is there a circuit i can build to get 12 volts off of a 5v usb connection? i want to run my ramsey fm 25 off my laptop.
But you might not have much current when all is said and done. Apple USBs are particularly low current, WinTel platforms seem to have a bit more oomph but likely not near enough for the Ramsey.
You didn't specify if your FM 25 is the old unit (the A) or the new unit (the B). I have an A and checked the schematic. The 12 V. input is regulated down to 5 volts and then to 2.8 volts. The only part of the circuit which is powered by 12 V. is the output stage driver and final. This would probably work on 5 volts with reduced output power. Perhaps you could strap the +12 and +5 busses together and connect them to the USB +5 V line.
I have no idea about the FM 25 B however.
Maybe a better option would be to get a Belkin or similar device made for USB attachment.
Neil
In short ... I'd say no.
Not enough amperage. In fact, there isn't enough amperage to charge some devices at 5v (what they are when charging off USB port) while they are on and running at the same time. I had a wireless hotspot run down after a day or so because the drain while it was on was slightly more over time than than the charger could keep up with ... because of low current. Assuming 12v, any circuit you make will still require available current for the Ramsey @ 2.4 times as much as at 5v.
You see the problem ...
That said, it's great to be back after a long wait constantly getting screens telling me the part15.us server was down.
Another option which is not as convenient as using a USB port is to use a 12 Volt battery. I use a 12 V 3 Amp-Hour sealed lead acid battery made for alarm system backup for many of my portable experiments where I need 12 VDC. I got mine at Batteries Plus for about $17 new. They also have larger and smaller Amp-Hour rated batteries. You can predict the operating time by dividing the A-H rating by the current drawn. This gives the hours expected and for most of my equipment this is tens of hours.
I charge this using a constant voltage current limited lab supply set at 13.8 V and the A-H rating divided by 10 for the current. This is 300 mA for this battery. From zero to full charge takes about 14 hours.
Since the lab supply is current and voltage limited I can connect the battery and forget it with no fear of damage or overcharging. Lacking a lab supply you could build a simple charger for this, or perhaps use one of those trickle chargers sold for automobile batteries. This requires that you monitor the voltage or milliamps and stop the charging manually.
Neil
You can take an old pc power supply and tap the 12 volt high current output,usually good for 5 amps or more. Also 5 volts at 10 amps or so. I stacked 10 power supplies together to power an amateur band fm repeater at 100 watts,another stack runs my bench supply,I can power up as many as I need to supply the proper current. Cheaper to run than a linear supply and easy to get from old pc's.
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i was looking at this for mobile/portable use. i want b,cast to like a bus or a restaurant or similar public places using my fm25 and my laptop.