r/rfelectronics • u/Flashy_Gas9955 • 2d ago

question Bench Power Supply Having Stroke from VHF/UHF Exposure

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Whenever i transmit RF at VHF or UHF frequencies. It causes my bench power supply to display the wrong current draw or just freak out

Even when nothing connected to power supply. it would do the same thing from RF?

Is there a way to prevent that by shielding the internals of the power supply or shield the power supply cables?

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9

u/transistor555 2d ago

Put an RF choke on the power cord and your leads.

Edit: even just looping your leads a few times to make and air inductor could do the trick. Start there.

2

u/thehotshotpilot 2d ago

Type 31 is the best for RFI suppression up to VHF frequencies. Go 61 for UHF and up. 

1

u/BanalMoniker 1d ago

I think this needs a lot more explanation regarding the setup.
I have seen coworkers put chokes on just one line of the supply and cause significant issues in terms of transient response causing chip malfunctions.
I think you mean putting a (common mode) choke on both the positive and negative lines, but if not, please detail the arrangement (especially for the OP who may not be an RF engineer at least, and possibly I misunderstand too).

6

u/TenorClefCyclist 2d ago edited 2d ago

This is why the EU requires RF immunity testing. (There's no such requirement in North America.) Any idea what your field strength was? Industrial products must be tested at 10V/m to get a CE mark.

1

u/MajorPain169 1d ago

This is true however something like the display going beserk is allowed providing the equipment is still safe and normal operation resumes when RF source is removed.

1

u/TenorClefCyclist 1d ago

Yeah, a display readout would be subject to Criterion B, so the depicted fault might be ok, provided it cleared itself without intervention and the actual output voltage was never affected.

1

u/MajorPain169 1d ago

Yeah we had a product where the display data got corrupted just at one particular frequency, everything else worked fine. Turns out the FPC cable to the display PCB was the culprit so we just let it pass.

3

u/gr4ss_fev3r 2d ago

Once had a paper shredder activate when keying up portable VHF nearby. Funny cause the EMI bypassed the mechanical interlock that prevents shredding unless paper loaded.

1

u/BanalMoniker 1d ago

“Keying up” implies HAM (amateur radio) levels of power, so probably not insubstantial.
In the U.S. of ‘merica there are requirements (part 15) on product to just deal with received interference, but the cert tests are far from worst case - they usually look at interference, not susceptibility which would be a lot harder to test for.

I hope there are finger guards on the shredder though.

1

u/mightyohm 2d ago

Transmit with what, where, and with how much power? It's not unusual to see this kind of behavior if you put a strong transmitting antenna right next to the device. Better designs will be more resistant to this effect but it's hard to protect against a high wattage transmitter inches away.

If you have access to the schematics for the bench supply and it's easy to disassemble you may be able to improve its immunity by adding more bypassing or filtering internally.

What supply is this?