Cable short circuit

xxxgaryxxx

Novice Member
A bit of a long shot.. The cable to the LNB on my flatsat easy auto skew motorised dish is short circuit measured with a volt meter with nothing connected either end.

I've run in a new cable in and still nothing. Will the short on the original cable have damaged the LNB and / or the comag box?
 
Damage to lnb highly unlikely but all other connected equipment highly likely
 

grahamlthompson

Distinguished Member
A bit of a long shot.. The cable to the LNB on my flatsat easy auto skew motorised dish is short circuit measured with a volt meter with nothing connected either end.

I've run in a new cable in and still nothing. Will the short on the original cable have damaged the LNB and / or the comag box?


Very unlikely. The problem is likely to be a whisker of screen touching the centre core in one of the end connectors.

Near impossible to have a short circuit mid cable unless it's mechanically damaged.

With what and how are you measuring the core to screen resistance ? If no connectors and you connect a 1.5V battery and a torch bulb is there is sufficient current returned to light the bulb ?

If you are using high impedance digital multimeter it will show a short circuit for a few seconds. It's just measuring the current charging the core to screen capacitance, of the cable.



No way it will damage the lnb. It;'s the receiver that delivers the power. The power output should be current limited so the voltage output is reduced to a level that the current being deilivered from the power supply can cope with and generate a lnb short circuit message. Powering off the receiver and fixing the problem should reset the current limit lock out.
 
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TJT1

Member
But OP has run a new cable. Unlikely, but always possible, that both are short circuit. But my guess would be (although OP hasn't said) that he has actually checked the new cable for absence of a short cct.
 

grahamlthompson

Distinguished Member
But OP has run a new cable. Unlikely, but always possible, that both are short circuit. But my guess would be (although OP hasn't said) that he has actually checked the new cable for absence of a short cct.


Which is why I asked how he was testing the cable for a short circuit. Also why I suggested that he uses a battery and a bulb. If there is a true short circuit the bulb will light. If he is using a high impedance electronic meter than the capacitance of the cable will produce a current until the cable capacitance is charged. The initial surge in current could be misinterpeted as a non existence short circuit.

My brother in law once asked me why he could get a voltage from the remote end of twin cable when only the live conductor was connected at the near end. He was using a digital multmeter whic was reading a voltage derived from the live to neutral capacitance between the live and neutral.

A AVO meter had no such reading due to the much higher current it used.
 

TJT1

Member
OK you win on a technical knockout until I get my DVM onto a bit of coax tomorrow. I have never seen an open cct. piece of coax read short cct. but there again, I don't generally look at the meter for a few micro seconds after connecting the meter.
 

grahamlthompson

Distinguished Member
OK you win on a technical knockout until I get my DVM onto a bit of coax tomorrow. I have never seen an open cct. piece of coax read short cct. but there again, I don't generally look at the meter for a few micro seconds after connecting the meter.


How long was it ? Try a 50m drum ? The core capacitive reactance in ohms of a short length of coax is minute.

50M is much larger.

How hard is it to use a 1.5V battery and a bulb. ? Do I have to post a diagram how to light a bulb with a short circuit somewhere on it ? The OP gives no indiication how he measured the short circuit . Nor the cable type and the aproximate length.

If the bulb lights it is short circuit if not it's not at DC voltages there is no short circuit.

The bulb test will work at any length. It measures the current from a 1.5V DC battery into a known resistance source. If the bulb lights there is a low DC resistance short circuit somewhere,

If not any current passing between core and screen has to be via the screen to core capacitance.

Check the relavent screen to core specification per metre.

I think you need to refresh your knowledge. Re Simple AC circuits and how frequency and capacitive reactance affects the attenuation.

WF100 has a core to screen capacitance at 55pF/M. So the 50M drum will have a capitance of 2750 pF.


So the 50m drum will have a DC resistance of just 26 ohms/1000M. Just 0.026 ohms.

Pretty close to a shorrt circuit a A DC source of 1,5V should still light the bulb.
 
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TJT1

Member
Round two.
How you dare be so bloody condescending in suggesting that I don't know how a battery and bulb work, yes I do know how a bulb and battery work. I also fully appreciate the effects of capacitive and inductive reactance have in circuit design. So I am not as idiotic as you seem to give me credit for. And the use a battery, bits of wire and a bulb is far more difficult that clipping a multimeter to the inner and outer. I also fail to see what captive and inductive reaction has to do with a short circuit cable.

You are now wandering off into the realms of fantasy now GLT. Your assumptions seem to be based solely on the fact that a DVM has very high internal impedance on the ohms range, thus, when multiplied by the capacitance of the cable produces a cable time constant (charging time) long enough to be noticeable.

That assumption is absolutely 100% wrong. Most consumer DVMs do NOT have a high impedance in the ohms range. FACT. You have to drive sufficient current through the 'load' to measure the voltage drop across the known internal resistance of the measuring cct. From this you can calculate the 'load' voltage and cct. current to calculate the 'load' resistance.

I have just measured both my DVMs and one has an internal resistance of about 1.9 kOhms and the other has 1.3k Ohms (I'll not bother to say how I did it as it's about "O" level electronics and Google knows(But you do need two DVMs))
With a mean Ri of about 1.5k (But lets round it up to 10k to make the maths easier. And that's about seven times higher in your favour). The time constant of your 50m bit of cable is 2750pF =2.75 nF * 10kOkms = 0.0000275 seconds That's a whole 27.5 microseconds. Hardly a long enough time interval to notice 'by eye'.

Why would anyone in their right mind want to screw around with a battery and bulb when they have a multimeter?

Your brother in law's experience is irrelevant here. He would have been using the DVM in Voltage mode (not Ohms) which WILL have a high input impedance as you say.
I rest my case
 

John

Moderator
Let's just turn it down a notch , thanks
 

pedro2000uk

Distinguished Member
A bit of a long shot.. The cable to the LNB on my flatsat easy auto skew motorised dish is short circuit measured with a volt meter with nothing connected either end.

I've run in a new cable in and still nothing. Will the short on the original cable have damaged the LNB and / or the comag box?

Is the flatsat motorised dish 'moving'?

What receiver / controller are you using with the flatsat.
 

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