# Limit current used of large resistive load, PWM???

I have a 240v 2000w (underfloor) heating element. I would like to limit the amount of current that the device uses, based on the output of my solar PV panels. I'd like to use between 250w and 2000w depending on sun, so approx 10-100% duty.

I'm assuming I'd need this to be at least 50hz so my electricity meter sees a constant load, does anyone know???

I was thinking I could simply use a solid state relay, is this possible??

Any ideas or thoughts would be great.

Thanks

Jim

You won’t do it with a simple solid state relay as you will need point on wave switching to vary the average current .
I’m not sure how a solar panel would react to this , but I’m assuming you would still be connected to the mains .

It would be simpler to use your soldid state relay to say switch heat on for a minute at full rate then off for 10, to give 10% power - the meter will average it out for you .
The higher duty cycle on your heating elements may shorten their lives , and your room stay cold.

Hammy, thanks for taking the time to reply, to explain a little more in detail. My solar panels are connected to the house with a grid tie inverter, they output upto 2.5kw of energy, but often less than this. If my power consumption is 1kw, and the pannels are generating 2kw, then 1kw is pushed back into the grid. I'd like to use this "spare" energy to heat my floor for free.

If you want it to be 250W seen per cycle of the wave you need to dim it aka phase control it.

BUT, if you're goal is to just use less energy you can control is slower. 5 seconds on, 35 seconds off is 250W on average as well Upside of this, it doesn't mess with the power factor. Also, a simple (solid state) relay can be used.

And heaters are slowwwwwww so you will not notice the difference (depending on the thermal mass and cycle time). But think about it, that's exactly the way a normal thermostat controls the temperature in your house.

As you're trying to regulate AC, you need to use phase cutting instead of PWM. That's working at 100 Hz (cutting each half wave - that's why the 100 Hz). You're using a TRIAC to regulate the current.

Now there are several concerns.
First of all the regulation itself. Straightforward but not a beginner project due to the high voltage and current involved. Your TRIAC will get hot.

The frequency of just 100 Hz is quite low, much lower than MPPT type trackers for PV panels typically use. That may be a problem. There's also the question of how your inverter reacts to such powerful current spikes (you're chopping almost 10A of current).

Then, why doing it in the first place? Power pushed back into the grid should make your meter run backwards, so it's not free energy.

Sadly my my meter doesn't run backwards, when I'm pushing power back in the grid the meter just stops

Mm, so an electrical dumb meter? Pretty said to see in the 21th century...

But yeah, you would need phase dimming to trick the meter. Downside, terrible power factor. But upside of that, you're probably not billed for the reactive power ut yeah, controlling high mains loads like that isn't particular easy / safe...

Other option with perfect power factor would be a variac or controllable (or switchable) auto-transformer... But yeah, to control those from software is not easy either...

You can't split the heater in parts?

Neither phase cutting nor PWM nor packet switching effectively limit current through a resistive load, they only limit power consumption over time. But if the load is rated for the full voltage, it also is rated for the maximum current, so the current is nothing to worry about.

V=I*R

R is fixed, trying to reduce I, then reduce V.

Since this works on AC, how about a multi output transformer connected between the inverter and the floor warmer? Transformers work on Power in = Power out. Thus selecting a lower voltage output = less current out = less Power out = less Power in = less I from the solar panel.

Eventually simpler: split the heater, and combine both parts into R, 2R or R/2. That's at least cheaper than a transformer.

Yeah, it isn't really about limiting the current but power if you talk about efficiently using PV power. The (auto)transformer/variac option is the only solution with the same power factor. But phase dimming should be able to do the trick as well.