Hi, hoping someone can help me - I have a transceiver that relies on having a good solid 14-15v voltage supply. I am running it from 12V
7.2A batteries ( its a portable system ). When the batteries are fully charged and giving out a 13V+ all is fine. However, when it starts slipping down below or near to 12V, the performance of the radio really suffers. Whats the best way of maintaining a regular, say, 15V supply?
I guess some kind of 'inverter' would be required, but not sure what, if anything else, needs to be added. The radio is protected with a 3Amp fuse, and does draw quite a lot of current - I have had a quick look at a few voltage regulators from RS, but they all seem to be for 5V Vin, not 12V.
Thanks in advance for any help, apologies if I'm just being stupid....electronics is not my strong point.
What kind of pack are you using and or is it made for the radio? If it not made for it or say you are using nicad's in place of alkaline in the internal holder that is, I would simply look for some lithium recharable's and a charger for them. Nickle cadmium batteries are 1.2 volt as to alkaline or lithium are a full 1.5 volt. From what you said, I suspect you have an internal battery holder for 10 AA cells, and you are using NiCads in place of alkaline's. You will spend a little for the lithium rechargables but they will run your radio much longer. JTT
You're looking for a decent (3 Amp plus) DC-DC converter. Last time I was looking for such beasts, it seemed that 30-40v -> 12v were available, and even 9-12v -> 30-40v, but doing the 9-12v -> 14-16v always seemed somewhat troublesome to find. If anything, I would see about putting 2 of your batteries in series to get ~24v, and then find a 24v -> 14-16v dc-dc converter. Then again... it's been some time since I've looked.
It runs other equipment - so I'm stuck with the supply voltage as is. I have tried looking for DC to DC converters on RS, but they seem to have poor output current (I need 1.2Amps on transmit, the ones I've found only seem to go up to 400mA ). Any other ideas? Cheers, Jez
Hi, J. The obvious solution is to replace your 12V battery and 12V battery charger with three 6V batteries and a 6V or 18V battery charger, and then use a 3 amp linear regulator to cut the power supply voltage down to size. Apart from being the most expensive solution, it would work pretty well.
If you want to get a steady 13.8VDC out of a 12V battery, you're not going to be able to do it with a linear regulator -- they can only reduce the voltage, taking the difference between input voltage and reduced output voltage times the current you're using, and dissipating it as heat. You can't use a saw to make a piece of wood bigger.
Another possible solution would be to use an automotive inverter to get
120VAC, and use that to power a small bench 13.8VDC supply. Altogether, though, that would be less than 50% efficient (you'd be using more energy to power your apparatus than to power your transceiver). Probably not the way to go, if you're concerned about battery life.
Switching voltage regulators can boost the power supply voltage up, and are pretty efficient (typically 75 to 87% of the input power is actually used to drive the load). But building a good one requires a little more technical skill than you admit to having.
One easy solution might be using a commercially available DC-to-DC boost converter. This has the switching regulator built-in. The device assures a steady input voltage to devices powered from a car cigarette lighter. Typically, they'll give you 12.5VDC or 13.8VDC from an input voltage varying between 10V and 15V. They're also fairly inexpensive, so they'd be a one part solution to your problem. Here's an inexpensive one ($30) that will give you a steady 12.5V output, if your steady-state power requirement is less than 25 watts (2 amps):
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There is a downside to this -- you'll have to be careful about running your battery into the ground. Your setup has the advantage of becoming unuseable before the battery becomes deeply discharged. Lead-acid motorcycle batteries generally don't react kindly to being deep-discharged repeatedly. You'll have to watch this, because these DC-to-DC converters generally don't shut down until your battery voltage is below 8 volts or so. Unless the battery is made to handle deep discharge, this might reduce battery life. And the additional power requirement of the DC-to-DC boost converter will probably discharge your battery that much faster, meaning you probably won't get that much more useful time out of the battery.
Your best solution might be to just buy a bigger battery. You also might want to look for a "marine"-type battery, which is made for deep discharge service.
You're not being stupid. The only stupid question is the one that you don't ask. :-)
But, you seem to have a situation here - the simplest answer is to get better (i.e., bigger) batteries, and have a couple of sets on hand, to switch over when one set gets low, so you can charge it, and so on. The thing about trying to boost the ~12V from a discharging battery is that it's already almost to the end of its useful charge, and any kind of boost regulator would draw even more current than the equipment. I'd look for good marine deep-cycle batteries - they'll probably maintain their charge better than motorcycle starter batteries anyway.
The most practical solution may be the one Chris gave you: a 6v battery + a 12 volt battery for 18 volts total, and a linear regulator to drop it down to about 13 - 14 volts. Usually, standard car battery chargers have a 6 volt setting so you can charge the 6V battery as well as the 12 volt battery (not at the same time). You need 2 resistors, 2 capacitors and an LM350:
The LM350 will handle up to 3 amps - mount it on a good heatsink. You could use an LM317 which will handle up to 1.5 amps, but the extra current capability of the LM350 gives you a nice extra safety margin for less than a dollar more. ($1.26 for a 350).
Monitor the voltage across the battery with the lowest Ah rating. When the 6 volt battery reaches 5.75, or when the 12 volt battery reaches 11.5, it's time to recharge, as a general rule for you to follow with this arrangement.
That might get you a bit too much juice, if your battery is fully charged; you could put a diode in series with the 15V output to drop it by 0.7V. Make sure the diode is rated for the current and power dissipation you need (0.7V * 2A = 1.4W).
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