How can I run 9V instruments off my 12V supply?

[MJBorley]

I have two old but serviceable instruments that each run off their own 9 volt disposable batteries (they are a Stowe trailing log and a Seamaster depth gauge). I would like to run them off the boat's 12 V domestic battery. What is the best way to do this? I have thought of the following options:

1) Purchase a 12V to 9V converter. This seems the best solution from an engineering point of view, but is also the most expensive as these units cost approx £50.

2) Put some sort of resistor in the feed that reduces the 12 V supply to 9 V - I don't know if this is possible, I certainly don't know how to do it, and intuitively it doesn't feel like it would be a very effecient solution (I don't want to increase my battery drain any more than I have to).

3) Run the instruments directly off the 12 V supply. This is without doubt the easiest, but presumably a voltage overload of 33% (perhaps more - don't batteries get up to around 14V while they are being charged?) would damage the instruments (I don't want to do anything that risks this). Has anyone done this with success?

There may be other options I haven't thought off - my criteria are reliability, cost, simplicity.

Mark

 

[rickp]

Knock up a simple 12v to 9v convertor using something like an LM7809. How much current do you need to supply?

Rick

 

[petersto]

voltage regulator from Maplin or similiar

 

[William_H]

It probably won't matter in your case but often with ear designed for battery the negative is not necessarily connected to the earth or ground or case of the instrument. So make sure there are no other connections to anything on the boat and use a double pole switch ie break both + and - wires or pull a plug out. The regulator integrated circuit from RS or Maplin are very cheap but need to be soldered to the wires and also have capacitors fitted from input and output to negative. Give us a PM if you wan to go that route or find one of those cigarette lighter plug type regulators that will give 9V. It should be OK to run both from one regulator but disconnect both when you leave the boat. (you could get accelerated corrosion from stray current paths and earths) good luck olewill

 

[rjp]

Have you checked that there are no terminals for an alternative 12v supply already present? I have a Seafarer echo sounder that takes a 9v battery OR can be directly connected to the main 12v boat supply via two screw terminals near the battery compartment.

John

 

[Evadne]

It is quite possible that the log will happily run on 12V DC, but you would need to have a look inside to find out if it is regulated. A lot of electronics of 10+ years ago will be running on 5V, and will have a DC-DC converter or a linear regulator to step the battery voltage down, or will run happily on any voltage up to 12V anyway. The transducer driver for the echosounder runs at a higher voltage, and it depends on how this is achieved whether or not you'll do any harm. It is probable that the echosounder will be ok as well, but there is an outside chance that it may not.

The safest way is to put in a 12V to 9V DC-DC converter, using something like the NMH series of devices, or a simple circuit like this one , or a linear converter such as the LM7809. The latter will only be acceptable if the current draw is much less than an amp, which it is in this case. None of these should cost more than a tenner to make.

Regarding the log, I have one of these so if you are not in a hurry I will have a look at mine when I'm next on the boat and report back.

 

[Nickcf]

a cheap way to do this is to buy a few rectifier diodes (say 6 amp jobbies) and wire in series in the +12v supply lead. each diode will lose 0.7 volt so assuming 13v average battery volatge, 5 or 6 diodes in series will suffice.
Maplins do some suitable ones
http://www.maplin.co.uk/Module.aspx?ModuleNo=46413&criteria=uk59&doy=30m1

total cost less than 4 squid plus ten minutes with a soldering iron or a connector strip

 

[wooslehunter]

It all depends on how sensitive the equipment is to input voltage. Diode drops are fine as long as they are set up right and the battery voltage doesn't change. If they're set up to drop 3v from 12v to 9v and the battery voltage goes up or down by 1v then so will the output 9v. That assumes the current load is constant. 7809 regulators are the proper solution but if you needed to ask the question, you probably don't have the ability to build one up. You need to know the current load to pick the right one and make sure that the capacitors refered to are the right types otherwise the regulators can oscillate.

The easiest solution is to buy rechargable 9v batteries. Get 2 sets and keep one charged. Then swap over when one set goes flat.

 

[lydiamight]

Just solved the same problem by buying a "Powerline Car Cord Adapter" Input 12 volts, output adjustable from 3, 4.5, 6, 7.5, 9, 12 volts also polarity adjustable. Cost me $9.99 in WalMart (Happen to be on holiday in the USA at the moment) Try car accessory shops in the UK for something similar.