PCB works! .... Diode question ?

[RickAlty]

Thanks for the help on my PCB layout a few days ago. I finished making it last night, and it works flawlessly. I was pleasantly surprised to find that two of the three PIC-08's that I have, and that reported 'hardware not found' on my solderless breadboard lash-up, are in fact good!

However, I have a question. I don't have, and couldn't find at my local Radio Shack or Fry's, a BAT65 diode. That space on the board is therefore empty. Doesn't seem to be affecting the operation any. What is the function of that diode in the 'enhanced' circuit? If I can't find one, would a 5V Zenner diode do the same thing?

 

[hippy]

The BAT65 is a low-voltage dropping, fast acting, diode ( about 0.3V ) which is used to prevent the Serial In pin ever getting below -0.3V which can disrupt the ADC readings.

In most circumstances it isn't necessary, but if you are using analogue inputs it will remove some slight discrepencies there may be in readings which occur when the cable is plugged in and when it isn't.

The BAT65 is a negative voltage clamp, and a zener replacement would make for a positive voltage limiter doing a different job.

 

[RickAlty]

Thanks, Hippy. If I ever see one, I'll stick it in there.

 

[RickAlty]

I went to Fry's again at lunchtime, still no joy on the BAT65 (they sell NTE parts there, and the equivalent is listed as an NTE 585)

They did have an NTE 586 though - the only difference seems to be the maximum voltage. While the BAT65 / NTE585 lists 750mA max, the 586 lists 3A.

Both diodes though list the forward voltage at 0.525V at peak though, so would this bigger diode have the same function? I figure it's never going to get anything like 3A applied to it anyway, so it shouldn't make a difference - or am I misunderstanding again?

Richard

 

[RickAlty]

Ummmm, that would be "Maximum Current"...... The Max voltage is listed as 40V for both.

 

[hippy]

I'm afraid I really don't have that much of a clue when it comes to the details of such things

 

[BeanieBots]

I'm not familiar with that specific diode but 0.525v volt-drop at 3A does make it sound like a schottky diode which is the type you need.
Like Hippy said, the purpose is to prevent serin from going negative. A regular diode will drop about 0.6v which is too much. It MUST be a low drop type such as a schottky diode. There is virtually no current involved so the current rating does not matter.

I get around the problem by fitting a regular 1N4148 in SERIES with the output from my PC. The diode is fitted inside the download cable and thus works for all the Rev-Ed project boards without the need for any mods to the boards. The only issue with my method is that it can cause problems when using a Laptop that only puts out 5v serial. A desktop that uses 12v will not notice a loss of 0.6v.

 

[eclectic]

Rick.

1. Have a look at Technical’s reply in this thread.

<A href='http://www.rev-ed.co.uk/picaxe/forum/Topic.asp?topic_id=1652&amp;forum_id=12&amp;Topic_Title=BAT85%2Bschottky%2Bdiode&amp;forum_title=No+new+posts+please%21+6
' Target=_Blank>External Web Link</a>

and

2. Hippy&#8217;s reply and website reference, in this thread.

<A href='http://www.rev-ed.co.uk/picaxe/forum/Topic.asp?topic_id=2305&amp;forum_id=15&amp;Topic_Title=program%2Bstops%2Bworking%2Bwhen%2Bserial%2Bcable%2Bis%2Bunplugg&amp;forum_title=No+new+posts+please%21+9' Target=_Blank>External Web Link</a>

e.


Edited by - eclectic on 04/08/2006 08:16:29

 

[Dippy]

How about digikey?
They're full of BATs.
Do Farnell or RS have a direct outlet in US?
Newark are a bit limited.

 

[RickAlty]

Thanks for the help. Looks like the one I have will be fine, so I'll whip it in there. The board is really only for programming anyway, I'll make a board for the actual application.

Richard

 

[RickAlty]

People a couple of doors down from my office make smart pigs, and make their own electronics in-house. I asked one their design engineer and he said no problem.