5Vdc from 48Vdc - Options?

There are many DC-DC converters that will do that. For example the Maxim 5090B (not a recommendation, just a handy reference), will take DC input from 6.5 to 76V, and produce 2A of 5V. About $2.50 in 1K. These need a handful of external components. More complete/self- contained packages ("bricks") exist, but cost more. Many other parts exists, with various tradeoffs in current output, input range, output options (single vs. multiple outputs, fixed vs. adjustable), degree of regulation, output noise, efficiency, size, weight, cost, etc.

FYI:

And a band around 48V is a commonly supported input range.

Self switching power supply with a couple of MOSFETs and R L C.

"Results 1 - 10 of about 479,000"

don

Beware - I've noticed that the folks at Google are prone to exaggeration. My count for that link says 519,000. However, once you start stepping through the pages it narrows down to 526, even when you 'repeat the search with the omitted results included'. Should still be enough to satisfy the OP though ... ;-)

-- Chris Burrows CFB Software Armaide: ARM Oberon 07 Development System for Windows

Nat Semi LM2574HV - if you're getting near 500mA go for the bigger TO220 devices in the range

Be careful. It is usually -48Vdc battery.

Of course, even with the best of these packages you should add a certain amount of input protection and filtering. Even when the DC-DC converter states it can handle the full input voltage it may lack the level of protection against excess voltage inputs, ESD and other extreme transients.

True enough, although it's not a given that the protection belongs at the inputs to the DC-DC converter in question. If it does, there are DC-DC converters with substantial amounts of isolation built in. For extra cost, of course.

Depends on the environment. In some +48 is far more common (if not exclusive).

Robert

I guess I'm showing my age...

ller,

out

The -48V supply is a telecom standard. That came from the backup battery for the telephone central office. I don't know where +48V power is used, but it may well be in large vehicles. I know that large trucks and earthmoving equipment uses +24V battery voltage to reduce the current. I expect +48V is not uncommon there as well.

Rick

You just don't work much with EVs maybe? AFAIK they've mostly (always?) used the negative rail as the reference. It's certainly true of all the ones I've worked with and heard about but there are some oddities out there so who knows what you might run across.

Robert

If you need isolation, have a look on "power over ethernet". It uses the input voltage you need. Normally it is used in an ethernet environment with a managed power source a ethernet cable and a powered device. But you dont need all the ethernet stuff. Have a look f.e. on TI the TPS23753 datasheet. You get all the parts off the shelf (especially the transformer)!

I think in part it is a national/regional thing. I know here in Britain cars are (or at least were) usually positive ground. I also recall hearing Americans complaining about the electrics on British cars for that very reason so presumably it isn't the case over there.

I'm not completely sure why the positive ground is preferred (it is after all unintuitive) but I always assumed it was an anti-corrosion thing. I'm sure many people on this group will have noticed the "black lead blues" at some point, where the negative side of wiring is more likely to corrode than the positive side. With positive ground you avoid that effect corroding the chassis.

It is plain chemistry/pysics. Ever had a look at the positive terminal of a car battery?

This was also the reason for positive ground in telephones at the time when there was only one wire and earth return.

s?)

the

Cars have not been positive ground in the UK since the 1960's. (at the introduction of semiconductors in alternators).

One (supposed) reason that positive ground was initially selected is that since the hot centre electrode of the spark plug emits electrons it gave a slightly lower breakdown voltage to have a negative pulse rather than a positive one from the ignition coil. Since the ignition coil was arranged as an auto-transformer this gave more reliable ignition with poor quality insulation materials available then to have positive ground rather than negative ground.

kevin

If this is for telecom use you will also need 1000V isolation as well.

Luckily there are dozens of off the shelf solutions - search Digikey and you will find many (e.g. TDK CC3-4805SF-E 600mA at 5V 36-72V input isolating converter for about $11 in unit quantities).

kevin

EV's usually float their batteries. The only large scale exception I know of are 24V scissor lifts. For UL certification any industrial tractor over 24V nominal must be isolated from the frame. I expect they would apply similar rules to other EVs.

Robert

ys?)

the

I would like to know the details of this. I have to say that I don't understand how it would make a difference in corrosion unless dissimilar metals were involved. I'm trying to think of where dissimilar metals are used in one leg of the current flow, but not in the other. The battery terminal is lead to the lead cable end typically. The cable end is lead to the copper cable, but that is the same on each side. I suppose the copper cable to the steel chassis is one that does not occur in the "hot" side. So is this one point of contact what this is about??? Let's see, positive flow from steel to copper vs. positive flow from copper to steel... That seems like a pretty trivial reason for picking a ground polarity.

This all sounds like a load of malarkey to me.

Rick