a good point. This wall wart is one of those "combo" deals that supposed to work with either Genesis or NES (it has two barrel jacks). It feels cheap and almost hollow. I have authentic PSU's from other consoles and they always feel really heavy. I personally feel this PSU is probably "good enough" to work as long as there are no external peripherals. However, the Genesis had external peripherals but they almost always had their own PSU. I can see "1A" being a safety margin but "1.2A"? Seems odd. – cbmeeks Jul 14 '15 at 14:26
• BTW, anyone ever see the PSU for a ColecoVision? I have one and those things are gigantic! I hear NASA keeps a few of them around to jump-start the space-shuttle from time to time...that is, before they canceled the space shuttle. lol – cbmeeks Jul 14 '15 at 14:30

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.

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.