Using logic level Mosfets to auto switch taps on DC low voltage transformer...

Thread Starter

mbohuntr

Joined Apr 6, 2009
430
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
 

Alec_t

Joined Sep 17, 2013
10,355
What is the transformer rated input voltage range and what taps are available?
Is there just one secondary winding?
 

Thread Starter

mbohuntr

Joined Apr 6, 2009
430
What is the transformer rated input voltage range and what taps are available?
Is there just one secondary winding?
I am probably going to have to roll my own.. 18awg, 50 turns secondary, 50 turns primary, tapped at 40 turns,45, etc. up to full turns. There might be a good reason why nobody does this, that is why I'm asking... Solar panel feeding the transformer.
 

Reloadron

Joined Jan 15, 2015
4,501
There has to be more to this picture. Aren't the outputs of solar panels DC? So less any inverter we can not use a transformer with DC. This would likely go much better with a detailed explanation and even a rough circuit drawing of exactly, in detail, what you are trying to do?

Ron
 

Avid0g

Joined Apr 1, 2018
21
How about putting in a zero current crossover trigger? At low loads, poor precision is less important. At high loads, crossover will still occur just after minimum. I was thinking of a shunt which is across back-to-back diodes. It goes linear below 0.7 VAC. You could use both a non-inverting and inverting op-amp (to create the complement) and then an OR gate. Use gain to move closer to zero. Just stay away from the offset range w
 

cmartinez

Joined Jan 17, 2007
6,462
There has to be more to this picture. Aren't the outputs of solar panels DC? So less any inverter we can not use a transformer with DC. This would likely go much better with a detailed explanation and even a rough circuit drawing of exactly, in detail, what you are trying to do?

Ron
I was wondering the same thing... you need an inverter circuit before you connect a solar cell to a transformer for things to work
 

Avid0g

Joined Apr 1, 2018
21
I wonder if the output is from a solar micro inverter.? These are normally very steady, and smoothly fall into sync with external voltages. No need for regulation. Majority rules.

On the other hand small Windmills do put out unregulated AC in both frequency and voltage. Maybe he misspoke?

Now there is a good case for a regulator before the battery charger. However, I would lean towards a rectifier and DC/DC switcher instead. Just tweak it to minimize reactance.
 

Thread Starter

mbohuntr

Joined Apr 6, 2009
430
A solar panel cannot drive a transformer directly. The primary winding will appear as a dead short and damage the panel.

ak
I guess I don't understand... you can drive solenoids with DC, and a battery only has milliohms of internal resistance, and yet it isn't shorting the panel...
 

cmartinez

Joined Jan 17, 2007
6,462
I guess I don't understand... you can drive solenoids with DC, and a battery only has milliohms of internal resistance, and yet it isn't shorting the panel...
What it means, is that transformers must be driven with AC for them to work, and will also output AC. So you need a circuit that will convert from DC into AC, plug its output into a transformer, and then the AC output of that transformer would have to be converted back into DC (possibly regulated, depending on what you want).

I suggest you take a look at DC-DC converter modules instead. It could save you valuable time and money. Look for them in Digikey, Mouser or Newark, for example. Also, I've bought modules from these guys in the past. They're quite handy.
 

crutschow

Joined Mar 14, 2008
23,314
I guess I don't understand... you can drive solenoids with DC, and a battery only has milliohms of internal resistance, and yet it isn't shorting the panel...
A solenoid has sufficient resistance to draw its rated current from the DC source.

But a transformer can only operate from AC.
So to increase the voltage from a DC source it first must be converted to AC by a switching circuit.
Suggest you do some reading on transformer theory.
 

Papabravo

Joined Feb 24, 2006
12,389
I guess I don't understand... you can drive solenoids with DC, and a battery only has milliohms of internal resistance, and yet it isn't shorting the panel...
Transformers don't work with DC. Transformers only work with AC.
For solenoids look at the DC resistance of the coil and notice that they do get hot when held in the ON position. For batteries you can charge SOME types with a DC source that is greater than the battery voltage. The current draw is a function of the voltage difference between the charging source and the current battery voltage, not the internal resistance. Internal resistance does limit the rate of discharge, and has an effect on the voltage delivered to the load.
 

Thread Starter

mbohuntr

Joined Apr 6, 2009
430
I was thinking about solar panels and MPPT controllers. I bought a 12v system for my camper, and was looking for ways to keep the batteries charging when the panel voltage drops on overcast days. I really didn't want to series/parallel the panels to raise the voltage to run an MPPT controller, and thought "What if I raised the panel voltage by stepping it up when needed." I guess I now know why..... thanks everyone!
 

AnalogKid

Joined Aug 1, 2013
8,102
I guess I don't understand... you can drive solenoids with DC, and a battery only has milliohms of internal resistance, and yet it isn't shorting the panel...
Depends on the battery and the panel. Counting on the solar panel's output impedance or ESR (Equivalent Series Resistance) and the battery's own ESR to limit the current into a battery to a safe level without a charging circuit can be OK in a limited range of combinations. However, it *always* is bad design practice to assume that two parameters that are poorly documented, uncontrolled, and vary significantly with both temperature and aging, will magically combine in your favor ever, let along always.

Back to the original point, while a transformer and a solenoid both are electromagnetic devices based on a coil of wire, all similarities end there. Ampere's Law has no differential operator in it, and allows for the creation of a non-varying magnetic field (DC into a coil produces a steady-state field). But it is Faraday's Law that governs/describes how energy is transferred from a magnetic field into a wire, and it does have a differential operator. This means that energy in a field produces a current in a wire *only* if the field is varying over time. The varying can come from moving the field past the wire, moving the wire through the field, or having both the wire and the field be physically stationary, but increase and decrease the strength of the field. This last one is how a transformer works.

When you switch DC into a transformer primary, there will be a voltage spike on the output as the current in the primary increases. But once the primary current is steady and unchanging, the voltage on the secondary drops to zero.

ak
 
Last edited:

Reloadron

Joined Jan 15, 2015
4,501
I guess I don't understand... you can drive solenoids with DC, and a battery only has milliohms of internal resistance, and yet it isn't shorting the panel...
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.
Cooked Coil.png

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
 

Thread Starter

mbohuntr

Joined Apr 6, 2009
430
Depends on the battery and the panel. Counting on the solar panel's output impedance or ESR (Equivalent Series Resistance) and the battery's own ESR to limit the current into a battery to a safe level without a charging circuit can be OK in a limited range of combinations. However, it *always* is bad design practice to assume that two parameters that are poorly documented, uncontrolled, and vary significantly with both temperature and aging, will magically combine in your favor ever, let along always.

Back to the original point, while a transformer and a solenoid both are electromagnetic devices based on a coil of wire, all similarities end there. Ampere's Law has no differential operator in it, and allows for the creation of a non-varying magnetic field (DC into a coil produces a steady-state field). But it is Faraday's Law that governs/describes how energy is transferred from a magnetic field into a wire, and it does have a differential operator. This means that energy in a field produces a current in a wire *only* if the field is varying over time. The varying can come from moving the field past the wire, moving the wire through the field, or having both the wire and the field be physically stationary, but increase and decrease the strength of the field. This last one is how a transformer works.

When you switch DC into a transformer primary, there will be a voltage spike on the output as the current in the primary increases. But once the primary current is steady and unchanging, the voltage on the secondary drops to zero.

ak
I think I understand... It is the "changing" field that induces current in adjacent coils...right? I work in Power generation, and deal with solenoids ,rectifiers and high voltage battery systems all the time, but I guess I never really understood the differences between magnetic fields, and inducing them.. I thought they were pretty much the same. and, yes, I sucked at physics... Thank you for teaching me!
 

Avid0g

Joined Apr 1, 2018
21
I was thinking about solar panels and MPPT controllers. I bought a 12v system for my camper, and was looking for ways to keep the batteries charging when the panel voltage drops on overcast days. I really didn't want to series/parallel the panels to raise the voltage to run an MPPT controller, and thought "What if I raised the panel voltage by stepping it up when needed." I guess I now know why..... thanks everyone!
I have never used a MPPT, but, don't they do exactly what you want: buffer the solar panel output to maximise current to the battery? Even on cloudy days? From what you said, I infer that all your panels are in parallel for 12 Volts nominal, and you need a 12 Volt MPPT model. Sounds do-able.

I certainly understand why you want to keep batteries fully charged. I put a trickle charger on my car battery last week and it took three full days to de-sulfate back to a normal resting voltage. I got behind on that chore.
 

Thread Starter

mbohuntr

Joined Apr 6, 2009
430
From what I read, the MPPT charger needs 5 volts higher than the battery to operate, the advantage is they are very efficient, and work well in cloudy areas.
I would need to double the panels, or replace them with higher voltage ones to use that controller.
 

dendad

Joined Feb 20, 2016
2,990
With the correct panel you will charge the battery via a MPPT controler ok. When the volts on the panel drops because of low light, so does the available power. Adding a boost converter of some sort may get you some charge but it will be very limited and probably not worth the effort. My vote goes to more panels to start with. They are cheap nowadays.
 
Top