Digital Pot setup

[gorgon]

Hi Justin
The zener in your case will drop the voltage supplied to the ts7815. As you state in your quote, the zener will start to conduct only when the zener voltage is reach. This means that the supply voltage on the anode side of the zener will be 5.6V less than on the cathode side of the zener diode. With a 30V supply the 7815 will not see more than 24.6V on the input. This is done to make the headroom up to the 35V limit greater and allow for some voltage spikes, if present, without destroying the 7815.

Regarding the breakdown of the zener diode. This is a result of the power disipated in the zener. As a product of the voltage over the zener and the current through it, you should select at least a 1W zener for your design. This should support a current of up to 130 - 150mA, if necessary, but personally I would sett the limit to around 100mA continuous.

In my proposal I said the next diode(your 1N4002), should be a second zener of 30V. This would have protected the 7815 further, against both overvoltage and reversed spikes. As it is now the 1N4002 only protects against reversed voltage spikes.

Hopefully that is good enough.

Good luck.

TOK

 

[JPU]

Hi Gorgon

Thank you for your reply. I was very worried after reading about the Zener. I have faith in you guys on this forum but another forum had made ae slightly concerned. If your interested it's the picaxe forum and I have the same name there, JPU.

There is a thread on capacitors I got involved in andentioned the fix. They were all surprised to see a Zener in series.

I do have one last question and this is simple I'm sure.

Why don't we need a resistor to limit the current through the Zener.

Thanks again to both you and Chris.

 

[gorgon]

..
Why don't we need a resistor to limit the current through the Zener.

The direct inrush current into the 4uF7 tantal capcitor will normally be the highest current at startup. As the capacitor is not a low ESR type and of low value, it should limit it's own current at startup. We're talking about some hundred us up to ms in duration. After that the current is limited to the normal working current of your circuit. Most zeners will not crash due to a short spike of current, if it is of a decent power rate.

But it is a valid question, and if you feel uncertain about it, a small resistor like 10ohm in series does no harm. This will reduce the peak current to max 2.5A for a 30V supply. However, it will also drop 1V at 100mA current, and should be rated at least 1/4W for normal use at 100mA continuous current.

TOK

 

[JPU]

Hi Gorgon

Thaks for that explanation. That makes sense to me now.

I'm in work and your anawer has put my mind at rest.

Cheers.

JPU

 

[CDRIVE]

In my proposal I said the next diode(your 1N4002), should be a second zener of 30V. This would have protected the 7815 further, against both overvoltage and reversed spikes. As it is now the 1N4002 only protects against reversed voltage spikes.

Hopefully that is good enough.

Good luck.

TOK

Yes Gorgon, good point. I replaced the Diode with a 30V Zener and added a 10 Ohm resistor for current limiting.

Justin, Zeners drop their rated voltage in their regulating direction and drop .7V in their reverse direction. So, as Gorgon said, this circuit should limit the + voltage to the regulator to 30V max and also clamp any negative spikes that might sneak in there.

Please keep in mind that we don't know what killed your chip. We're guessing.

Chris

 

[JPU]

Hi Chris

Thanks for the schematic. I have already sent the PCB off with the original revised design. It will be interesting to see what happens.

Never the less, this whole experience has meant that I have learned allot about electronics and thats great.

Thanks

Justin

 

[CDRIVE]

Sorry Justin, I've been and still am very distracted with projects of my own that are not electronics related. I've only been answering posts that don't require much brain strain on my part. Compounding this, your circuit may be fresh in your mind but I design quite a number of circuits here. Your's is just one of many.

I suggest rephrasing the next post to not exclude all the great minds we have here. By now you should be quite familiar with who they are.

Chris

 

[CDRIVE]

Justin, I just noticed that you deleted your last post, including the schematic. Now my last post makes no sense at all. This is not the first time you've deleted your posts. CC has commented about this practice (by others) with negativity. I tend to agree with him.

Chris

 

[JPU]

Chris

I deleted the post yesterday, as it was directed at you and as you hadn't responded to it or the PM I sent you I decided to remove it so as not to waste your time.


Justin

 

[CDRIVE]

As I said, my mind has been preoccupied.

Sorry for the delay,
Chris

 

[JPU]

Hey Chris, you help and others here help people out for nothing more than a thanks. You never need apologise to me as I am in your debt..

As I have said before if you lived closer I would definitely have taken you out for more than a pint or two by now.

Thanks

 

[CDRIVE]

Hey Chris, you help and others here help people out for nothing more than a thanks. You never need apologise to me as I am in your debt..

As I have said before if you lived closer I would definitely have taken you out for more than a pint or two by now.

Thanks

You wouldn't have to twist my arm.

Chris