How can I prevent mineral buildup in my small humidifier?

I find vinegar and water works really well. I use an old toothbrush to get into crevices and to dislodge mineral buildup.

You didn't specify the brand or model, but for generic advice, try using a strong solution of white vinegar and water and soak overnight or for several days (if very scaled up).

If you were using hard water in the humidifier, try using a commercial humidifier cleaner such as BestAir products (or similar - there are many to choose from): Amazon com BestAir 3US 6 UltraTreat Ultrasonic Vaporizer Water Scale...
(Make certain it's certified for steam and/or ultrasonic use, if your using that type of humidifier).
Follow directions on box. You will have to let soak for a period of time (hours/days) to allow chemicals to do their thing.

If you are using "soft" conditioned water, just refill with soft water and allow to sit for several hours, then discard and refill. The hardened minerals will re-disolve and will be removed when you flush the unit. For hardened minerals clinging to the sides, etc, just use your fingers to break up, possibly a brush or Dolby pad and break them loose.

(Soft water has just as much minerals as hard water, but in hard water there are mostly calcium based and turn rock hard and in soft water they are sodium based (sodium bicarbonate actually) and have a weak structure).

It's important to keep a humidifier clean of deposits, not just to keep the unit running, but to avoid air quality problems (bacteria will cling to scale buildup, which make excellent breeding grounds for them).

If you use "hard" tap water, clean (at a minimum) once a week. Use white vinegar and water - 50/50 mix. If your using "soft" conditioned water, clean once a month. Same with distilled water.
You could also consider using a small amount of bactericide in the water to prevent bacteria from growing. (Okay for filter type humidifiers, but DO NOT use in ultrasonic or vaporizer type humidifiers). 1/4 cap full should suffice. BestAir 3BT 6 Original BT Humidifier Bacteriostatic Water Treatment 32 oz

It all depends on a couple of factors.

First is the type of humidifier your using. Portable or whole house connected to the furnace. Steam, ultrasonic or filter. Your unit's instructions is the best guide as to how often you need to clean it and this can vary from every day to once a heating season.

My recommendation is to watch and clean the unit as soon as you see scale forming. Then in the future, reduce this to just before you see scale forming.

The second is the type of water you are using. Hard tap water is the worst and usually will require every day for small portable steam or ultrasonic units to once a week for filter models. For central whole house humidifiers, once a month (perhaps as little as every two weeks if your water is really hard, >10 grains hardness). There is no fixed formula for this, the idea is to perform on-going, preventive maintenance.

Soft or conditioned water will contain the same amount of minerals as hard, but the softener has replaced the calcium minerals with sodium minerals. (Sodium bicarbonate actually).The result is the humidifier will still form scale but it much easier to remove. With soft water scale, you can just break up the scale with your fingers and the filters can just be submerged in clean water and the salt scale will re-dissolve.

Third, with distilled water, you may only need to clean any of these units annually. (No minerals to for scale).

With any filter humidifier, consider using a commercial bactericide to prevent the growth of bacteria when the water sits for long periods (>14 days). Not so critical with soft water as the salt will retard the growth of bacteria, but very important with hard or distilled water. Don't use this product in ultrasonic or steam humidifiers as it will be carried into the air along with the water vapor, and can be considered as toxic.

With hard water, use a 50/50 solution of white vinegar and water to dissolve the scale buildup. This works best if done on a regular basis. If left go too long, it become impossible to clean properly. You need only use regular water when using soft or distilled water in your humidifier.

are you seating it right after fill ck the micro switch , may be misaligned

The best solution is to buy distilled water or deionized water.

Another alternative to vinegar is to use lemon juice. About 1 cup to a tank of water should do the trick. It smells much better than vinegar and also has the acid content to clean the vaporizer.

From The Hunter Manual
1. Turn the humidifier ‘OFF’ and unplug the unit.
2. Lift the main housing and remove the water dispersion tray and the PermaWick™ Filters. The motor is attached to the main housing. Do not place the motor near water. Do not use a wet towel on the motor or motor housing as electrical shock may occur.
3. The PermaWick™ Filter should be cleaned in a vinegar and water solution to remove any mineral buildup. Place in a solution of one (1) gallon of water and eight (8) ounces of undiluted white vinegar. Allow the filter to soak for 20 minutes.
4. After soaking, rinse it several times with clean water. Place the PermaWick™ Filter in a location where it can drip dry while you clean the other parts of your humidifier.
5. Remove the water tank from the base and empty any water that is left in the tank. Soak the base in a mixture of one (1) gallon of water and eight (8) ounces of vinegar for 20 minutes.
6. After soaking, wipe the base with a soft cloth or sponge to remove any mineral build up. Rinse with clean water several times. If you are planning on storing the humidifier, be sure the base is completely dry.
7. Fill the water tank with one (1) gallon of water and eight ounces of undiluted white vinegar. Replace the tank cap and shake the tank vigorously until all interior surfaces of the tank have been exposed to the water/vinegar mixture.
8. Empty the tank of the water/vinegar mixture. Rinse it several times with clean water and then replace the tank cap. If you are planning on storing the humidifier, dry it inside and out, leaving the cap off to allow the tank to completely dry. Continue to clean the other parts of your humidifier.
9. Remove the water diffuser cap by squeezing the two tabs as shown in Figure 3. Rinse the cap in clean water to remove any debris. If the cap contains hard water residue, soak it in a mixture of one (1) quart of water and two (2) ounces of vinegar for 20 minutes.
Rinse well and reinstall the water diffuser cap. Do not expose motor or other electrical components to water.
10. Wipe the surfaces of the main housing with a soft cloth.
11. Once all the parts are clean, reassemble the humidifier, and follow the USING YOUR HUMIDIFIER to continue use. If you are storing the humidifier, make sure the parts are completely dry, reassemble, and store in a dry location

From The Hunter 3322 manual.
1. Turn the humidifier ‘OFF’ and unplug the unit.
2. Lift the main housing and remove the water dispersion tray and the PermaWick™ Filters. The motor is attached to the main housing. Do not place the motor near water. Do not use a wet towel on the motor or motor housing as electrical shock may occur.
3. The PermaWick™ Filter should be cleaned in a vinegar and water solution to remove any mineral buildup. Place in a solution of one (1) gallon of water and eight (8) ounces of undiluted white vinegar. Allow the filter to soak for 20 minutes.
4. After soaking, rinse it several times with clean water. Place the PermaWick™ Filter in a location where it can drip dry while you clean the other parts of your humidifier.
5. Remove the water tank from the base and empty any water that is left in the tank. Soak the base in a mixture of one (1) gallon of water and eight (8) ounces of vinegar for 20 minutes.
6. After soaking, wipe the base with a soft cloth or sponge to remove any mineral build up. Rinse with clean water several times. If you are planning on storing the humidifier, be sure the base is completely dry.
7. Fill the water tank with one (1) gallon of water and eight ounces of undiluted white vinegar. Replace the tank cap and shake the tank vigorously until all interior surfaces of the tank have been exposed to the water/vinegar mixture.
8. Empty the tank of the water/vinegar mixture. Rinse it several times with clean water and then replace the tank cap. If you are planning on storing the humidifier, dry it inside and out, leaving the cap off to allow the tank to completely dry. Continue to clean the other parts of your humidifier.
9. Remove the water diffuser cap by squeezing the two tabs as shown in Figure 3. Rinse the cap in clean water to remove any debris. If the cap contains hard water residue, soak it in a mixture of one (1) quart of water and two (2) ounces of vinegar for 20 minutes. 41553-01 01/03/2008 6 © 2008 Hunter Fan Company
Rinse well and reinstall the water diffuser cap. Do not expose motor or other electrical components to water.
10. Wipe the surfaces of the main housing with a soft cloth.
11. Once all the parts are clean, reassemble the humidifier, and follow the USING YOURHUMIDIFIERto continue use. If you are storing the humidifier, make sure the parts are completely dry, reassemble, and store in a dry location

Have you cleaned the unit lately? The following is from the owners manual.
1. Turn the humidifier ‘OFF’ and unplug the unit.
2. Lift the main housing and remove the water dispersion tray and the PermaWick™ Filters. The motor is attached to the main housing. Do not place the motor near water. Do not use a wet towel on the motor or motor housing as electrical shock may occur.
3. The PermaWick™ Filter should be cleaned in a vinegar and water solution to remove any mineral buildup. Place in a solution of one (1) gallon of water and eight (8) ounces of undiluted white vinegar. Allow the filter to soak for 20 minutes.
4. After soaking, rinse it several times with clean water. Place the PermaWick™ Filter in a location where it can drip dry while you clean the other parts of your humidifier.
5. Remove the water tank from the base and empty any water that is left in the tank. Soak the base in a mixture of one (1) gallon of water and eight (8) ounces of vinegar for 20 minutes.
6. After soaking, wipe the base with a soft cloth or sponge to remove any mineral build up. Rinse with clean water several times. If you are planning on storing the humidifier, be sure the base is completely dry.
7. Fill the water tank with one (1) gallon of water and eight ounces of undiluted white vinegar. Replace the tank cap and shake the tank vigorously until all interior surfaces of the tank have been exposed to the water/vinegar mixture.
8. Empty the tank of the water/vinegar mixture. Rinse it several times with clean water and then replace the tank cap. If you are planning on storing the humidifier, dry it inside and out, leaving the cap off to allow the tank to completely dry. Continue to clean the other parts of your humidifier.
9. Remove the water diffuser cap by squeezing the two tabs as shown in Figure 3. Rinse the cap in clean water to remove any debris. If the cap contains hard water residue, soak it in a mixture of one (1) quart of water and two (2) ounces of vinegar for 20 minutes. 41553-01 01/03/2008 6 © 2008 Hunter Fan Company
Rinse well and reinstall the water diffuser cap. Do not expose motor or other electrical components to water.
10. Wipe the surfaces of the main housing with a soft cloth.
11. Once all the parts are clean, reassemble the humidifier, and follow the USING YOURHUMIDIFIERto continue use. If you are storing the humidifier, make sure the parts are completely dry, reassemble, and store in a dry location

The instruction manual suggests using the water vinegar solution.
The following is from the manual.
1. Turn the humidifier ‘OFF’ and unplug the unit.
2. Lift the main housing and remove the water dispersion tray and the PermaWick™ Filters. The motor is attached to the main housing. Do not place the motor near water. Do not use a wet towel on the motor or motor housing as electrical shock may occur.
3. The PermaWick™ Filter should be cleaned in a vinegar and water solution to remove any mineral buildup. Place in a solution of one (1) gallon of water and eight (8) ounces of undiluted white vinegar. Allow the filter to soak for 20 minutes.
4. After soaking, rinse it several times with clean water. Place the PermaWick™ Filter in a location where it can drip dry while you clean the other parts of your humidifier.
5. Remove the water tank from the base and empty any water that is left in the tank. Soak the base in a mixture of one (1) gallon of water and eight (8) ounces of vinegar for 20 minutes.
6. After soaking, wipe the base with a soft cloth or sponge to remove any mineral build up. Rinse with clean water several times. If you are planning on storing the humidifier, be sure the base is completely dry.
7. Fill the water tank with one (1) gallon of water and eight ounces of undiluted white vinegar. Replace the tank cap and shake the tank vigorously until all interior surfaces of the tank have been exposed to the water/vinegar mixture.
8. Empty the tank of the water/vinegar mixture. Rinse it several times with clean water and then replace the tank cap. If you are planning on storing the humidifier, dry it inside and out, leaving the cap off to allow the tank to completely dry. Continue to clean the other parts of your humidifier.
9. Remove the water diffuser cap by squeezing the two tabs as shown in Figure 3. Rinse the cap in clean water to remove any debris. If the cap contains hard water residue, soak it in a mixture of one (1) quart of water and two (2) ounces of vinegar for 20 minutes. 41553-01 01/03/2008 6 © 2008 Hunter Fan Company
Rinse well and reinstall the water diffuser cap. Do not expose motor or other electrical components to water.
10. Wipe the surfaces of the main housing with a soft cloth.
11. Once all the parts are clean, reassemble the humidifier, and follow the USING YOURHUMIDIFIERto continue use. If you are storing the humidifier, make sure the parts are completely dry, reassemble, and store in a dry location.