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Non-isolated LED Driver

11/30/2015 4:56 AM

hi everyone, i am from led lighting industry, i found one problem that with non-isolated led driver i found some leakage current in housing of led luminaier (feel current by touching) at which led is mounted, so for safety how we can protect housing from this unwanted current and most of the time my non-isolated led driver AC input side fusible resistance got burned suddenly when i switched it ON, first i am using 4.7E 1/2 watt wirewound fusible resistance and after some complain of same i change it to 10E, but still this problem happen with this change too..

thanks to all in advance your suggestion and guide will appreciate...

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#1

Re: non-isolated led driver

11/30/2015 5:27 AM

Maybe it's time to contemplate the advantages of an isolated LED driver?

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#2

Re: non-isolated led driver

11/30/2015 5:51 AM

Sounds like you should hire a licensed electrical engineer.

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#3
In reply to #2

Re: non-isolated led driver

11/30/2015 5:56 AM

Do you really want to go to India?

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#4

Re: non-isolated led driver

11/30/2015 6:19 AM

Darwinian evolution will sort this one out.

https://en.wikipedia.org/wiki/Darwin_Awards

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#5

Re: Non-isolated LED Driver

11/30/2015 7:42 AM

" (feel current by touching) " - really? Yikes!

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#6

Re: Non-isolated LED Driver

11/30/2015 8:44 AM

That is a permanent flaw of non isolated power supplies - the output is live to ground at both polarities because if you ground either the + or - output, current will flow through the external circuit = you, and if you are standing in water = more will flow.

Such items can not be sold in most countries due to the risk they represent to the public.

This is usually solved with a high speed switching power supply with a true transformer to isolate the out put, versus the lower flyback type - which is not isolated.

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#7

Re: Non-isolated LED Driver

11/30/2015 12:20 PM

Most likely there is some connection from an element of your circuit to the housing. You must locate the connection and remove it.

Another possibility is capacitive coupling. A high voltage node in the circuit may be too close to the housing and the field causing a potential on it. You must ensure that your circuitry is properly isolated from the housing.

Lastly, your housing must be properly grounded. I'm sure that you would rather have the wayward current passing through the housing than your body. This is also the means to trip the circuit breaker if the current is too high.

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#8

Re: Non-isolated LED Driver

11/30/2015 5:14 PM

I expect that the leakage is coming through your heat sinks.

Try electrical isolation of the rectifier diodes (and possibly the LEDs) from the heat sink, or the heat sink from the housing (if they are not one and the same). This will obviously affect the thermal transmission from the diodes to the sink, so adjustments in diode capability may be necessary.

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#9

Re: Non-isolated LED Driver

12/01/2015 9:12 AM

To avoid or minimize shock hazard, you may consider changing the housing or encapsulating your product using plastic... A total plastic enclosure may suffice for your cheap product! It is known as double insulation.. Good luck!

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#10

Re: Non-isolated LED Driver

12/01/2015 11:49 AM

This is a well known "normal" AC mains problem and is due to the design of the unit, which is why many companies have a mains "wall wart" plug and only feed low voltage DC to the unit.

The good ones are switched mode and very economical.

If you want to retain the current design, you need to add a ground wire and a 3 pin plug, with the unit frame always earthed/grounded.

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#11

Re: Non-isolated LED Driver

12/04/2015 3:28 AM

Thank goodness for CE_marking!

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#12
In reply to #11

Re: Non-isolated LED Driver

12/04/2015 4:03 AM

Even that is no guarantee as I have had tiny shocks from several metal framed (German made) Sat receivers over the years, just when I remove or replace the (grounded shield) aerial cable, without either pulling the mains plug, or operating the rear ON/OFF switch. Just with it "switched off" from the remote, which as you know, is not really FULLY off.

I can tell that its only a very low voltage AC, nothing to worry about, as I have had extensive (accidental!) experience with both AC and DC shocks over about 67 years....but I'm still here!!!

It is not the DC voltage used to switch and power aerial bits and pieces...as that was REALLY off, always checked as it could damage earlier style receivers, or blow the fuse!

Most painful shock was in Germany, because someone had painted over the earth pins on a Euro socket in our new kitchen, which I had not noticed as I was installing a Washing up machine, on a warm day in June 83.

My sweaty wet (short) hair touched the machine frame (enamelled) as I had my hands on the hot water pipe/Ground.....

I saw stars.....

Checking of the insulation with a megger showed it to be well above 1 Meg Ohm limit!!

After the pins were cleaned, no other problems.....

Maybe the OP is feeling something similar?

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#13

Re: Non-isolated LED Driver

12/07/2015 2:15 PM

Perhaps you went the wrong way with your resistor choice. It really depends on how you are deriving your DC power. Without some sort of magnetic isolation a full wave bridge will experience a short circuit through at least one diode if you tie AC and DC grounds together.

In another situation, involving a fluorescent lamp ballast, I came across a specification about shock hazard when the output was hot and I touched a wire and ground at the same time. Yet, when I reduced the resistance (by licking my finger and then touching the wire) I found that lower resistance actually loaded the output enough to remove the shock hazard.

The spec. said that the output with respect to ground could not exceed 50 volts. With a high internal and external resistance the circuit was likely to exceed 50 volts. But with a low external resistance, the output voltage was forced down below the threshold for a shock. You should have seen the expression on the face of my boss when I proved this. He was sure I would be electrocuted.

No, he was not able to understand why it was safer to lick my finger first. Granted, I allowed a small amount of current through my body, but I could not feel it without the presence of a voltage delivered from a small internal resistance source. The large internal resistance made this possible.

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#14
In reply to #13

Re: Non-isolated LED Driver

12/07/2015 4:19 PM

You have, I believe, understood the problem the wrong way around.

The internal resistance of the human body is VERY low. Only skin has a bit of resistance, more, the dryer it is.

When a small current is passed through the skin and it is resistive, you feel it.

By licking the skin, you probably reduced the skin resistance to almost nothing, exactly the same as the internal body resistance.

Think about two resistors in series, one say 1000 Ohms, and one 100 Ohms. Lets put 11 volts across the two. The 1000 Ohm resistor will have 10 volts across it, the other will have 1 volt across it.

Dropping the 1000 Ohm to 100 (licking it!), so its now 100 + 100, means that each resistor will have 5.5 volts dropped across it!!

Do you now understand what actually happens.

By the way, if the skin has 1000 Ohms resistance, the internal resistance may only be one or two Ohms in reality....so the difference will be far greater....

I hope this helps.

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#15
In reply to #14

Re: Non-isolated LED Driver

12/07/2015 4:58 PM

No, he is describing the situation where there is a high resistance connection to ground, via capacitance - which drops as frequency rises. When this high resistance short meets his high resistance connection to earth, 95% of the drop occurs across his skin. When he wets his finger, he then forces 95 ++% of the drop to be across the equipment and the voltage across his skin can not be felt

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#16
In reply to #15

Re: Non-isolated LED Driver

12/08/2015 3:28 AM

Then he described it badly.....

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#17
In reply to #16

Re: Non-isolated LED Driver

12/15/2015 12:13 PM

Oh, excuse me your honor......Who died and promoted you to Judge?

You know what you can do with your opinions? Yeah, I think you do.

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#18
In reply to #17

Re: Non-isolated LED Driver

12/15/2015 3:04 PM

HaHa!!

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#19
In reply to #17

Re: Non-isolated LED Driver

12/27/2015 6:19 AM

You now know how to maximize your OT Score, so you (may) have learnt at least one thing this week!!!

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