RF stopping axial fan.

[davelectronic]

Just looking for some information, if anyone has any idea what is going on with a linear 12 Volt power supply. I'm using a high current linear power supply to run a 100 watt linear amplifier. But when I key the microphone it's stopping the two cooling fans, if I let the key go, the cooling fans come back to life. I've checked for voltage drop, there isn't any that would stop the cooling fans. If I try a different load (halogen lamps) The fans stay on and run fine. Do you think this could be an RF thing, where the linear amplifier is near the power supplys cooling fans. Is it possible for stray RF energy to interfere with the axial fans functioning ? I'm really at a loss to why the cooling fans stop on transmitting on a HF transceiver and linear amplifier. Thanks for reading, any thoughts appreciated.

[TimFox]

Assuming the fans run on 12V DC, they probably have a cheaply-designed transistor inverter inside the plastic case to drive an AC motor (rather than a brushed DC motor).  With no RF shielding from the amplifier and its tank circuit, it could easily be interfered with, since this is an untested susceptibility case.  Can you install a “cane metal” or other perforated metal shield between the RF stuff and the fans?

[Benta]

All 12 V fans I know of use brushless DC motors with integrated controller. Brushed DC motors I've only seen in equipment from the 60s and 70s. Even the early PC-XTs had brushless fans.

Yes, your RF can certainly disturb the BLDC controller (and a lot of other things as well).

[TimFox]

Also, in the vicinity of an RF power amplifier, you are almost certainly in the “near-field” region, where the energy stored in the EM field is large.

[davelectronic]

Thank you for your replies, I think it must be RF interference. I will try and put together some shielding, the fans where cheap ones, but they cool the PSU well enough. I've noted other weird stuff using linear amplifiers over the years, from wiping out televisions to affecting sound systems etc. But will try the shield idea.

[Bud]

Put an  EMI choke on fan  wires, if the wires long enough, make a couple turns through the choke.

[davelectronic]

Thanks for the tip, I will try that as well, but doubtfull that will work. As the RF interference is not a DC line type of interference. But I'm not 100% sure it wouldn't work.

[fourtytwo42]

Just a thought, if possible change to dumb induction motor AC fans.

[Bud]

Thanks for the tip, I will try that as well, but doubtfull that will work. As the RF interference is not a DC line type of interference. But I'm not 100% sure it wouldn't work.

Fan wires act as an antenna. The induced RF voltage can travel through it to both - to the fan  where it may affect the fan tachometer circuit, and into the power supply, where it may affect many things. The EMI choke has high impedance at RF frequencies and will reduce the magnitude of RF sneaking into the circuits connected to both ends of the fan wires.

[davelectronic]

Ok I didn't realise that could happen from RF energy. AC fan sounds like the best option to go for. Thanks for that, it's not something I'd thought of.

[coppercone2]

try a metal fan if you can find one, the problem is plastic, and also screen the control cable.

real equipment uses these. https://www.mcmaster.com/emi-rfi-shielding-fan-guards

in an rf environment you want a shielded box for your power supply. also keeps dusts and bugs out

[davelectronic]

Finding a metal blade fan would be quite difficult. It's to late to shield the power supply case now, but certainly worth a try in the future. Maybe just moving the linear amplifier might fix this problem ( picture included showing how close to PSU it is) I'm almost certain it's to close to the power supply. I will moving it first, then if that fails, try some other techniques like shielding.

[coppercone2]

be careful with this, I know it looks like wasted effort but the last thing you want to do is cause problems with a RF power supply while you are transmitting, it can probobly do all sorts of nasty and possibly non-fcc friendly stuff or otherwise impede communications. Getting a solid RF supply is important

those boxes look moddable for EMC protection with some sheet metal tools

The shields are not super tight, its not like a mosquito window screen usually, the ones I see are kinda loose, so if you have a slight protrusion from the chassis i.e. the back of a fan, they will cover it like a blanket. IDK about all the holes in the chassis,

what you can also try to do is increase the number of bolts/screws holding the top and bottom together, it makes an antenna on the gap near the screw hole.

https://interferencetechnology.com/doing-things-that-usually-do-not-work/

Maybe RLC filters on the power supply output too, if you can handle the heat

[antenna]

Jump to minute 18 on this video. 

[coppercone2]

you can put a emi screen over the whole top,  i wonder if that would work,  like a cooking collander

[davelectronic]

I'm just going to remove the linear amplifier from the vecinity of the power supply. I will add the a heavier gauge cable to supply power to the amplifier. Shielding would be a headache. I've seen audio devices and visual devices affected by RF energy, but never a fan. I have played about with CB radio for years, never seen a small DC fan stopped by stray energy from an amplifier.

[coppercone2]

just keep in mind you can make a square extension that goes over the whole thing and screws into the main chassis , out of bent punctured metal (the stuff with holes), to simply make the chassis taller and perforated

[davelectronic]

Yes I would consider a shielded enclosure if moving the linear amplifier some distance from the power supply fails to work.

[coppercone2]

it would be ok in a screened rack too probobly but it would need to be closed if transmitter is running

[T3sl4co1l]

Ground your feedline and balun your antenna.

Tim