Help with a very basic electronics question - Page 1
Forum section: electronics / beginners

author: DrunkPelican
author_rank: Newbie
timestamp: May 29, 2023, 11:38:53 pm
content: Hi, all. I'm not much of an electrician and had a question. I have 2 9v fans powered by 6 D cell batteries. I want to basically hard wire them in my sailboat, which runs off of 12v, and not use batteries at all. How hard would it be to wire them in? Would I need any special adapters or anything? Thanks.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 30, 2023, 12:06:59 am
content: I think you can just wire them in as if the 12V is the same as the 6 batteries. I think the fans would take the extra voltage OK. Otherwise buy a bucking regulator to drop the 12V down to 9V.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 30, 2023, 12:11:52 am
content: Or, if you are worried about running themabovetheir designed voltage just wire them in series across the 12v supply so they will be running a bitbelowat 6v.They will run a bit slower, but that may be tolerable.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 30, 2023, 04:35:52 am
content: Both of the above answers are reasonable things to try. If the fans run too fast to suit you when wired directly to 12 V. you can wire a couple of diodes in series with the fan. The fans probably draw less than an amp and diodes which can handle an amp are cheap and easily obtained. This approach would be less efficient than the buck regulator mentioned above, but not horribly so, and will be far less expensive.If the fan has a built in speed control you are at some small risk of burning it up operating off the higher voltage, but you would then have full control over fan speed with no other components needed.One thing to be mindful of when wiring these into your boats 12V supply. A fan is an easy thing to leave running and you could easily deplete the boats batteries if you were docked for a few days. Less of a problem if you have solar panels but it is something to think about and plan for.

author: mariush
author_rank: Super Contributor
timestamp: May 30, 2023, 08:22:27 am
content: The easiest would be to just have a bunch of diodes in series with the input voltage wire. For example, a 1n4007 diode will drop around 0.7v so if you want to get 9v out of 12v, you could just have 4 or 5 diodes in series : 9v + 5 x 0.7v = 12.5v - that will also account for potentially higher voltage than 12v from the boat batteries.The next easiest would be to buy a DC-DC converter and convert the 12v from your boat to 9v.You can buy small converter circuit boards from eBay, which cost a few dollars.Here's an example :https://www.ebay.com/itm/115807093955- you just adjust the output voltage by turning that potentiometer until you get to 8-10v, whatever you want.If you don't want to solder, there's boards with connectors that you can screw wires into, for example :https://www.ebay.com/itm/263812185309If the fans are low current, a simple linear regulator would also work.. for example select LM317 module from the drop down list :https://www.ebay.com/itm/312240978252

author: Ian.M
author_rank: Super Contributor
timestamp: May 30, 2023, 10:56:53 am
content: '12V' is rarely 12V on a boat! You'd normally only see a Lead acid battery voltage that low (under light load) if the battery was well discharged, and if you are going much lower on the house battery on a regular basis, expect to have to replace it sooner than you'd like. OTOH with the engine running (or on a charger), it will typically reach around 14V. That's 55% overvoltage for a nom. 9V fan.Unless these fans are dollar store grade, its probably worth getting a buck converter to run them.N.B. Automotive and small craft marine electrical systems can be pretty hostile, with large voltage transients. To survive long-term, you probably want something with a significantly higher max. input voltage rating than the 35V to 40V typical of the commodity far east bare board buck modules.

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 30, 2023, 12:58:42 pm
content: Put a single series resistor in the +ve lead to each fan and drop 3 or 4 volts across it. They may have to be a 5W resistors though.

author: radiolistener
author_rank: Super Contributor
timestamp: May 30, 2023, 01:08:15 pm
content: 12 V battery usually has 12.7 V and up to 14.5 V during charge, so there is need 14-9=5 V drop. There are several ways:1) insert 8-10 diodes in series, each diode has about 0.5-0.7 V drop2) use linear regulator to get 9 V from 12-14 V3) use DC/DC step down converter to get 9 V from 12-14 VDC/DC step down converter usually more efficient than linear regulator.

author: antenna
author_rank: Frequent Contributor
timestamp: May 30, 2023, 01:49:05 pm
content: A boat is no place for inefficiency. Get 2 12v fans. Cheap regulators or diode solutions will drop the excess voltage at the fans current and thats wasted power. An efficient buck converter will cost you more than the 12v fans. Alternator is going to be running about 14.4v so thats 14.4-9=5.4v dropped across diodes. A good third of the power is wasted keeping the 9v fans.

author: Ian.M
author_rank: Super Contributor
timestamp: May 30, 2023, 02:37:35 pm
content: OTOH a buck module per fan makes it fairly easy to add a variable speed control. You'll need to do a bit of experimenting to find the minimum voltage at which the fans start reliably (and allow a bit of margin for the bearings deteriorating with age), then its just a matter of calculating appropriate resistor values to 'end-stop' a pot to fit in place of the existing feedback divider, to limit the output to the desired range.