Constant humidity is like extra money. Our collections can get to be quite valuable too. Besides the materialistic concerns, I hate worrying about the right humidity. A relatively small desktop humi most likely came with a container filled with florist's foam - that green stuff. The foam is the least expensive way to humidify and is a functional equivalent of a bowl of water, except that its harder to spill.
I advise you to get serious about your collection and install absorbent polymer crystals, or "beads". The foam will crank out water vapor until the air is saturated or it runs out of water. The beads are designed to maintain a specific RH and are available in several percentages. I use 65%. So long as the beads are charged with water, they will drive the RH in your humi to their programmed limit, but not beyond it. If the RH in the hum is above the programmed limit and the beads are not saturated, they will absorb moisture, again driving for the programmed limit. With beads, you could literally dispose of your hygrometer. They are that trustworthy.
Try
Cigarmony and
Heartfelt (Viper). Both are reputable vendors and can answer any other questions you have.