The AM Forum

Hi All,

 This is not exactly radio related, but, here goes. I have a bilge blower that I need to use to move some air through a 4" duct. As it will be inside the house, noise is a factor. If I run the blower at the rated 12Vdc, the noise it generates is much like a very loud hair dryer. If I reduce the voltage to 7Vdc, the noise is tolerable and the air movement is adequate.
 
 I have a wall wart type power supply that is rated 9.5Vdc which, if used as is, puts me back into the noisy area of the fan curve. Can I use a string of silicon diodes to lower the voltage? I have read conflicting opinions on this.

TIA, Roland........

You are on the right track and will get about 0.7 volts drop per diode, but they will dissipate heat so be sure the diodes are each rated for at least twice or so the current rating of the blower.  Your blower may be rated at several amps since it's for marine service and I assume it has a 4" discharge.

How did you make the trial drop to 7 volts in the first place?

I am surprised the wall wart has enough current capability to run the blower but if so then diodes of sufficient current capacity will work.

If this is for long term service and it is approaching the wall wart current capability you might consider building a dedicated supply using a 6.3 volt transformer with rectifier and filter.  With proper filtering you should end up with around 7.5 volts under load.

 I cobbled together a test power supply using a 6.3Vac(3amp) filament transformer, a full wave bridge rectifier and a 4400uF capacitor. This supply gave me ~7Vdc. The blower draws 1.3 amps at this voltage. It would work as such, but, I don't have a suitable enclosure. The 9.5Vdc(2amp) supply is almost plug and play except for the higher voltage. It's a switching type with short circuit protection, another plus.
 I found a couple of silicon diodes(of unknown ratings) in my spares pile and put them in series. It dropped the voltage enough to make the noise tolerable,but, they became alarmingly hot after about 20 minutes.
 Take two. I scrounged up a bridge rectifier (800Vdc@5amps) and used two legs which gave me two diodes in series again. Worked O.K. and the heat was managable with a small heat sink.
 This blower is to run intermittently and unattended 24/7 so I'm not too concerned about anything overheating and frying, but, you just never know.

Roland............

A bilge blower?  That peak my interest.  What does it look like.  Does this take the place of a sump pump?

 :)

Roland

In my "shack", I have a mix of modern, commercially manufactured equipment, homebrew equipment, and vintage equipment (some of which has been modified). I always turn off the AC power for homebrew and vintage equipment when I'm not using it. If something starts to smoke or burn (unlikely, but possible), I can take immediate action to deal with the problem before it gets to be a major problem.

Modern appliances (including "wall warts") include things like thermal fuses to reduce the possibility of overheating/fire. There are historical cases where manufacturers have made major recalls of things like "wall warts" and appliances because of prospective fire hazards.

Using a homemade power supply to power a blower in an unattended application, is ... in my opinion... a very bad idea.

Stu

 I agree with the caution about leaving homemade power supplies unattended.
Back in my callow youth, I used to breadboard car battery chargers on plywood. I'd hook it to a battery and walk away. When the battery spat electrolyte, I figured it was charged. I did get an acid facial from the 'rapid oxidation' of free hydrogen once. From then on I moved my operation outdoors. No GFCI's back then, took my chances if it rained.

R...............

I wouldn't recommend one using "hammy hambone" construction but I will take a properly constructed (aluminum chassis, good quality transformer, 3 wire cord, fused, etc.) homebrew supply over questionable modern Chinese junk supplies any day.  Some manufacturers do provide very good quality wall warts but the quality control (and design) of minimal cost recent vintage stuff is all over the place.

Why not just buy a duct fan that is 120vac from Grainger.

Grainger blower  -  $120.10  not enough C.F.M. (81 max)
Bilge blower       -  $19.11    Will blow utility bills off the dining room table from 10' away.

R....

if the walwart unit is a switcher, can you change the feedback ratio to get 7.5V?

Probably. If you can get at the electronics. Unit is put together with what look like slotless screws, one way trip.
I sawed open another unit to find the pcb wrapped in plastic and copper foil with all seams nicely soldered. I don't think they want me mucking about in there.

R.......

As in a nautical bilge?

   

Yes. It's a nautical bilge exhaust blower.

I'm going to use it to supply make up air for my heating system boiler which pulls conditioned air out of the house for combustion, not an efficient use of my heated air.

R......