Looking For 12v Regulator

SpiderMan

I have a conflict in our RV between the solar charge controller and a furnace. The Propex furnace goes into an error mode if it senses input voltage above 15v. Unlike some furnaces, the thermostat on the Propex is not a simple mechanical switch which interrupts power, so high voltage is a problem even when the thermostat is in the OFF position. During warm weather (when I don't need the furnace) this isn't a problem since I've got the absorption voltage for our flooded batteries set at 14.6v. But our coach batteries are exposed to the weather with a battery temperature sensor. Thus, in cold weather the charge controller pushes voltage well above what the furnace will tolerate. My only solution so far has been to reduce the maximum voltage output of the solar controller (Blue Sky 2512iX w/ ProRemote). That keeps the both furnace and the wife happy, but leaves the batteries undercharged. 

Nothing else in the RV is affected by this problem - only the Propex furnace. I'm looking for a device that will limit voltage supplied to the furnace to about 12 or 13v. It doesn't need to boost low voltage, but simply to "clip" high voltage. Operating load is under 2 amps (the furnace is rated at 1.7 amp, but I've never seen it actually draw more than about 1.3 or so). The device needs to be able to limit the voltage even when no current is being drawn because the furnace will go into an error mode even if it senses high voltage when not running or turned off.

The furnace distributor (not manufacturer) has suggested the following gizmo, but unfortunately the sales page contains mostly hype with few details. Any thoughts? 

I've also heard about putting a diode in series with the load to reduce voltage, but it's not clear to me if a diode would limit the voltage when there's no current flowing. 

mcgivor

To get around this problem an Altronix SMP3, 5 or 10 can be used, there is a pot on the board which can reduce the load side voltage to near12V, the specs say AC input 16-28VAC but inputting DC is no problem, polarity is not an issue as the first components are a bridge rectifier, I know it works because I use one for the same purpose. There are other ways to skin the cat, this is just one device I had kicking around looking for a job to do.

mvas

SpiderMan said:

The furnace distributor (not manufacturer) has suggested the following gizmo, but unfortunately the sales page contains mostly hype with few details. Any thoughts? 

I've also heard about putting a diode in series with the load to reduce voltage, but it's not clear to me if a diode would limit the voltage when there's no current flowing. 

On the contrary, the detailed specifications (below) are not "Sales Hype" ...

• Input Voltage: DC 6V - 20 V
• Output Voltage: DC 12V
• Output Current: 12A (Max)
• Output Power: 144 W (Enough Power)
• Conversion Efficiency: Up to 93%

These are exactly the specifications you need for your "gizmo".
What else do you need to know?

And no a Diode is not a good solution.

BB.

Switch mode power supplies/converters are definitely not hype. Assuming that the unit is well designed and operated within it limits, they do pretty much what they say.

There is another unit (same picture, higher specs) that appears to have reasonable reviews--Although there were a few failures.

Just make sure you keep the wires short/heavy and connect them correctly (input connected with correct polarity/voltage, output not connected to battery instead, etc.).

And in this case, find out if the furnace support folks actually have used this unit, or just supplied a link to a product that could work.

-Bill

SpiderMan

Please forgive my skepticism. I'm not well-versed in electronic components and can't tell what it is. I wasn't able to find any information directly from the manufacturer, only sales sheets on Amazon for dozens of items spanning a wide variety of electronic applications, many of which appear to be poorly translated into English, most of which have few or no reviews. From my viewpoint, it's a $35 "black box".

Estragon

Our host sells dc/dc power supplies, and they can probably answer any questions you have (in English).

BB.

Yea--There is a huge issue with "cheap" import DC converters that may or may not work/be reliable. I have no idea if the linked unit is a reliable unit or not.

I looked for a good primer on understanding DC to DC converters, but I was not very successful. DC converters are a complex subject with many different implementations (circuit types) and also involved some pretty high level math (control theory). Here is a relatively complete PDF paper that may help (skip over the math and pay attention to current flow diagrams):

https://www.eecs.umich.edu/courses/eecs373/readings/dc-dc-primer.pdf

The issue is that there are "buck" or take high voltage and down converter to low voltage. "Boost" converters which take low voltage and make a higher voltage. And "Buck-Boost" converters that take a wide range of input voltages and make a stable output voltage (both buck and boost function in one regulator).

To a degree--Just like we "accept" that AC transformers work. You can treat DC to DC converters sort of like a "DC Transformer" or "Black Box". If your application is within the abilities of the converter, they work.

The basic idea is that you have a switch (transistor) that controls current flow to an inductor. Then usually through a blocking diode and filtered with a capacitor. The inductor/coil stores energy and allows energy to be "converted" from one (from high voltage/low current or low voltage to high current--more or less like an AC transformer) to another voltage.

With switch mode power supplies, they are more efficient than "linear" regulators (for down regulating--Think of opening/closing a water valve to regulate water pressure). And linear regulators cannot "boost" voltage. The inductor can increase voltage in a boost circuit (think of "charging an inductor" by getting current flowing, then you cut the current--And the you get high voltage across the inverter, or even a spark, as the inductor tries to keep the current flowing--That current flow is directed through a blocking diode to a higher voltage output+filter capacitor).

The key component is the inductor and its ability to store energy.

-Bill

mike_s

The big concern, since it's not specified, is whether it uses a common ground configuration. Some such devices are designed for isolated input/output. If the furnace is grounded to chassis, it may not work as expected.

SpiderMan

Thanks, that article helps greatly to de-mystify the black box and ease my mind about ordering one. 

Would a unit closer to my power needs be more efficient, especially related to standby losses with no load? Such as This One? It's also a tad less expensive and actually has reviews (all of which are positive). Due to the transient nature of furnace use, it's off far more than on. Or are these power losses so low that I shouldn't worry about them. Neither unit lists numbers for standby losses. The furnace itself is rated at 0.01 Amps on standby.

The furnace manual makes no mention of grounding its metal casing, which is mounted to the bottom of the wood floor of the RV. It's quite possible that there is a grounding path through the gas supply line or through inadvertent metal-metal connections on shielding I've installed around the furnace touching the vehicle chassis. 

BB.

I do not know any details about your furnace. Many RV furnaces take ~8 amps, mostly to run the fan. Also, depending on the ignition system (i.e., spark or glow rod) can affect the starting load too. If your specs/measured usage is around 1.7 amps, then any converter >~3 amps should be fine (I would go, at least, 2x the expected maximum current). And remember to fuse/breaker the input wiring too.

Yes, you are correct that you do not want too large of converter--They do have idling current and a smaller unit, should have less idle draw. However, without measuring, I could only guess (something like 1-2% of the rated current??? i.e., 12 amp unit could draw as much as 0.12 to 0.24 amps on standby--Not trivial). You will want a manual on/off switch for when the furnace is not needed to kill standby power.

Grounding the case is always a good idea (just use a green wire back to a good ground point). The case is probably isolated, but things can go wrong.

-Bill

SpiderMan

The Propex draws much less than typical RV furnaces because it's both smaller and more efficient at moving air (less noise, which is why we like it so much). 

I just measured and sure enough a significant portion of the current (about 2/3) is returning on the ground path rather than through the negative wire. I've ordered the voltage regulator and will find out by experiment if the ground path current is an issue or not. 

Thanks to all for your responses. I'll try to remember to post a follow-up once I've got the regulator installed. 

BB.

I will try and remember to suggest the Propex for our next RV poster(s). 8 amps for an RV furnace always seemed way too much power (to waste). Sorry to see the "stock unit" of the Propex needs a 12v to 12v converter to make it work in solar power systems--But, there are many devices out there that have difficulties with 14.5-15.0+ input voltages (12 VDC on deep cycle storage batteries should be good to 16+ volts during charging/equalization).

Many folks find out that high voltage difficulty when they go into cold climates (and Canada, etc.). 16-17 volts is within the "nominal" cold battery (near/below freezing) charging voltage range. You can find AC inverters that will run at this "elevated voltages" if you look around hard enough. And one reason I suggest running efficient 120 VAC appliances than trying to run everything off of 12 VDC (electronics, lighting, etc.).

The issue with ground paths through chassis/sheet metal is common for vehicles. Many devices won't care (just ground the device to a good ground/negative battery bus with tie to a good local chassis ground). Hopefully, you are connect that the DC converter will not care either (they can be designed either way).

Please let us know how it works out for you.

-Bill

SpiderMan

I installed the Drok power supply yesterday and it seems to be performing exactly as expected. Output voltage remained steady at 11.9v with and without furnace running, with solar charge controller both at float and through a 15.4v equalize cycle. Furnace happy = wife happy = life will be better this winter. 

Thanks again to all for your input.