Current limiter

[mojoe]

I have one of those sealed 12 VDC to 24VDC boost modules that are in an aluminum, finned case. It is rated for 3 A output at 24 VDC. I am using it to charge one of those "solar generators" while traveling. The minimum input charge voltage is 14 VDC, which is borderline from a lighter socket, even with the engine running.

The problem is, the solar generator is drawing up to 112 W, which is too much, and the boost converter gets quite hot. Apparently, the boost converter does no current limiting, to the 3 A that it's rated for.

The AC charger that came with the solar generator is rated for 3 A, and that's what it puts out. So, it must have current limiting.

My question is, can I use a LM196 as a current limiter in this application? I would place it on the 12 VDC side, to limit the current going into the boost converter, and adjust as needed to get 3 A out, maximum.

Is there a better way to do this?

[TMM]

The problem with what you propose is that the LM196 (or any pre-regulator you use configured with a current limit) will reduce the 12V input voltage to your boost converter until the set current is achieved. What will probably happen is that the current is too high, so it begins reducing the 12V to the boost converter, the boost converter responds by increasing it's current draw which the pre-regulator responds to by even more rapidly reducing the voltage until the voltage becomes so low that the boost converter shuts down completely and stops drawing significant current. The pre-regulator responds by rapidly increasing the voltage back to 12V and the whole process repeats over and over - i.e. the system is oscillating. This will probably put undue stress on all the components (due to rapid charging/discharging of any storage capacitors and current pulsing above the set limit) and you might even get an erratic 24V output from the boost converter.

Putting the regulator on the 24V side might work better but it's hard to say given we don't know how your solar generator responds to changes in voltage. If it had to reduce to the 24V to say 15V to get the current under control, that's also a lot of power dissipated in the LM196.

If the main concern is that the boost converter gets too hot, a cheapskate solution to your problem is to get a simple thermo switch and mount it to the heatsink. When it gets too hot, the thermal switch cuts the 12V input until it cools down (seconds, minutes) after which it cycles back on. Since the cycling is very slow, it shouldn't cause any problems. As to if your boost converter is going to burn up if it is run at say 150% of it's rating intermittently, who knows.

Best solution is to just buy a boost converter that has current limiting built in, or is rated for 150W+ to suit your load

[mojoe]

If only I could know before buying, which boost converters had current limiting. This is my second one. The first one was rated for 10 A output. The cigarette lighter plug got hot enough to burn my hand. That's why I bought a smaller boost converter.

I guess I'll have to only use the solar panel for charging when on the go. This isn't a problem if I'm going to be camping, and it is sunny. There is a separate input for the solar panel, which does MPPT. I wanted to also be able to charge from the vehicle, if needed.

The main purpose of the solar generator is to run a 12 V fridge overnight. The fridge can be powered from the vehicle when the engine is running. I didn't realize when I bought it, that the solar generator needed a higher voltage for DC charging. The AC charger works fine.

[David Hess]

Nothing prevents having a converter with current limiting on the output *and* input.  The problem is getting past the marketing to find one.

Update, just to be clear, what is required is a converter which monitors its input current and controls its output current.  This is trivial to do but it cannot be done by adding a current limiter to the input.

[Terry Bites]

https://www.aliexpress.com/item/1005001733018098.html?spm=a2g0o.productlist.0.0.4ce01eaazD6lLR&algo_pvid=20f55358-60db-43c9-8072-e09bd9df675b&algo_exp_id=20f55358-60db-43c9-8072-e09bd9df675b-17&pdp_ext_f=%7B%22sku_id%22%3A%2212000017381889115%22%7D&pdp_pi=-1%3B1.44%3B-1%3B-1%40salePrice%3BGBP%3Bsearch-mainSearch

Worth a punt. Has volt set and Imax pots.