Dometic SMP207-01 24V PSU for Caravan floorheating - high output voltage.

[rune72]

Hi.
A friend of mine sent me these images of a 24V PSU for floorheating in his caravan.
I can recognize it as a switchmode PSU. But the plate on the unit does not make sense..  It it stating that the output is 24Vrms AC and also that it is dual voltage in - there is no input terminals for 24V AC.
The lead is the mains input. P3 is the output.

The problem is that it is measuring with no load 34.5V DC and 84V AC on the output, with load - it is measuring 35,5V DC and 77,5V AC. It is making a high pitch humming noice at load.
The load is measured to 400 ohms and it is a floorheating mat.  24V into 400ohms would give a current of 16.6 A, so this unit is clearly not up to the full task but is factory installed.
A new unit from the caravanshop is around 400 USD (4000 Norwegian Kroner).

I do not have the unit at hand at the moment, I might get it later today.
Do any members have any tips in what I should check.

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Thanks in advance.

[coromonadalix]

Stupid sticker to say the least,  well  its supposed to be 24vac    it should oscillate around 100khz,   maybe  you could check the small 90 degree board chip and write us the part number

You measure the output with what meter ??  some meters cant meausre properly with high frequencies ...  you should not get dc at the output.

[rune72]

Yeah right.....

I would still bet on that the output is supposed to be 24V DC..  because if this was an AC to AC converter - a 230V to 24V transformer is all that is needs..

[coromonadalix]

Edited my earlier response  ... please check

Seems a pretty cheap psu,  found nothing more at dometic

Or maybe something is wrong with the heated floor mat and the psu goes into current protection ?   or overload ?

[rune72]

I will get my hands on the PSU later today, and do some more tests with my 121GW..
Will post my findings.

[HackedFridgeMagnet]

400 ohms at room temp does not mean 400 ohms when heating. Does it say how many watts on the psu or the heater?

It may indeed be a 24 Ac output, just a really crappy one.

[rune72]

The PSU is rated at 350W.. (14A @ 24V).

[madires]

It's a simple and crude SMPSU. Not even active PFC. Seems to be a classic design with a PWM controller and voltage feedback by a TL431 plus optocoupler. Since there's a much too high output voltage under load my guess would be a broken feedback loop. Also check the electrolytic caps.

[rune72]

New update:
The PSU is now on my bench.
I did a few quick checks before dinner...

It does only output 38V DC on the P3 connector. - there is very little AC that my 121GW can measure. - I have not put the oscilloscope on yet.
I did have a a 20V battery powered LED load to load the PSU - and the voltage droppes to about 33V. - still no AC present, but blue transformer starts to make a hissing noice.

The chip on the riserboard if an HEF4069UBT (Hex inverter). - .
The Optocoupler is marked L1215 817 C , googling it, it seams to be an EL817C.

[james_s]

AC output switchmode power supplies are very common, notably for driving low voltage halogen lamps. You said that PSU is rated for 350W, do you realize how large and heavy a 50/60Hz transformer would be to deliver 350W? The SMPS is compact and cheap. If it says the output is AC then the output is AC.

[coromonadalix]

HEF4069UB is an   Hex unbuffered inverter, oops   could be a crude pwm oscillator ??


c19 should be checked, in some smps  the capacitor goes bad and the smps cant start,  maybe some cold solders on the main transistors / mosfets,  check the big coil and the big switching transformer too

[TheMG]

For such a high power output, that power supply has a very small amount of capacitance in both the primary and secondary sides.

It is one of those power supplies which essentially "rides the sinewave", in other words when looked at on a scope you would see the output (under full load) resembles a rectified sine wave, and not pure DC. That is why the nameplate says the output is ac and gives the voltage in Vrms.

This is very common for low-cost power supplies that are intended to run purely resistive loads such as incandescent lamps and heaters, as it keeps the component cost down and also is a cheap way to improve power factor of the unit.

Since such power supplies are quite crude, the only real way to measure the true output is under the full rated load, using a true-RMS multimeter. Even then, the regulation is not going to be great.

Some hissing/buzzing is normal from such crude power supplies.

If the heating mat is working properly and not overheating, then I would say the power supply is working as intended. If noise is an issue, will likely have to find a better quality 24VDC power supply.

[james_s]

If I were in that position, I'd pick up a couple of surplus hot swap server PSUs and mod them for series operation. It's what I've used to power the larger chargers I use for my RC airplanes. I have some IBM branded 650W units that are compact and very quiet, I've run a series pair for 24V for years and the setup has been bulletproof.

[fzabkar]

The load is measured to 400 ohms and it is a floorheating mat.  24V into 400ohms would give a current of 16.6 A, ...

P = V x V / R = 24 x 24 / 400 = 1.44W

The floor element is not measuring correctly.

[TheMG]

The load is measured to 400 ohms and it is a floorheating mat.  24V into 400ohms would give a current of 16.6 A, ...

P = V x V / R = 24 x 24 / 400 = 1.44W

The floor element is not measuring correctly.

Good catch. 24V into a 400 ohm load is only 0.06 amps, which is hardly anything. That would also explain why under "load" the voltage of the power supply doesn't drop very much, as that is very little load.

That would suggest a fault with the heating element itself not the power supply, or perhaps if there is a thermostat, thermal fuse, or other control device in series with the element that might be faulty.

[james_s]

Wow it didn't even occur to me to check the math. Looking at that now how does one even calculate 16 Amps starting with 24V and 400 ohms? Even if you mix the numbers up it doesn't come out to 16.6.

[fzabkar]

Wow it didn't even occur to me to check the math. Looking at that now how does one even calculate 16 Amps starting with 24V and 400 ohms? Even if you mix the numbers up it doesn't come out to 16.6.

400 / 24 = 16.6

[james_s]

Ah, yeah that makes sense, R/V instead of V/R. Brain fart on my part too apparently, it's been a long week.

[coromonadalix]

you can snatch some server psu on fleabay and others at some ridiculous prices sometimes,  but  the problem is controlling the heated floormat  ??

[Manul]

So I don't understand, was floor heating with this PSU or not? Transformer noise may (but probably not) reduce if switching frequency is tuned up or down a little for some sweet spot.

[fzabkar]

A 240VAC 2200W electric kettle should provide a 26 ohm load at 24VAC, or 22W.

Or you could have two 12V 50W/100W automotive headlamps in series, assuming the cold current doesn't trip the PSU.

[rune72]

I want to thank you all for the feedback  .
We have found a used one from the same brand and are going to trying it when it arrives.

I am still looking into this SMPSU when I have the time, but now the 6A input fuse is blown, and it's a 5x20mm sand filled with legs, and I don't have any with legs in my parts collection.. so now I need to bodge up some kind of input protection before I can start looking at it again.

It was important that the PSU is silent, as this is mounted in a cavity in the caravan and as it's running all the time when connected to mains power, there is no thermostat for regulating the temperature, and hissing or fan noice would make him crazy...

[james_s]

If the input fuse is now blown then you've got a bigger project on your hands than when you started out. Fuses very rarely blow on their own.