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Pinguino 26J50 kit
24-02-2013, 02:39 PM,
#1
Pinguino 26J50 kit
Hi,
Firstly this is meant to be constructive criticism so don't take it personally!

I'm not sure if I have an incorrect zener diode but the '3.3V' on my board measures 4.02 on my DVM. I'm reasonably confident of the DVM as USB 5V reads OK.

From personal experience I've found low voltage zeners do not have a particularly sharp 'knee' so can vary in voltage more from sample to sample than you'd expect.

And being even more nit picking, R2 (10R) is going to run a little warm as it's dropping nominally 1.7V it will dissipate a bit under 0.29W.

I think I'll replace that arrangement with a little fixed-voltage regulator.

It might be worth thinking about if you do a new PCB layout.

Best Regards
Keith
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25-02-2013, 09:55 AM,
#2
RE: Pinguino 26J50 kit
Thank you for this feedback.
Régis

(24-02-2013, 02:39 PM)keithfuller Wrote: Hi,
Firstly this is meant to be constructive criticism so don't take it personally!

I'm not sure if I have an incorrect zener diode but the '3.3V' on my board measures 4.02 on my DVM. I'm reasonably confident of the DVM as USB 5V reads OK.

From personal experience I've found low voltage zeners do not have a particularly sharp 'knee' so can vary in voltage more from sample to sample than you'd expect.

And being even more nit picking, R2 (10R) is going to run a little warm as it's dropping nominally 1.7V it will dissipate a bit under 0.29W.

I think I'll replace that arrangement with a little fixed-voltage regulator.

It might be worth thinking about if you do a new PCB layout.

Best Regards
Keith
It is easier to complain than it is to do, but it is better to do than it is to complain.
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07-04-2013, 06:46 AM,
#3
RE: Pinguino 26J50 kit
(24-02-2013, 02:39 PM)keithfuller Wrote: I'm not sure if I have an incorrect zener diode but the '3.3V' on my board measures 4.02 on my DVM. I'm reasonably confident of the DVM as USB 5V reads OK.

Out of curiosity I checked mine.

Reading from the three onboard power pins: USB (nominally 5V) reads 4.94, but 3.3V reads 4.14.
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07-04-2013, 12:56 PM,
#4
RE: Pinguino 26J50 kit
Hi Pinguino,
I decided to replace zener diode D2 and resistor R2 with a 3.3V fixed-voltage regulator. The one I selected was MCP1700-3302E/TO which satisfied my requirements on several levels; it had 'legs' so I could see to solder it, it was easily available from CPC in UK and was cheap at £0.32 each.

I also added a 4u7 6.3V capacitor from 3.3V to ground as some regulators are prone to oscillate without one.

I (think) I've attaced a picture of how it looks.

If you need any more information please let me know.

Best Regards
Keith
   
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07-04-2013, 02:09 PM,
#5
RE: Pinguino 26J50 kit
Very neat! Mine's not nearly so neat... see attached.

The only smallish 3V3 regulator I had on hand was an AMS1117 surface mount to which I attached flying leads with Kynar wire, removed R2 and D2, and connected the regulator's flying leads to the 3V3, GND and 5V header pins.

Thanks for the heads up in the first place.


Attached Files Thumbnail(s)
   
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07-04-2013, 04:32 PM,
#6
RE: Pinguino 26J50 kit
4.14 sounds liable to cause damage!

John
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07-04-2013, 08:09 PM,
#7
RE: Pinguino 26J50 kit
It seems some models have a bad zener Sad.
Replacing the 10 Ohm resistor with a 150 Ohm is a solution.
If you need one tell me and I will send it to you.


(07-04-2013, 06:46 AM)Pinguino Wrote:
(24-02-2013, 02:39 PM)keithfuller Wrote: I'm not sure if I have an incorrect zener diode but the '3.3V' on my board measures 4.02 on my DVM. I'm reasonably confident of the DVM as USB 5V reads OK.

Out of curiosity I checked mine.

Reading from the three onboard power pins: USB (nominally 5V) reads 4.94, but 3.3V reads 4.14.
It is easier to complain than it is to do, but it is better to do than it is to complain.
Reply
08-04-2013, 12:56 AM,
#8
RE: Pinguino 26J50 kit
(07-04-2013, 08:09 PM)regis Wrote: It seems some models have a bad zener Sad.

I checked each of the 18 3V3 zener diodes I had on hand in the circuit and found not one that was close to 3V3 - the values ranged from 4.00 to 4.19V.

Quote:Replacing the 10 Ohm resistor with a 150 Ohm is a solution.
If you need one tell me and I will send it to you.

Thanks for the offer, but I have used a voltage regulator as noted above.
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20-07-2013, 01:40 AM, (This post was last modified: 20-07-2013, 02:06 AM by Pinguino.)
#9
RE: Pinguino 26J50 kit
I built another board last night (thanks to Regis for the usual fast delivery), this time using a 150 ohm resistor.

On this board the 3V3 measures 3.53V when the USB voltage is 5V and 3.64V when the USB voltage is 5.16V. So we're still exceeding the absolute maximum voltage for the 18F26J50 of 3.6V depending on the USB voltage.

The two USB voltages are from two different computers and within the USB 1/2 spec of between 4.75V and 5.25V and USB 3 spec of between 4.45V and 5.25V.
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23-07-2013, 02:22 AM,
#10
RE: Pinguino 26J50 kit
The resistance solution was to increase the 150 ohm resistor to 150K. To arrive at this, I tried 330R, 470R, 750R, 1K, 2K, 12K, 18K, 47K, 82K, 100K and finally 150K.

150K produces these results:

o USB input voltage is 5V, 3V3 is 3.30V
o USB input voltage is 5.17V, 3V3 is 3.43V
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