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,

 

Hello,

Any potentiometer will bur, as they are not rated for the high current.
Better use a PWM circuit.
Here is an example:
Project: Simple PWM circuit

When you use a logic gate mosfet, the circuit will work on the mentioned 6 Volts.

Bertus

 

A rheostat connection will be better than pot.

Look at the size and decide if it's what you really want to do.

"(RWA) M22/03-0052
Rheostat, 6 ohm, 25 watt, 2.04 amps. 1/4" slotted shaft. 1-5/8" diameter body, 1-3/8" behind panel depth. Made by Ohmite."

Could be $25-$50 on the surplus market if you can find one.

This post explains why you got the (good) answer that you did.

 

Thank you folks ! I'll think about all this...

 

Been there, done that. inwo has the right idea.

The otoscope I fixed had a 3.5V center tapped transformer and a 5 ohm rheostat. The center tap was only used as a ground point to avoid static electricity build up or leakage from the 120 volt side. The strange part is that the transformer was one of those ferro-resonant types with an oil filled capacitor to kind of regulate the voltage. What can I say? You have to put something complicated in it to charge a doctor $300 for a flashlight.

Probably an incandescent bulb for a (2) D cell flashlight.

The microscope used a Radio Shack multi tapped supply transformer. 3V, 4.5V, 6V, 7.5V.
I put a 6.3V bulb in it so he could over-drive it if he needed dazzling white light.
The doctor liked it.

 

Can someone give me acceptable ranges (ohm, watt, amp...) for my rheostat ? I have problems finding the exact model suggested by Inwo.
Thanks

 

I'll post a link and maybe #12 can pick one.

https://www.surplussales.com/Potentiometers/Rheostats/PotsRheost-1.html

The 6 ohm is sold out so I'm thinking 8ohm 25 watt?
Or 10 ohm 50watt or 25 watt with the knob.

 

How about a 6v wall wart on a light dimmer?

Would have to be an old style with a transformer. ac or dc if unregulated.

 

6 ohms..not in stock
Mouser: $35 to $40

I have to go to the bank within 80 minutes. Be back later.

 

Just a thank you, INWO for the link to rheostats!

 

The 6 ohm is sold out so I'm thinking 8ohm 25 watt?
Or 10 ohm 50watt or 25 watt with the knob.

You're going too high on the watts. Basically, the math says that when the pot resistance matches the lamp resistance, that is the highest wattage on the pot. For 6 volts into a 10W bulb, Rbulb = 3.6 ohms. As long as the voltage is not increased, any pot that can get to 3.6 ohms will be going halvsies with the bulb for power dissipation.

6V, 7.2 ohm, 5 watts, and the pot gets half of that. Double the wattage for the safety factor and a 5 watt pot will work all day. Then again, sometimes you can buy a 10 watt pot for less than a 5 watt pot. Go figure.

 

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

 

You're going too high on the watts. Basically, the math says that when the pot resistance matches the lamp resistance, that is the highest wattage on the pot. For 6 volts into a 10W bulb, Rbulb = 3.6 ohms. As long as the voltage is not increased, any pot that can get to 3.6 ohms will be going halvsies with the bulb for power dissipation.

6V, 7.2 ohm, 5 watts, and the pot gets half of that. Double the wattage for the safety factor and a 5 watt pot will work all day. Then again, sometimes you can buy a 10 watt pot for less than a 5 watt pot. Go figure.

That's what I thought at first.

Then when I did the math it came out to 22 watts.

I squared R

If it gets set to the last turn, meaning near lowest resistance, the full lamp current will flow thru that turn.

That turn however is only a small percentage of the total resistor.
Ergo it can handle only that same small percentage of the total watt rating.

In practice the heat may be distributed somewhat more than that.
I believe this to be the reason rheostats are rated in amps, not watts.

You may notice that the 25 watt ones are only rated at a couple amps.

 

I see your point.
It might be too much to expect a 5 ohm pot to be set at 1/2 ohm, in series with 3.6 ohms, and dissipate 1.07 watts (20% of its rating) with 10% of its internal resistance wire.

 

Exactly!

How this plays out in the real world I don't know.

That's why my conservative guess.

My first post. " decide if it's what you really want to do."

If this is his choice, I will actually try it before recommending less watt rating.

It may be fine!

 

A rheostat connection will be better than pot.

Look at the size and decide if it's what you really want to do.

"(RWA) M22/03-0052
Rheostat, 6 ohm, 25 watt, 2.04 amps. 1/4" slotted shaft. 1-5/8" diameter body, 1-3/8" behind panel depth. Made by Ohmite."

Could be $25-$50 on the surplus market if you can find one.

This post explains why you got the (good) answer that you did.

I thought a rheostat would be better than a pot.
As long as you’re pretty sure the potentiometers you are suggesting won’t burn, as I already experimented it with a higher resistance (100kΩ), I will take both: 5Ω-5w and 5Ω-10w and test them. I’d just like to have your comments about the potential overheating of the pots before I order.

 

I thought a rheostat would be better than a pot.
As long as you’re pretty sure the potentiometers you are suggesting won’t burn, as I already experimented it with a higher resistance (100kΩ), I will take both: 5Ω-5w and 5Ω-10w and test them. I’d just like to have your comments about the potential overheating of the pots before I order.

I mean : 5Ω-5w and 10Ω-5w

 

My opinion is that they will overheat.

My calculations show greater than 15 watts needed.

I'm not sure I have #12 convinced. Or myself for that matter.

 

For an example using a 5 ohm pot and 1.75 amps.

When adjusted to 10% (.5ohms) draws 1.5 amps.

10% of the 5watt rating (where the heat must be dissipated) is .5 watts.

Actual watts to be dissipated is 1.5A X 1.5A X .5R= 1.25 watts

Too small by a factor of over 2. 3 would be safe = 15 watt pot minimum.

My math is probably not worst case. I'd still go 25 watt.

It may be that the dissipation percent may not follow the pot setting (percent) exactly. May be wrong by a factor of two or more.
I don't know how to predict other than field testing or specific pot data.

I welcome other opinions or experience.

 

That link I put up has 5 ohms 10 watts, and 10 ohms 10 watts, too.
My math says 1/2 ohm in series with a 3.6 ohm lamp puts 1.07 watts on the 1/2 ohm.
A 5 ohm pot set at 1/2 ohm is at 10% of its resistance and 10% of a 10 watt pot is 1 watt.

That's my story and I'm sticking to it.