I'm thinking it's cut too close.

I wouldn't load a 1/4 watt resistor to 1/4 watt.
Depends on the duty cycle.

I'll sign off at double the watt rating. 15-20 watts.

Then there is the value, also.
Incandescent is still pretty bright at 40%.
I'll vote for 10 ohms.

 

One more passing thought: an 8 ohm L-pad (for a stereo) could be used and might be more available than single core rheostats.

 

Ohmite likes my calculation for maximum current.

http://www.ohmite.com/techdata/rheostats203.pdf

I= square root of W/R

1.4 amps for the 5 ohm 10 watt

1 amp for the 10 ohm 10 watt.

 

I thought a rheostat would be better than a pot.

A pot can be a rheostat. It depends on how you wire it up. This fact leads to sloppy habits about how you use the words.

 

I'll order 2 or 3 different pots when I have time this week considering your opinions. I'll make some tests and you will learn about my observations probably in a couple of weeks, maybe more. I thank you, Inwo and #12, for your advises and infos. You both agree about wiring the pot in series ?

 

I'll order 2 or 3 different pots when I have time this week considering your opinions. I'll make some tests and you will learn about my observations probably in a couple of weeks, maybe more. I thank you, Inwo and #12, for your advises and infos. You both agree about wiring the pot in series ?

If you choose the pot solution, yes. Wire it as a rheostat. In series.
I'm sorry we are not in total agreement.
Was hoping to get more opinions.
I have never had luck, running things at maximum ratings. That does not mean it won't work for you.

 

I finally got time between interruptions to read the references.

Your reference in post #23 shows that a 5 ohm, 10 watt rheostat has a 2 amp limit. The lamp alone will limit the current to 1.66 amps. Therefore, any setting of the rheostat can not have more than 1.66 amps and the 5 ohm, 10 watt rheostat will be safe under these conditions.

The curves on one of the graphs tells me that a 5 ohm, 5 watt rheostat should survive, but with very little safety margin and the 10 ohm, 10 watt will also have this very slim safety margin.

This firms up my decision that 5 ohms, 10 watts is the right one of the 4 options I linked in post #12.

 

I finally got time between interruptions to read the references.

Your reference in post #23 shows that a 5 ohm, 10 watt rheostat has a 2 amp limit. I calculate 1.4 amp The lamp alone will limit the current to 1.66 amps. Therefore, any setting of the rheostat can not have more than 1.66 amps and the 5 ohm, 10 watt rheostat will be safe under these conditions.

The curves on one of the graphs tells me that a 5 ohm, 5 watt rheostat I show 1 amp max hould survive, but with very little safety margin and the 10 ohm, 10 watt will also have this very slim safety margin.

This firms up my decision that 5 ohms, 10 watts is the right one of the 4 options I linked in post #12.

I only see 5 watt pots in the link.
Rheostats are offered in 12 watts and up.

http://www.digikey.com/product-search/en?pv1=1166&pv1=1114&pv1=1551&pv1=1120&pv1=1350&pv1=1028&pv2=602&pv2=77&pv2=198&pv2=12&FV=fff40004%2Cfff80123&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

 

Oh $%^&. You caught me being wrong again.

I falsely remembered a different page because I had to do 3 or 4 searches to get anything under $35.

Sorry. My bad.

 

Here are some at Digi-key for under $5
5 ohms, 5 watts should probably work unless you want to get very dim. Then choose the 10 ohm, 5 watt. You can experiment with fixed resistors to see whether you want 5 ohms or 10 ohms.

http://www.digikey.com/product-sear...t=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

This link was good ?...

 

This link was good ?...

Link is good, but only shows the 5 watt pots.

Here is a Digikey link to larger.

http://www.digikey.com/product-sear...t=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

I'm sorry our discussion has proved more interesting than helpful.

 

Hi,

I try to make a simple electronic circuit.
So I have an instrument (medical ophtalmoscope) that contains a light bulb 10w, 6v. The adapter is noted 6v, 1.7A . I want to bypass the small rechargeable battery pack that isn’t working well. So I want to dim my bulb using a potentiometer. I tried a 100kΩ that was smoking (I chose that resistance because that's what is noted on the potentiometer of my original batt pack . So I guess there was a more complex assembly of resistors connected to it.)

My question: what is the ideal resistance needed for my potentiometer in this circuit (and does it have to be linear or logarithmic) ? I want the brightness to vary ok but not necessarily perfect. (A range of acceptable resistances would be fine ). I want it to work without burning it this time.
Thanks a lot,

I am tripping over one minor point in your drawing...

What is the actual configuration of the contacts on your switch ?

It Appears to be a SPDT switch, which would dictate your battery + should be hooked up to the center spade, but then again, Looking at the switch rocker, says it is a sequential SPDT...

I am going to have to cobble up the circuit to know if you have it wired right... 'cuz somehow your drawing doesn't look right... A 2 watt pot should handle that all day long and then some...

Take that information from an experienced " pot smoker "

 

I am tripping over one minor point in your drawing...

What is the actual configuration of the contacts on your switch ?

It Appears to be a SPDT switch, which would dictate your battery + should be hooked up to the center spade, but then again, Looking at the switch rocker, says it is a sequential SPDT...

I am going to have to cobble up the circuit to know if you have it wired right... 'cuz somehow your drawing doesn't look right... A 2 watt pot should handle that all day long and then some...

Take that information from an experienced " pot smoker "

You are right. It's a mistake. But trivial, as shorting load to ground isn't an issue.

The black connection to the switch is unnecessary.

 

I haven't smoked a pot since I tried to fix the cruise control on a 1986 Aerostar. You gradually give up your bad habits as you get old in exchange for surviving longer.

 

"You gradually give up your bad habits as you get old in exchange for surviving longer".



Wise words, #12!

 

Just an observation. Just as there are no old, stupid electricians, there are probably no 400 pound people over 50 years old and there are no 80 year old alcoholics. We all contribute to our own death with bad decisions and over-indulging, but you have to learn to make better decisions if you're going to be 80 years old before they nail the lid on. Some of those events include realizing when the hangover simply makes drinking cost too much, or realizing that I should sell my 32 foot ladder because I won't survive another fall from that high, or refusing to work a live circuit breaker box without a safety man present. I'm 63 years old and I just don't bounce as well as I used to. I won't survive another 120 RMS across my chest like I did when I was 20 years old. A serious car accident with multiple broken bones...I'd probably prefer to die than spend another 13 months in a cast, and I probably would die.

So, I drive like a granny, I quit drinking some time in the 1990's, I don't plan on more than 15 minutes in a 125 F attic, I use jack stands when I'm under a car, and I literally break out in a sweat if I don't have at least 3 layers of safety between me and anything higher than 50 volts. You figure these things out about the same time you get tired of going to funerals.

 

I am tripping over one minor point in your drawing...

What is the actual configuration of the contacts on your switch ?

It Appears to be a SPDT switch, which would dictate your battery + should be hooked up to the center spade, but then again, Looking at the switch rocker, says it is a sequential SPDT...

I am going to have to cobble up the circuit to know if you have it wired right... 'cuz somehow your drawing doesn't look right... A 2 watt pot should handle that all day long and then some...

Take that information from an experienced " pot smoker "

I did not execute what's on my drawing yet. Except for the pot (100k) connections. My question is still : what's the ideal pot for the parameters I mentioned ? If someone can give the right link to find it, it will help a lot. I'll create à box with connectors to the adaptor and the instrument. I may or may not add a switch. And I will find out how to wire that right.

 

Just for the record guys... An experienced POTentiometer smoker, as I think I have flamed more than my share...

Good thing I'm a packrat and get them relatively free...

 

I don't know how you can go wrong with this one from post #7.
It's not open to speculation about watt rating as the amp rating is listed in the specs. as good for 1.95 amps.

It may be argued that it's too large. While I don't agree.
I think we can all agree that with rheostats, too big is a good thing.



Rheostat
10 Ohm

Enlarge Image​

(RWA) AN3155-25-10
Aircraft rheostat, panel mount. 10 ohm, 1.95 amp, 0.30 min., 25 watts nominal. 2 terminal. Black bakelite bird's beak pointer knob included. Sealed. Fully CCW position is off (open). Clockwise rotation from off position proceeds from 10 ohms to zero ohms. 1-3/4"D x 2-1/2" deep. Made by Ohmite.

 

Are you in the States?

I can throw ten 1ohm 1watt resistors in an envelope for you.

You won't have to risk $40 on a guess.

The ten in series will simulate the rheostat and give you an idea of dimming and dissipation.

Say for example 4 of them dimmed to the level you need, Then get a 4ohm rheostat, and you will get full range.

Also show:
A bunch of my 10ohm 25watt pots.
If 10 ohm works for you, I may be able to supply you.