Inexpensive, low current buck-boost converter design wanted

[EK701]

I'm looking to power a digital volt meter from 1.8-60VDC and the meter requires 5-25VDC for the display.  Current is <50ma.  I've searched here and google, but haven't been able to find what I need that isn't expensive.  Any suggestions on how to achieve this?

[qwaarjet]

The cheapest way would probably to get separate supply voltage to run the meter with a wall wart, battery, ect. Otherwize you would mostly likely need to build your own converter like around something like  a LM50750, but that would require a PC board and magnetics so not very cheap.  Also many meters, like what I think you are describing, require separate grounds between power and signal which would add an isolation requirement as well. I have used  Recom Power isolated DCDCs in this application but I supply power to the DCDC from a common 5V or 12V bus in the system and not the measured signal.

[T3sl4co1l]

What is "cheap"?  1.8 to 5 is easy, or 60 to 25 is easy, but 1.8-60 to (something) is not.  It's not impossible, but doing truly the full range isn't going to be easy.

I don't think there are any controllers handy that do such a wide range, so you'll have to roll your own solution there.  For example, a 1.8-whatever input controller plus an LDO (so it saturates to Vin with minimal drop at low inputs, but regulates for excess Vin) would be a possibility.

I'd be happy to design it for ya, but it's probably easier (and cheaper) to solve the wide range problem first!

Tim

[EK701]

The meter is just a cheap digital volt meter from ebay with 4-30vdc in, gnd, and V sense leads.  Ground is not isolated.  It will read voltage between 0-100vdc.  Most of the time the upper voltage will be below 30vdc, so I think some type of voltage clamp would work at the upper end.  If it does, I just need a boost circuit for the 1.8-4vdc range.  Thoughts? 

[qwaarjet]

With an appropriate heatsink you could use a linear regulator set for ~3V. Note: At 60V and 50mA it going to be dissipating ~3W
Then feed that voltage in a cheap boost converter and use the output to the boost converter to run the meter. When the voltage goes below 5V the linear regulator should just pass the input voltage (with some diode drop) to the boost converter. Not the most elegant design but cheap

cheap boost converter I do not vouch for quality
http://www.ebay.com/itm/DC-DC-Converter-Step-Up-Boost-Module-1-5V-to-5V-500mA-USB-Charger-For-Phone-YU-/151333666945?pt=LH_DefaultDomain_0&hash=item233c308081

[EK701]

Hmm, that is an option.  What happens with a typical boost converter when the input voltage exceeds the output voltage?

[qwaarjet]

Depends on the specific type of boost converter and the components used but problems could include failure to regulate output voltage stability, input voltage just passing through to output, magic smoke, ect.

[T3sl4co1l]

When Vin > Vout, a boost stops boosting (at least if it's not an idiotic design that somehow keeps going regardless), and Vout is never less than a diode drop below Vin.

Tim