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Zenor Diode application confirmation.

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tuurbo46
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Username: tuurbo46

Post Number: 54
Registered: 09-2006

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Posted on Wednesday, 28 October, 2015 - 11:50 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi All,

I am after some guru design advice.

Currently my circuit is battery operated, and battery Vin is 3.6V. I have now modified this circuit so the battery charger is on board. But after reading the datasheet again the charger has an over voltage protections at 6V. This is not possible on my circuit because one of my IC will blow at 4.5V (This IC can run upto 1A). So is what I would like confirmation on, can I clamp Vin with a 4.3V 5W 2% zenor diode (I=P/V therefore 5W/4.3V=1.16A). The charger operates/ charges at 4.2V but may have a clitch and shot up to 6V, and my zenor diode will catch this glitch.

Also am I correct in thinking under 4.3V the Zenor will consume little current and therefore not effect battery life?

Look forward to your reply.

Thanks,

Tuurbo46
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hackinblack
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Post Number: 655
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Posted on Wednesday, 28 October, 2015 - 01:30 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

if the voltage is below 4.3 volts the zener won't be conducting,so wont be drawing any current.
pushing the IC's supply to 4.3 volts may be a bit risky if the IC blows at 4.5!
it depends on whether it is its maximum rating or a 'tolerant to...voltage'getting close to either is bad practice;it can over-stress the chip.
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tuurbo46
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Post Number: 55
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Posted on Wednesday, 28 October, 2015 - 03:10 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi hackinblack,

I suppose the answer would be then to have two zenor diodes. One at Vin 4.3V, to prevent Vin going above 5V and taking out the micro, and a second one next to the IC at say 3.9V. This will then protect the micro and the IC. A bit of a messy way of doing it. Is there a better way of doing this?

Regards,

Tuurbo46
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ian_field
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Post Number: 309
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Posted on Thursday, 29 October, 2015 - 08:47 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Its hard to guess from your description - but if the zeners end up in parallel - the rail will simply clamp at the lower Vz. The way to do it; is have each zener protected rail split and fed by a current limiting resistor.

If the PSU can survive a short circuit - a crowbar SCR will protect the equipment much more certainly.

For oddball zener voltages, its hard to beat the TL431 programmable zener. It can handle up to 100mA and at the low voltages you mentioned, you may not have to worry about device dissipation.
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tuurbo46
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Post Number: 56
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Posted on Thursday, 29 October, 2015 - 10:09 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Ian field,

Thanks for your input. This is an interesting problem, so many of my IC's on the board have different voltage requirement.

OK, so keeping things simple and trying to explain in words, so you mean have two resistors in parallel, the top of the resistors connect to my battery Vin rail, and the bottom of the two resistor, the first will have the 3.9V zenor to ground, and the second the 4.3V zenor to ground. And the center of these two resistor zenor combinations will be my IC output supplies. So at none battery charge state my IC's will run at 3.6V, and when the battery is being charged, each IC will be clamped to there associated zenor voltage?

Sorry for the wording, it is quite hard to explain in words.

Look forward to your reply.

Cheers,

Tuurbo46
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chippie
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Posted on Friday, 30 October, 2015 - 09:16 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Simply put, the answer is yes......but...

Your statement ' have two resistors in parallel' is incorrect....


The resistor/zener diode combination is connected in parallel across the supply charging the battery.
The voltage for feeding the two ic supply rails is taken across the zener diodes.....

(Message edited by Chippie on 30 October, 2015)

(Message edited by Chippie on 30 October, 2015)
eccentric millionaire financed by 'er indoors
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ian_field
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Post Number: 310
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Posted on Friday, 30 October, 2015 - 05:46 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Back in the days of full height 5-1/4" hard drives, I often found protection diodes in the same package style as 1N540x, these were designed to sit on a supply rail and do nothing - until an over voltage event. Then they would fail short circuit and crowbar the supply.

These devices were invariably 5.2V and I don't know whether anywhere still stocks them - but its worth having a look, there may be parts in all the usual CPU rail voltages.
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tuurbo46
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Post Number: 57
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Posted on Sunday, 01 November, 2015 - 09:18 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi,

Thanks for your input. I am using the Microchip MCP73113. Below is my zenor maths, but I think my unit will consume/ waste to much current when charging - I am getting a bit lost now. I have cut the services down on my IC and now it only consumes 0.5A. At standard battery voltage 3.6V the unit is fine, and at charge it all goes wrong.

IC Vin range 3.4V - 4.5V and consumes 0.5A.

3.9V 3W zenor maths.

Izenor Max = P/V = 3W/3.9V = 0.77A

I consumed at charging MIN = V/R = (4.2V - 3.9V)/0.5R = 0.6A

I consumed at charging MAX = V/R = (6.5V - 3.9V)/0.5R = 5.2A

What do you reckon, does this look OK or do you also think it will consume to much current.

Look forward to your reply.

Regards,

Tuurbo46
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gordon
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Posted on Sunday, 01 November, 2015 - 07:37 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Tuurbo, it is a bit confusing as to the charging and battery supply you have. The ic you quote has input protection if the voltage supply to it goes above 6 volts. The output charging voltage should not go above 4.2 volts when the lithium ion cell is fully charged. If your circuit is connected to the lithium ion cell, then the voltage will never be higher than 4.2 volts. For a bit more safety you could connect a schottky diode to drop the
voltage by about another 0.3 volts from the lithium battery to your circuit. Extra safety could be provided by using a 5 volt USB supply to power the charger ic.

(Message edited by gordon on 01 November, 2015)
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ian_field
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Post Number: 313
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Posted on Sunday, 01 November, 2015 - 08:45 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Full charge terminal voltage is critical for safety on lithium cells - some older generation cells shouldn't exceed 4.1V, most current types its 4.2V, but I've seen one type that requires 4.3V.

My home brew e-cig chatger is based on the TL431 programmable shunt regulator, I put a Shottky diode in series with the cell so the TL431 doesn't dump the cell if it fails short circuit. Obviously the Vf has to be taken into account when calibrating.

The 100mA limit of the TL431 can bee boosted by adding a PNP emitter follower in the manner of a Szicklai pair.

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