Maker Pro
Maker Pro

coaxial and voltage/amps

Someguy

Nov 6, 2015
5
Joined
Nov 6, 2015
Messages
5
Hi,

I am new to electronics , there is something that ha been bothering me for a while (amongst many other things). How do tvs handle a high voltage coming in through a lightning bolt into the aerial? Or / and how would they react to say a few hundred volts getting into it? Obviously it will disrupt the signal but will it blow anything out? Is it different now with 'flat' tvs?


How much can tvs handle? Is this something manufacturers take into consideration, or is it something that is so unlikely it is not worth thinking about?
 

Someguy

Nov 6, 2015
5
Joined
Nov 6, 2015
Messages
5
..and the same for cable/satellite boxes If it had a few hundred volts put to it would it blow something?
 

davenn

Moderator
Sep 5, 2009
14,254
Joined
Sep 5, 2009
Messages
14,254
How do tvs handle a high voltage coming in through a lightning bolt into the aerial? Or / and how would they react to say a few hundred volts getting into it? Obviously it will disrupt the signal but will it blow anything out? Is it different now with 'flat' tvs?

They don't .... anything more than a couple of volts into the receiver section of the TV is likely to permanently damage it
anything more than 100V or so leaves visible damage. The induced 100's to 1000's of volts of an indirect to very close lightning strike totally blows them apart.
Those really high voltages arc all through the TV and other items and generally destroys them
Induced ot direct strikes via the antenna is not the only source. much severe damage is done to household electronics
and the buildings as well that are connected to the mains power wall outlets. Direct and induced strikes on power lines along the street can feed 10's of 1000's of volts into buildings where it vaporises power cables in the walls and causes building fires. That is over and above the high voltages that flow into your TV, DVD, computer fridge/freezer and anything else connected to the main power

here's an example of lightning damage to a FM transmitter installation .....
FM Transmitter after storm damage.jpg



Dave
 

Gryd3

Jun 25, 2014
4,098
Joined
Jun 25, 2014
Messages
4,098
Thanks a lot for the reply, Dave.
There are solutions, or at least partial solutions to help mitigate or prevent damage though.
Take a look at lightning suppressors. While these do not provide a guarantee to protect your equipment, they can certainly help.
Wikipedia said:
When the local electric field exceeds the dielectric strength of damp air (about 3 million volts per meter), electrical discharge results in a strike
The voltages you are dealing with are immense, as is the current.
Not only is there 10s of thousands of volts, but 10s of thousands of amps may be present which can vaporize and obliterate objects that are struck.
 

Someguy

Nov 6, 2015
5
Joined
Nov 6, 2015
Messages
5
Thank you.

So could just 9 to 50 volts really do any damage to a tv or cable box?
 

davenn

Moderator
Sep 5, 2009
14,254
Joined
Sep 5, 2009
Messages
14,254
Thank you.

So could just 9 to 50 volts really do any damage to a tv or cable box?

yup, very easily if coming in the antenna line. That input is designed for microVolts ... not Volts


Dave
 

Gryd3

Jun 25, 2014
4,098
Joined
Jun 25, 2014
Messages
4,098
Thank you.

So could just 9 to 50 volts really do any damage to a tv or cable box?
yup, very easily if coming in the antenna line. That input is designed for microVolts ... not Volts


Dave
This is the case with pretty much all electronics.
If you buy a 3V toy and put 9V into it, you can cause damage. It could be minor, perhaps a fuse, capacitor or resistor, or it could very well make it's way further into the device and damage key components resulting in a device that is more expensive to repair than to replace.

Antennas expect very tiny voltages, as do microphone inputs. Other audio inputs and video inputs expect 1-3Volts depending on the input type.
Exceeding the expected voltage in any case may need to damage, even if it's only by a couple volts.
 

Someguy

Nov 6, 2015
5
Joined
Nov 6, 2015
Messages
5
ok. I have to assert that I am talking about adding DC to the coaxial input to the tv. I was confused because although it i clear to me that the components expect microvolts not multiples of volts. I figured that because of the tuned circuit it might not 'let DC through' to do any damage. IDK, I have an old CRT tv that I haven't used for a while. I intend to put a 9v battery across the coaxial cable and see what happens. It's going to be powered on and tuned in to a channel, then I will connect the 9v battery.

So, tuned circuits will still 'fry' if the unexpected high voltage input is DC?

I'll post results if any when I've done it.
 

davenn

Moderator
Sep 5, 2009
14,254
Joined
Sep 5, 2009
Messages
14,254
ok. I have to assert that I am talking about adding DC to the coaxial input to the tv. I was confused because although it i clear to me that the components expect microvolts not multiples of volts. I figured that because of the tuned circuit it might not 'let DC through' to do any damage. IDK, I have an old CRT tv that I haven't used for a while. I intend to put a 9v battery across the coaxial cable and see what happens. It's going to be powered on and tuned in to a channel, then I will connect the 9v battery.

So, tuned circuits will still 'fry' if the unexpected high voltage input is DC?

NO DONT do that

Far out !! why does it always take so many posts for someone to get to the point ??

you use a bias-Tee circuit to separate the DC and the RF

untitled.GIF

OK that was a quick draw up for you
now depending on how the amplifier is arranged, depends on if you need the reverse at the amp end
or if for some amps the DC voltage can be fed into the amplifying device
again, you have told us nothing about that

cheers
Dave
 
Top