Need electronics help adapting Milwaukee m12 battery to a 9v load...I think

[Roger Feeley]

Iv had a triton powered respirator for years. The ni-cads gave out a long time ago and I thought I was being clever when I converted it to run off of an ac power supply. I only used in the shop when I was turning or when I emptied the dust collector so being tethered to an extension cord didn’t bother me. My power supply puts out 9v and 500ma. The fan runs ok but a bit slow. I’ve put up with that for, heck, 5 or 10 years.

then along comes Coronavirus and I’m dealing with my own short sightedness. If the thing was mobile, it would make the ultimate mask.

i found a guy on Etsy that is three-d printing power docks for various portable tool batteries. I have one for the Milwaukee m12 battery.

using a resistor substitution box, I found that about 5 ohms between the battery and the fan is about right.

so my thought is to put a 4 ohm resister in series with a 10 ohm potentiometer.

what I don’t know is the required wattage.

new battery power supply is 12v and 5 ohms seems to make it happy.

i know that an ac powered 9v .5a supply will run the thing but a bit slow.

i didnt want to run it very long as my resister substitution box is 1w.

Need a little ohms law coaching here.
9v x .5a = 4.5 watts, right?

should I be looking at 10w components?

be gentle, guys. I’m doing my best to remember stuff from a 1 semester electronics class in 1976.

[Paul F Franklin]

It would be ideal if you could measure the actual current as you might get by with smaller parts, but assuming it's .5 amps max. since your .5 amp supply ran it ok, then the total power dissipated by the resistor(s) will be I*I*R or .5 x .5 x 5 or 1.25 watts. If you have a 4 ohm resistor and the pot is set to 1 ohm, the fixed resistor will dissipate 1 watt and the pot .25 watts.

Now keep in mind that potentiometer power ratings are with the full element in use (max resistance). If you are only using part of the element (which you will be) you would have derate the power rating. If you want a 10 ohm pot, you would need one with a power rating of at least 2.5 watts (5 would be better). You might be better off with a 5 ohm pot as more of the range would be used.

But I suggest you just buy a few 1 ohm resistors and a few .5 ohm resistors (I'd go with 1/2 watt since they are basically free) and just put a few in series to get what you need. Unless you want to be able to change the pot during use, but I'm thinking you will find a good setting and leave it alone.

One more point: You will be wasting power in the resistors (as heat; they will get warm). Your battery life would be a bit longer if you used a voltage regulator to lower the voltage to what you need. But that would be more complicated, and the M12 battery will likely last a long time either way.

[Roger Feeley]

Thanks,
yeah, the battery should run a while. It also has the virtue of being swappable. The original batteries ran for a few hours and then you had to charge the thing overnight. These little m12s would be a big improvement.

it would just break my heart to discard the whole thing because of a power issue. It has the sealed helmet, the ear protection and a pristine face shield. It would be like chucking a Unitas because the motor went bad.

[John Lanciani]

Have you tried just using the battery directly? The 12 volt lithium tool batteries are really 10.8 volts when they have a load on them, they only became 12 volts when marketing got involved. You might find that it works just fine without the added complexity.

ETA - if the original battery was in-cad it was probably 9.6 volts nominal, 10.8 should be fine.

[Roger Feeley]

Have you tried just using the battery directly? The 12 volt lithium tool batteries are really 10.8 volts when they have a load on them, they only became 12 volts when marketing got involved. You might find that it works just fine without the added complexity.

ETA - if the original battery was in-cad it was probably 9.6 volts nominal, 10.8 should be fine.

i measured the voltage across the battery with no load and it was mid to upper 11 volts. I thought about just hooking it up but if I blow the motor, I’m done. I am proceeding very carefully.

There are battery rebuild kits on eBay. I can see 5 batteries about the size of D-cells.

heres a SMC thread about the batteries.
https://sawmillcreek.org/showthread....ton-respirator

[Jim Koepke]

Somewhere out in my shop is a 100w 5Ω potentiometer. It is ceramic, about 4" across by 2" deep counting the shaft and weighs about a pound and a half.

John's suggestion sounds good to me.

jtk

[Jim Koepke]

i know that an ac powered 9v .5a supply will run the thing but a bit slow.

Looking at this again, you do not mention what the original batter was.

If it was 9V and is now running slow with the power pack, it might be trying to pull more current than is available.

jtk

[Bruce Wrenn]

From flea bay, you can buy adjustable dc to dc transformers for a few bucks. Coming from China, it takes a couple weeks to get them. While on fleabay, order a couple dc volt meters. I buy these for less than two bucks delivered.

[Roger Feeley]

Here’s a link to a rebuild kit. Does this tell you anything?
https://www.batterynrebuild.com/prod...-collector-air

[Paul F Franklin]

In the spec sheet, which I found here: https://www.routerforums.com/triton-...espirator.html

it says the battery is 6 volts. That would be 5 1.2 volt nicads in series, which kind of jibes with the rebuild kit since it includes 5 1.2 volt nicads, although it doesn't give any indication of how they need to be wired.

I think Jim may have hit on it....it may be your .5 amp supply can't supply enough current and its output voltage is dropping. If you can borrow (or buy a cheapie; they are very inexpensive) multimeter, you could measure the actual voltage your supply is providing under load. As you said, you don't want to blow it up. I definitely would not connect your battery directly.

[Roger Feeley]

Will do, Paul. I actually have an old but very serviceable Fluke digital that I rescued when my company was acquired.

[Rod Sheridan]

A diode has about 0:7 volts forward drop, which is reasonably linear with respect to current.

I would put 8 in series to drop the battery from 12 to 6 volts.

You can probably get 3 ampere diodes.locally....Rod

[Roger Feeley]

I found 10 1ohm 5 amp resisters on Amazon. The respirator came with a little gizmo to test fan output. It’s a little ball in a tube. You hold it to the fan and if the ball lines up with a circle, the fan is good.

[Roger Feeley]

A heartfelt thanks to all!
i got the thing running. I love the concept of it now. Originally, with the built in rechargabes, I could use it for a few hours and the I had to wait. Now, I use my Milwaukee m12 battery which I can simply swap out. No dedicated battery. No dedicated charger. Granted, I’m not a Milwaukee guy but I do have a couple of their cordless tools that share one battery. If I ever need a second battery, I won’t feel so bad.

i thank you and my lungs thank you.

[Jim Koepke]

If I ever need a second battery, I won’t feel so bad.

You will when you see the cost of a second battery. They are often as expensive as one of the cordless tools.

It might be cheaper to purchase a new tool on sale with two batteries included.

jtk