5V Regulator and Pickit2 Question - Electronic Circuits Projects Diagrams Free

gramo · 6th January 2008, 12:36 AM

Just a quick question on the 7805 5V Regulator

Would anyone know if raising the output pin to 5V would be to much if there is 0V at the Input pin. The reason why I'm curious is because I am powering a circuit with my Pickit2, which also has a 7805 circuit. eg;

Now the problem lies in page 11 of the 7805 datasheet found here in the "RAISING THE OUTPUT VOLTAGE ABOVE THE INPUT
VOLTAGE" section.

It describes the damage would be similar to having a large capacitance on the Output of the Reg also found on that page (page 11, 2nd paragraph). Towards the end of that paragraph it implies that;

Quote:

Generally no protection diode is required for values of output capacitance ≤ 10μF.

Which kinda leads to suggest that small reverse leakage currents are "ok"

When I'm prototyping with software, I simply leave the Pickit2 connected and the whole circuit is powered from the Pickit2. This is fine for upto 100mA (25mA recommended), but will my 7805 bite the dust with no power connected to its Input pin because of the reverse leakage current?

Leftyretro · 6th January 2008, 12:48 AM

Quote:

Originally Posted by gramo

Just a quick question on the 7805 5V Regulator

Would anyone know if raising the output pin to 5V would be to much if there is 0V at the Input pin. The reason why I'm curious is because I am powering a circuit with my Pickit2, which also has a 7805 circuit. eg;

Now the problem lies in page 11 of the 7805 datasheet found here in the "RAISING THE OUTPUT VOLTAGE ABOVE THE INPUT
VOLTAGE" section.

It describes the damage would be similar to having a large capacitance on the Output of the Reg also found on that page (page 11, 2nd paragraph). Towards the end of that paragraph it implies that;

Which kinda leads to suggest that small reverse leakage currents are "ok"

When I'm prototyping with software, I simply leave the Pickit2 connected and the whole circuit is powered from the Pickit2. This is fine for upto 100mA (25mA recommended), but will my 7805 bite the dust with no power connected to its Input pin because of the reverse leakage current?

I thought it was a no no to do something like that, but I'm not sure and I'm sure others here can state for sure.

However a simple solution would be to use a DPST switch that removes both the input and output voltages from the regulator circuit.

Lefty

audioguru · 6th January 2008, 01:16 AM

Why do you have two 5V power supplies? use only one.

gramo · 6th January 2008, 01:30 AM

More of a convenience thing really. The board is a small protoboard that I made up for developing. When I'm prototyping and simming code, its left connected to the PicKit2 on the PC bench, and when I'm working on the hardware side of things it is on my work bench

Its handy having the PICKit2 and 5V reg, as when its on the work bench, I just plug it into the mains via a DC socket...

Well thats 'why' I have two supplies, I haven't blown a reg yet, but surely this is not good. Using Schottky diodes is out of the question, as even their low forward voltage drop will play havoc with ADC references

Perhaps a mod is in order and I'll put that switch in

blueroomelectronics · 6th January 2008, 02:07 AM

Actually pins 2 & 3. The PICkit2 & Junebug both have a blocking diode. Your target can be powered and they'll keep their supplies off. They do need USB power but that's a given.

Ahh yes almost forgot, put a 1N4001 (in reverse) diode across the 7805 and it'll be fine.

**Nigel Goodwin** · 6th January 2008, 12:23 PM

Quote:

Originally Posted by blueroomelectronics

Ahh yes almost forgot, put a 1N4001 (in reverse) diode across the 7805 and it'll be fine.

Doesn't the 7805 already have an internal diode like that?.

Hero999 · 6th January 2008, 02:20 PM

No, look at the schematic on the datasheet.

There is normally no need to have both a blocking diode and a diode in reverse parallel with the though.

blueroomelectronics · 6th January 2008, 05:27 PM

It doesn't hurt. I recall the LM2940 doesn't need it but the old LM7805 might. I've used both diodes in the Inchworm design and have had no problems, it was there so the Inchworm could be target powered just like gramo wants to do.

Quote:

3..If your regulator is furnishing power to a capacitive load, and the primary power is removed -- like unplugging a PC card, or disconnecting an experimental setup -- the charge in that capacitive load will cause the secondary or output of the regulator to be more positive than the primary or input. If this reverse voltage exceeds the regulator's ratings it will blow up. To prevent this sort of failure, a diode is placed between the input and output, such that, when reverse voltages are present, the diode conducts preventing damage.

audioguru · 6th January 2008, 06:19 PM

Voltage regulators that are made for cars have protection built-in from reverse polarity.
I guess mechanics frequently connect the battery backwards.

**Nigel Goodwin** · 6th January 2008, 06:29 PM

Quote:

Originally Posted by audioguru

I guess mechanics frequently connect the battery backwards.

You wouldn't believe how stupid mechanics can be! - mind you, most of them seem to be young kids!.

Hero999 · 6th January 2008, 09:00 PM

Quote:

Originally Posted by blueroomelectronics

Quote:

3..If your regulator is furnishing power to a capacitive load, and the primary power is removed -- like unplugging a PC card, or disconnecting an experimental setup -- the charge in that capacitive load will cause the secondary or output of the regulator to be more positive than the primary or input. If this reverse voltage exceeds the regulator's ratings it will blow up. To prevent this sort of failure, a diode is placed between the input and output, such that, when reverse voltages are present, the diode conducts preventing damage.

That's not true if there's a reverse polarity protection diode and a capacitor before the regulator. At the you disconnect the power, the voltage on the input capacitor which will gradually drop to the same voltage as the output. The LM7805 will not be harmed by the tiny capacitor and reverse bias leakage current.

blueroomelectronics · 6th January 2008, 09:08 PM

Take a look at page 8 on TI's datasheet.
http://www.sparkfun.com/datasheets/C...nts/LM7805.pdf

But you're right, it's only if the output is >7V so the 7805 wouldn't apply.

audioguru · 6th January 2008, 09:17 PM

The diode from the output to the input on a regulator is to prevent a large output capacitor from discharging with a high current into the regulator when the input is shorted to ground.
Inputs are rarely shorted to ground.

blueroomelectronics · 6th January 2008, 09:25 PM

Well in a commercial product I'd add the diode, you have to plan for worst case and it's cheaper to add the diode then be servicing the stuff.
For the hobby bench I'd just leave it out.

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6th January 2008, 01:16 AM	(permalink)
audioguru	Why do you have two 5V power supplies? use only one. __________________ Uncle $crooge

6th January 2008, 01:30 AM	(permalink)
gramo	More of a convenience thing really. The board is a small protoboard that I made up for developing. When I'm prototyping and simming code, its left connected to the PicKit2 on the PC bench, and when I'm working on the hardware side of things it is on my work bench Its handy having the PICKit2 and 5V reg, as when its on the work bench, I just plug it into the mains via a DC socket... Well thats 'why' I have two supplies, I haven't blown a reg yet, but surely this is not good. Using Schottky diodes is out of the question, as even their low forward voltage drop will play havoc with ADC references Perhaps a mod is in order and I'll put that switch in __________________ Spency. PIC Micro's - Your mind is the limit PIC's and interfacing with other devices - a PIC Basic Guide @ digital-diy.net

6th January 2008, 02:07 AM	(permalink)
blueroomelectronics	Actually pins 2 & 3. The PICkit2 & Junebug both have a blocking diode. Your target can be powered and they'll keep their supplies off. They do need USB power but that's a given. Ahh yes almost forgot, put a 1N4001 (in reverse) diode across the 7805 and it'll be fine. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com Last edited by blueroomelectronics; 6th January 2008 at 02:10 AM.

6th January 2008, 02:20 PM	(permalink)
Hero999	No, look at the schematic on the datasheet. There is normally no need to have both a blocking diode and a diode in reverse parallel with the though. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez

6th January 2008, 06:19 PM	(permalink)
audioguru	Voltage regulators that are made for cars have protection built-in from reverse polarity. I guess mechanics frequently connect the battery backwards. __________________ Uncle $crooge

6th January 2008, 09:08 PM	(permalink)
blueroomelectronics	Take a look at page 8 on TI's datasheet. http://www.sparkfun.com/datasheets/C...nts/LM7805.pdf But you're right, it's only if the output is >7V so the 7805 wouldn't apply. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

6th January 2008, 09:17 PM	(permalink)
audioguru	The diode from the output to the input on a regulator is to prevent a large output capacitor from discharging with a high current into the regulator when the input is shorted to ground. Inputs are rarely shorted to ground. __________________ Uncle $crooge

6th January 2008, 09:25 PM	(permalink)
blueroomelectronics	Well in a commercial product I'd add the diode, you have to plan for worst case and it's cheaper to add the diode then be servicing the stuff. For the hobby bench I'd just leave it out. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com