Yes, you can drive solenoids with DC when they are designed and rated for DC. Below is an AC solenoid which was fed a steady diet of DC. It worked just fine for about 20 min and then stopped working.
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Designed as can be seen on what's left of the label 24 VAC 60 Hz. When supplied with DC the coil heated up, the housing blistered, and that was the end of life as the coil knew it. I used this image as a part of discussing with the technicians the importance of using the correct replacement parts.

Ron

I have successfully run a 24 volt AC solenoid on a bit less than 12 volts DC to power the gas valve on a steam boiler during an extended midwinter power outage. The DC came from a partly discharged 12 volt battery, but it was enough to operate the solenoid and run the boiler for three days.
BUT the concept of transformer tap changing requires a lot more, and a micro such as the arduino has no part of it. NONE!! The systems with tap changing are done by simpler logic systems that the utility companies use, and power fets have not much part of it. Definitely a switching power supply arrangement is the suitable choice.

 

Good Morning!

I am thinking about a project will sense the input voltage on a transformer, and automatically switch taps to maintain the output to a certain range... say 15-18Vdc. Output current is relatively unimportant.. Input current is already limited to less than 8 amps, and this is for stepping up voltage, so the output current will still be less than 8 amps.
If I use a micro controller, the first part is simple enough, the unknowns are a Large cap on the output to stabilize the voltage transients, and the inductive kickback. Should I set it up for a make before break switch, or break before make, and deal with the kickback?

Any thoughts or concerns would be greatly appreciated!
Mike

Petkan:
Classical transformers are not suitable for DC fed applications (solar power+battery). Based on your load requirements (say 12V solenoid 1A), you have to chose the battery and then make the charger work with this battery. If the solar panel produces lower than the battery voltage you need a boost stage. If it is always higher - Buck stage, if it could be either lower or higher - buck-boost stage. Ready made modules are available at aliexpress.com or on ebay for around $2. It is most flexible to buy a CV CC Buck-Boost module, so you can set the desired final voltage and max charging current. It is not clear whether your actuator solenoid is on/off or proportionally controlled. If it is On/off - most likely you can reduce the current after energizing. The simplest way to achieve it is by having in series with the solenoid a parallel RC circuit with low Ohmic resistor (defining the hold current) and electrolytic cap across (to provide during its charging the pull current). It is certainly possible to avoid losses in such resistor by switching mode controlling the voltage or current to the load for instance by a Buck module (manipulating the resistive divider defining the voltage). If the solenoid requires proportional control you need to continuously adjust the Buck by adding controlled current into its feedback pin. You can contact me at 'MOD: personal email address deleted' for more details.
This is achieved by replacing the voltage feedback by curent feedback

 

Most PV modules sold for low voltage battery charging have a maximum power point voltage a few volts above the voltage of a fully-charged lead-acid battery. MPP does not change very much with illumination until the light level gets very low, so unless a very efficient converter were used there would likely be almost no gain in using a converter with the intent of improving low light performance. MPP varies significantly with temperature, so the lower light with overcast may actually be more than compensated for by reduced array temperature. MPP tracking converters will deliver benefit with full illumination, particularly at low temperature where the PV output voltage is higher - assuming it truly tracks MPP in some fashion. If the MPP voltage of the array is very much higher than the battery voltage at all times, an MPP converter is essential to getting decent system performance.

A ordinary switch mode converter, be it buck, boost or flyback, will deliver no benefit (possibly negative "benefit") if the output load power requirement is greater than the input power available. When charging batteries with a PV array, it is almost always the case that the input power is insufficient to meet the output load requirement until the battery is nearing full charge. An MPP converter must have an additional control "loop" that prevents the input voltage from being pulled below the MPP of the PV array. Without this control, it is almost certain that a simple "bang-bang" (fully on or fully off) controller will be more efficient that any switchmode converter.

 

The solution can be simple, since for battery charging it is not vital to get the most possible power out of the solar array. Use a switchmode regulated power supply that will work with a wide range of input voltages and has an output current and voltage range adequate for charging your battery. Then adjust the output voltage up to charge your particular battery, and adjust the current down to the allowable max charging current for your battery. The beauty is that this kind of supply is a catalog item, or at least available from a number of different makers. AND the circuit is undoubtedly available if you want to build it yourself.