Bad luck with PF hack...

[legotux]

Following a successful build powering a seventies 4,5 volt locomotive via a PF receiver and a 4.8 volt 4xAA battery pack, I wanted to make a second one. But I had no spare 4.5V motors left, I have 4x 12 volt spares though. And the 12 volt motors run much faster than the 4,5 volt motors, so it would be great to build a similar thing with a 12 volt engine. 

I ordered a bunch of DC-DC converters, 3 pcs that convert 5V to 12V, and a few others, adjustable.  I did some trials tonight. So a 4.8V battery pack, followed by the DC-DC converter, verified it's output is 12V, feeding that into a PF receiver, and its output to a 12V old seventies motor. The train ran very well in the test setup, both amazingly powerful and very fast. So I set out to build it together in a model. That's where trouble started. Upon connecting the battery with the DC-DC converter with connected PF receiver, smoke came out of the DC-DC converter. Afterwards, I connected the PF receiver to a lab supply, and it still worked. I reckoned I had a bad unit and took one of the other 3 identical units. That at first worked, but then I noticed that the same IC that had smoked on the first unit, got extremely hot here too. And the sad end result a few attempts later was that now, the PF receiver is dead. No light, no sign of life. No smoke either, but... And the lab power supply conforms it: it's dead.

I don't think I did anything wrong. The DC-DC converter can handle 8 watts, which ought to be largely sufficient: I never observed a 12V motor drawing over 400mA current, not even when almost stalled. And the DC-DC converters started getting hot/smoking right away, so under no-load conditions. On the other hand, two devices failing in a row in the same manner seems no coincidence. I have other DC-DC converters that can handle more, but that is moot right now, because my PF receiver died so I can't continue doing tests.

Did any of you have success with attempting something like this? I'll do a new attempt when I receive a new PF receiver, but any tips are of course welcome...

Legotux

Edited August 7, 2021 by legotux

[MAB]

Did you connect the +/- the right way around? As wrong polarity can cause capacitors to fail and converters to smoke. 

I also don't think PF receivers can handle 12v continuously or if they have voltage protection for the IR receiver. They are meant to be 9v and I don't know if they are protected against continuous higher voltages. Maybe use a relay for the output instead.

[JopieK]

Here is a datasheet for the driver: https://www.onsemi.com/pdf/datasheet/lb1836m-d.pdf

Supply voltage should be between -0.3 .. +10V.

If the cap is blow (electrolytic capacitor) you could try to resolder them, but there is a diode for reverse power protection so I don't think that was the problem. If the driver IC is fried you could also try to replace it of course but you never know if more items were damaged.

@Mortymore tested the unit and found:

- With the 2 motors running freely, no more than 150mA was pulled.

- With the M motor 8883 stalled, the current went up at around 600mA.

- With the XL motor 8882 stalled, was around 800mA.

- With the 2 motor stalled, about 950mA.

- The maximum input current measured was 1A, and for the output 980mA.

See also this topic: https://www.eurobricks.com/forum/index.php?/forums/topic/45289-max-voltage-for-power-functions

[legotux]

13 hours ago, MAB said:

Did you connect the +/- the right way around? As wrong polarity can cause capacitors to fail and converters to smoke. 

I also don't think PF receivers can handle 12v continuously or if they have voltage protection for the IR receiver. They are meant to be 9v and I don't know if they are protected against continuous higher voltages. Maybe use a relay for the output instead.

Yes, I double checked. And it worked for a while.   I think I read they can handle 12 volt fine somewhere*, but with these experiences I'll play safe and keep to 10 volt max.

* here:

 

legotux

12 hours ago, JopieK said:

Here is a datasheet for the driver: https://www.onsemi.com/pdf/datasheet/lb1836m-d.pdf

Supply voltage should be between -0.3 .. +10V.

If the cap is blow (electrolytic capacitor) you could try to resolder them, but there is a diode for reverse power protection so I don't think that was the problem. If the driver IC is fried you could also try to replace it of course but you never know if more items were damaged.

@Mortymore tested the unit and found:

- With the 2 motors running freely, no more than 150mA was pulled.

- With the M motor 8883 stalled, the current went up at around 600mA.

- With the XL motor 8882 stalled, was around 800mA.

- With the 2 motor stalled, about 950mA.

- The maximum input current measured was 1A, and for the output 980mA.

See also this topic: https://www.eurobricks.com/forum/index.php?/forums/topic/45289-max-voltage-for-power-functions

Thanks for linking that topic, that was useful!  The motor I used was older, the venerable x550a.

Edited August 8, 2021 by legotux

[legotux]

I acquired a fresh PF receiver and rebuilt the setup with an adjustable DC-DC converter, set to 10.5V. So far, and it's been used a few laps round the room, that works splendidly.

I wanted to create a strong loc combination, what I settled on are 3 x550a 12V motors, two inside two yellow locs from the 724 set, and the third in a MOC tender holding the PF receiver and 4 AA batteries.

That set runs splendidly: it successfully pulled 16+ mixed original and MOC wagons up a 5% slope with a curve (old blue track). But with added neodymium magnets.

I'm not satisfied with how the modern wheel sets run on the blue track, they have too much play and thus derail easily. I added washers on the axles that improved it, but not enough.

I await a shipment of various washers. When that is sorted I'll try make some pics and videos.

Legotux