USB wiring

[cheddarman]

I've made a lens heater for my astrophotography comprising a 3S Lipo battery (12+ volts) powering a resistor ring that fits on the front of the lens. These resistors heat up and keep the dew off the lens.
I have been given a USB (5 volt) cloth heater that wraps around the lens and I'd like to add a USB A socket to my power supply to accommodate this new heater.
So, how do I wire this in, how do I reduce the 12+ volts down to the required 5v. What's the safest way?
According to its spec. the current for the new heater is about 1.4 Amps. I've forgotten all my formulae

 

[bertus]

Hello,

You could use a step-down regulator to bring the 12 Volts back to 5 Volts:
https://www.amazon.co.uk/ICQUANZX-0...19157031&rps=1&sprefix=buck+c,aps,159&sr=8-10
The shown converter is adjustable, so you can set the heath.

Bertus

 

[dave9]

There are variables involved that we don't know, but I would abandon this idea of adding an extra complexity and efficiency loss from a buck PSU module with its own waste heat, when the purpose was already delivering heat, considering your USB heater element is probably only worth $5 and USB A is capable of 1.5A at most (ideal conditions) so may become a fault.

Is 7W even the ideal amount of heat or you're just basing everything around a $5 *free* heater, that is only 7W because it's just within the envelope that maxes out USB A power delivery?

I'd search for 12V heater "film" or "silicone" (ebay, aliexpress, etc sources) near the wattage and element shape your application needs. As far as safest, put a fuse on it, or safest for your lens assembly would be to shoot for a little overhead on needed wattage for the heater element but then use a temperature controller to reduce output to the minimum required temperature.

 

[cheddarman]

Hello,

You could use a step-down regulator to bring the 12 Volts back to 5 Volts:
https://www.amazon.co.uk/ICQUANZX-0-8-24V-Step-Down-Regulator-Converter/dp/B07VQ89RZG/ref=sr_1_10?crid=TNSSV87WCNAH&dchild=1&keywords=buck+converter+12v+to+5v&qid=1628964502&refinements=p_76:419159031&rnid=419157031&rps=1&sprefix=buck+c,aps,159&sr=8-10
The shown converter is adjustable, so you can set the heath.

Bertus

Thanks for that, looks just the job complete with the USB socket!

 

[cheddarman]

Just an update, bought this one. Didn't need three!!
https://www.ebay.co.uk/itm/263228711535?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649
Many thanks everyone, very useful forum for future use!

 

[cheddarman]

Further update, item arrived yesterday and I installed it in my power supply box but didn't work. Put multimeter on the input and it indicated that the polarity was reversed.
Reversed the input and it worked fine! - GND was Vin+ and Vin+ was GND. Luckily there must be reverse polarity protection in it so no damage.