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Topic: need physically Small power supply from high DC voltage (Read 513 times) previous topic - next topic


Hi everyone,

I have a project that I'm planning to power off an existing DC source that could vary from 30 - 85VDC.

The project uses a 5V Pro-Mini.   

        Using my perf-board prototype, the largest draw I've seen is about 70mA.  That includes everything - sensors, switches, Mosfet, Display, etc.

What I want to do:

        Take the 30 - 85 VDC available and bring it down to a regulated 5VDC with at least 100mA capacity.

   Have this supply:
        Take up little physical space
        low cost
        reasonably efficient


     I can't find any OTS parts or modules that can handle the input Voltage range that isn't also physically large, Pricey, & way overkill on current (explaining the large size and cost)

Best Idea left I haven't looked into yet ... 2-step regulation.  Bring the higher input voltage down to something reasonable (20V?) with the small size/current/cost, then use another regulator to step down the rest.
            - still haven't really seen a module that can handle the input range that meets the size/current/cost requirements either.

I'm trying to avoid having to use a step-down resistor method but may have to resort to it if nothing else comes up.   (At least its low current, so not as wasteful as t could be.)

Any suggestions would be appreciated.



You might need to come down from 85 volt to 5. Current might be 100mA. As much as 80 volt needs to be dropped. Any linear solution will produce 8 Watt. For any small sized box that is much, too much.
How much power discipation can You allow? All linear 5 volt voltage stabs also produce heat.
A switched regulator looks like the best. Maybe You need a custome designed regulator if no rare and special one can be found.
Is there anything You can do to reduce the main voltage? 85 volt, where is that used?


Yeah, I know a full linear reg isn't going to work.   
    That's where the 2-stage idea come in ... just use the dropping Resistor to bring the voltage down to something a smaller module can handle.

Most of the switching modules I've seen can't handle the higher input voltage (they tend to max out around 25VDC)

The supply available is the battery charger unit &/or the Battery itself (same voltage range). 
                 I don't want to hack into the supply itself,
The project objective is to be a small device without needing an external supply.

I'm currently just using the 5V from the USB port and could run the project from a 9V battery for semi-portability in the short term/testing phase. 


I have experience from 48 volt heavy traction lead battories. 6, 12 and 24 starter battories are also common but battories close to 85 volt is new to me.  There are some 120 volt fork lift trucks and some with a voltage lower than 100 due to EU regulations.
What kind of things are You using? Where are they used etc.? Why this 85 volt?


Li-Ion Battery PAcks for e-bikes.

My current pack is 48V Nominal, so it's maximum is about 55.5V.

I'm making my project compatible with packs up to 72V nominal - 84V Max


Why this wide range compatibilty when it looks that difficult? Create alternative versions for various voltages might be an option.


Best Idea left I haven't looked into yet ... 2-step regulation.
Agree, look into it.

To reduce 85V to 20V at 70 mA using a resistor would generate about 5 W of heat. Not good in an enclosed space, and the resistor itself would be large and quite hot.


A common universal AC mains voltage 5volt phone charger will also work on 85volt DC.
The one I tested just started to work at about 50volt DC.


There is one solution thinking about 2 steps. You reduced the matter to an 8 - 50 volt issue.


Battery would be made up from cells so tap into a couple of cells and reduce from there.


Battery would be made up from cells so tap into a couple of cells and reduce from there.
Pre-made pack.   I don't have access to the individual cell voltages.


Well then, Arduino don't take that much current and can be put to sleep also.
Perhaps just add an additional Lipo 7v4 pack and charge occasionally.
Not difficult to add it's own supply monitor and alarm if necessary.


The resistor for 85-20V is a bad idea for more reasons than heat alone: if your current draw is not 70 mA but just 10 mA (it will vary that much depending on which components are active) your 65V drop becomes a 9V drop, and you may fry your linear regulator. On the other hand the resistor will be too large to handle even a 200 mA peak.

What you need is a buck converter, not a linear converter. Otherwise there's the power issue as pointed out above. They indeed can get quite small. A quick search turned up this little guy takes up to 55V on the input, and produces 5V on the output.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.


Telco-style dc-dc converters will typically handle inputs up to 72V or so (at high efficiency), and are pretty cheap on the surplus market.  They're usually higher output currents, and I don't know how they behave at low currents...


Maybe there are build Yourself kits to find? The rather low current You need makes the task a lot easier. Switching technic is not woodoo.

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