are Triac's RF sensative ?

[Kam]

Hi Guys

A friend of mine has a 13.8v 60a power supply which has no RF immunity
and when you put a load on it that generates RF the voltage shoots up
to 20 odd volts, I will make a new thread about this when I get round
to looking at it.

So that I can test the above PSU I have made a crowbar circuit using
a 14v zener and BTA41 Triac, and was wondering if the Triac could be
effected RF, I don't want the fuse blowing unnecessarily.

I am waiting on some things from eBay to finish the crowbar box off
soon as I get it finished ill put some pics up of it.

RF proofing is something I really want to learn more about, I think you can use
ceramic caps for it but don't know how that actually works or where to put them
and what values etc.

any advice greatly received

 

[(*steve*)]

Much of what you say seems confused.

"Noise immunity" refers to how well a device continues to work despite external RF interference. What I suspect you mean is that it has no RFI suppression and generates RF at some point in its operation.

I would need also to know where the "RF Voltage" shoots up to 20V and how you measured it. Knowing the frequency would also be very helpful.

A crowbar circuit is very likely exactly NOT what you need as it will short something (your power supply?) when it is triggered.

In short, you need to start right back from the beginning so we can determine what the likely problem is, and what the best solution is.

 

[Kam]

I meant immunity

The power supply works correctly untill you put a hf radio
or amplifier on it and transmit then output voltage
of the power supply increases, 20 volts is a guess because that's
whats commoing out of the transformer.

The crowbar is so that i can connect my hf radio to the psu and do some
testing, when the output voltage isnt effected by RF and working correctly
then the crowbar wont be used.


My origonal question about Triacs was beacause I am useing one in my crowbar
circuit and wasnt sure if that could be effected by RF too and if it could how could
i protect it from it, like take the RF down to ground somewhere.


Anyways this is something i need to crack on with as need to get the crowbar built
and finished so ill build it up and answer my own question i guess when i test it.

 

[duke37]

If the power supply is upset by RF getting into it, the obvious way is to stop the RF being sprayed around. The transmitter must be screened properly and fed either into a remote antenna or a properly matched, screened dummy load.

Any electronic circuit will repond to an RF signal, it just depends on the strength of the signal, the frequency and the type of component. The triac will need a drive circuit and the size, orientation and location of this will determine the sensitivity.

 

[Kam]

hi Duke

it is a home made power supply (not by me)

the problem isnt the radio or antenna system as mine are %110
and the power supply has had the problem at my freinds location
and mine both on an antenna and bird dummyload (10w at 28mhz)
the power supply is over sensative to RF

 

[(*steve*)]

Of course another question might be, "is it the power supply or the test equipment measuring the voltage that is being affected?"

 

[Kam]

im using an oldf fluke meter which has good rf screening,
i know this because i bout a uni-t from maplin and tried to measure
my own power supply voltage on it and watched the uni-t dvm reading drop a few volts,
but the fluke is rock solid.

i admit im not up to you guys standards when it comes to electronics but im not an idiot either.

 

[duke37]

So it is a home made supply, is it in a grounded metal case and does it have RF bypass capacitors with short leads on both input and output?
Do you have bypass capacitors at input and output of any regulators?
Is the circuitry built with short connections?

Show us a the schematic and pictures of the build and the case.

 

[Kam]

It is in a metal case
there is no schematic for it.

I need to finish the crowbar first as i'm not going to risk
my radio testing it.

Cant tell you any more info about it at the moment
which is why I said I will make a new thread about it
and I will include lots of good pictures.

This thread is about the Triac/crowbar and I don't want to include
the power supply in this as the info about that might be useful to others
as there is little practical information about increasing RF immunity in power supply's
on the web, so a seperate thread for that would be more helpful.

 

[duke37]

You could put the crowbar in the power supply, this how it is normally done. This will have less RF pick-up.

You could work out the schematic, it is hardly likely to be complicated if home built.

A simple analog meter would be better in strong RF fields.

 

[Kam]

the power supply isnt mine, i'm just trying to make it useable for a freind
never tried working out a schematic from a board, ill try.

got loads of analog meters will use one when it comes to testing.


so heres my crowbar circuit, it works and trips out at around 14.4v
do i need to add any caps anywhere ?

 

[Kam]

D1 14v Zener
R1 1k 1/4w resistor
T1 BTA 41 Triac
S1 fuse up to 30A

 

[(*steve*)]

Keep a good supply of fuses on hand

Another option is to use a circuit like this to power a relay which disconnects the output. That way, you disconnect the load, but you don't blow the fuse.

 

[Kam]

hi Steve

Yeah my original design did indeed use a normally open relay
but I was worried about the mechanical switching time of it
so went for the Triac instead,

 

[(*steve*)]

Like I said, keep a good supply of fuses on hand.

You're talking over-voltage, not over-current, so I'd be less worried about the switching speed. And I'd use the normally closed contacts of the relay too.

The triac should pull in the relay, opening the contacts.

 

[Kam]

if a hf radio is drawing say 20a and the voltage shoots up to 20-25v
wouldnt there be an increased chance of damage due to the high amperage ?

got tons of fuses you get like 60 assorted ones for 3 quid on ebay

 

[(*steve*)]

Probably no more than if it was drawing 20mA and the voltage shoots up.

Remember that the Triac takes time to turn on, and the fuse takes time to blow.

As long as you *never* expect the crowbar to trigger, this is a fine circuit.

As soon as you are using the circuit for more routine protection (i.e. you are expecting it to trigger in normal use) then it becomes a poorer option.

Have a look here. This shuts off the output if the input voltage rises to high. A disadvantage is that there is a voltage drop across the pass transistor. An alternative would be to replace the pass transistor with a mosfet (with appropriate gate protection). Note that this turns off the output only during an overvoltage.

I'm not convinced you're looking at the right end of this problem.

 

[davenn]

Kam...

Steve's final comment is correct

I'm not convinced you're looking at the right end of this problem.

you are still going about this the wrong way...

Duke commented some posts ago about the PSU and its lack of RF immunity
THIS is where you MUST start ....
Putting a crowbar on the output of the PSU may sound a good idea but it just means that every time you transmit ... the PSU is going to cut in and out all the time and cause big power spikes into the radio as it switches on and off constantly
seriously that isn't a good plan just as likely to do the radio just as much harm as over voltage

You need to be looking at the PSU ... its layout of components wiring etc and sort out the best ways to give it some RF immunity.
First thing you need to do is confirm how the RF is getting into the PSU ...
1) is it a radiated signal or...
2) is it being fed back to the PSU via the 13.8V power cable between the PSU and the radio

Using a dummy load on the radio HF output will instantly confirm this

you may need clamp on ferrites on the 13.8V cable right at the output of the PSU and also on the mains input to the PSU.
you will need to consider what decoupling capacitors you need to add to the PSU circuitry to bypass the RF to ground

I assume this is a ham radio transceiver ... what make / model ??

there are lots of problems encountered by hams with RF getting back into PSU's
yes even commercial PSU ... that are just not well built ... they would be a great PSU for any other use other than a transceiver
more often than not its from radiated RF ....
1) bad feedlines
2) incorrectly wired up connectors
3) transceiver isn't earthed with its own earth wire from back of rig to its own earth rod outside the shack
4) poorly matched antennas
5) non-use of a balun at the feedline antenna feedpoint ... this results in the feedline becoming part of the antenna and radiating RF all over the place its entire length all the way down and right to where it plugs into the antenna tuner or the rig

Dave

 

[davenn]

You also need to consider the fact that the PSU may just be faulty and or it cannot handle a 20A load ....
now normally most PSU's will voltage drop when they cant supply the current
but depending on the way the PSU is regulated that the type of fault it may be doing the opposite

here's a curly example for you ....
my mate recently bought a nice PSU for his Kenwood TS2000 HF-VHF-UHF radio
now I have a PSU the same for my TS2000X and no probs with it.

he has discovered that when the mains power drops from the nominal 240VAC we have here to anything between 220 - 230 VAC odd the PSU actually increases the output voltage from the 13.8 and takes it up to ~ 20 -25V !!! Figure that one out!!

fortunately he and I had invested in a couple of nice overvoltage protection units from a place in the USA that protects the radio from such things
the last thing either of us wanted was our $3000 radios zapped

cheers
Dave

 

[Kam]

Thanks for the input Dave and yes I know It's the PSU I have to look at but the below line is why I am making the crowbar

Using a dummy load on the radio HF output will instantly confirm this

I am not risking my radio while TESTING the PSU prior and during repair.
Would you stick a few caps here and there then risk your hf radio to see if the over
voltage problem is cured ?

My radio Setup is long established and is not causing stray RF that is upsetting the PSU.
The PSU was originally brought from a radio rally's bring and buy sale which when the new owner (a friend of mine) plugged it in he said it blew his CB up when he keyed up, so he gave it to another friend of mine who is not in a good financial situation and he tried it with a linear amplifier and said the volts went up too, then i tried it here and i had the same problem with a cobra 148 when i transmitted the signal meter went bright white, luckily no damage was done, so the PSU has a history of over volting when a load that creates RF is used on it.

The supply has been designed to supply high current so were not asking too much from it