Electrical theory - running AC off of DC supply and..

[Pj]

Ok, refresh my memory on how this works:

At work we have 74vDC power supplies. Before some things became illegal, people were able to use iphone, laptop power supplies and the like 110vAC chargers on 74vDC power to charge items. Seems to work, no one noticed any ill effects.

How does this work?

Second, which is/was considered a better power supply, and why? I know one is cheaper than the other and I think one produces less DC noise than another.. switching vs non-switching.

[Wowbagger]

Most modern power supplies are high frequency switching power supplies. The first thing such supplies do with the incoming AC is rectify it and turn it into high voltage DC, usually 170 or 300 VDC. They then switch that DC at very high frequencies (usually 1MHz or greater) to drive a very small transformer (transformer size varies inversely with frequency), and make the low voltage needed by the device. This is why most modern power supplies can deal with 120VAC at 60 Hz (North American power standard) or 240VAC at 50Hz (European standard) - they convert it to 300VDC, so frequency doesn't matter, and by a very minor change in the rectifier topology they can make 300VDC from either voltage.

Running a very low power device like a phone isn't very demanding on the switcher, so running from 74VDC vs. 170VDC simply means the switcher has to run a bit harder, but not so much harder that it blows up.

For radio power supplies, it *used* to be that switchers were bad - the outputs were poorly filtered, and that 1MHz signal would leak out. Moreover, since the switching was done on short pulses, they were rich in harmonics across the band, so such power supplies were very RF noise *in the old days*. However, modern power supplies are very well filtered, and basically put out no more RF than an old school "big lump of iron and copper" transformer power supply.

So which is better? I'd say good switchers, every time.

(Now, if you buy cheap crap with no good filters - you get what you pay for).

[Bill_G]

The bigger question is - who at your work put 74vdc on standard outlets that people could plug their chargers in?

[jackhackett]

It's all good until someone tries it with an old style wall wart... how much smoke would 74VDC through a transformer primary make?

[Pj]

The bigger question is - who at your work put 74vdc on standard outlets that people could plug their chargers in?

[Bill_G]

Ha!

[escomm]

The bigger question is - who at your work put 74vdc on standard outlets that people could plug their chargers in?

[Pj]

Ha!

[KE7JFF]

Also, some of the new power supplies I have seen in newer devices have a range of voltage they work on.

Then you have the AC tools with motors from Harbor freight that work on both AC and DC....

[KitN1MCC]

The problem there is you never see CSX on the Tracks like that


They may have a step down system or regulators down to 74volt. on the older loco we used to put a 12VDC alternator and battery for 12v radios