Simple question. What is the purpose of diode across 7805 - Electronic Circuits Projects Diagrams Free

William At MyBlueRoom · 9th April 2006, 03:50 PM

I've seen this in a few schematics, but not all. I do like to employ a diode inline with external power but I would like to know what the purpose of a diode across a regulator is.

justDIY · 9th April 2006, 04:11 PM

it is meant to protect the transistors inside the regulator when it is being used in a high capacitance circuit

for example, lets say you have a huge output capacitor for some reason, and then your Vsup disappears for some reason (crowbared) - now the output side if the reg is at a higher potential than the input side, so current will want to flow from the output cap, through the reg, reverse biased ... the datasheet explains how to calculate wether or not you need that diode, as the reg have some built in protection.

so, with the diode there, if the output somehow reaches a higher potential than the input, the diode becomes forward biased and conducts the excess voltage safely around the reg.

Russlk · 9th April 2006, 04:13 PM

if the input in shorted to ground, the charge on the output capacitor could damage the 7805. The diode discharges the output cap.

William At MyBlueRoom · 9th April 2006, 04:16 PM

Thanks, I'll have to consult the datasheets to see if I require it. Are only 78xx regulators affected?

justDIY · 9th April 2006, 04:21 PM

all linear regulators are affected, in varying degrees ... the datasheet is the best source for specific information

9th April 2006, 07:44 PM

but.... isnt that diode backwards???

Russlk · 9th April 2006, 07:53 PM

Quote:

Originally Posted by hyedenny

but.... isnt that diode backwards???

Of course! Read the discussion!

mramos1 · 9th April 2006, 07:58 PM

If you flip the diode, lots of power will go to the circuit around the regulator.

The way it is now, voltage on the output will run back to the input (with a diode drop) and keep the regulator happy.

You probably looked at it like I did the first time, I do everything left to right.
Power is on the right.

**Nigel Goodwin** · 9th April 2006, 08:13 PM

Quote:

Originally Posted by mramos1

You probably looked at it like I did the first time, I do everything left to right.
Power is on the right.

Yes, it's a confusing way to draw a circuit!.

9th April 2006, 08:22 PM

oh whoops - I DID look at it backwards.... and they dont even drive on the wrong side of the road in Canada!

**Nigel Goodwin** · 9th April 2006, 08:32 PM

Quote:

Originally Posted by hyedenny

and they dont even drive on the wrong side of the road in Canada!

NO?, I thought they did?.

mramos1 · 9th April 2006, 09:11 PM

Well we know he did not mirror it for a PCB my mistake because the text is still correct.

I stared at it. I went, OK power on the right. Do it that way. You can do it.
Only after I knew what the diode was for.

So he took the schematic from people that drive on the wrong side of the road... Or draws backwards.

Hey I learned something as I have never used a diode to feed the power back to the regulator (never had a big cap on that side I guess). I have used diodes to keep the user from attaching the power backwards many times.. So it all worked out.

So, what side do they drive on in Australia, just wondering? In Canada, it is on the "RIGHT" side..

justDIY · 9th April 2006, 09:30 PM

Quote:

Originally Posted by mramos1

The way it is now, voltage on the output will run back to the input (with a diode drop) and keep the regulator happy.

under normal circumstances, no current will be flowing through the diode at all ... since the input voltage will be of a higher potential than the output, the diode will be reverse biased.

if something happens to cause the input voltage to drop below the output will the diode will become forward biased, and then do its job, and provide an low resistance path for current to discharge from the circuit, rather than forcing its way backward through the regulator

I wonder if there is any benefit to using a high speed schottky barrier instead of a gp silicon in this application, or would that depend on the power levels involved?

**Nigel Goodwin** · 9th April 2006, 10:14 PM

Quote:

Originally Posted by justDIY

I wonder if there is any benefit to using a high speed schottky barrier instead of a gp silicon in this application, or would that depend on the power levels involved?

No, there's no benefit, fast recovery diodes are only really required for switchmode power supplies, or supplies fed from line output stages. If you replace them with normal rectifiers you soon find out why! 8)

DigiTan · 9th April 2006, 10:59 PM

I've got a question. Why does the diode go from to the output to input? Couldn't you just have the diode in series with the output to block reverse current and drop more voltage than the regulator in this scenario?

Also, if the diode in the diagram allowed too much leakage current, couldn't that cause a positive feedback loop and potentially make the regulator loose stability?

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9th April 2006, 04:11 PM	(permalink)
justDIY	it is meant to protect the transistors inside the regulator when it is being used in a high capacitance circuit for example, lets say you have a huge output capacitor for some reason, and then your Vsup disappears for some reason (crowbared) - now the output side if the reg is at a higher potential than the input side, so current will want to flow from the output cap, through the reg, reverse biased ... the datasheet explains how to calculate wether or not you need that diode, as the reg have some built in protection. so, with the diode there, if the output somehow reaches a higher potential than the input, the diode becomes forward biased and conducts the excess voltage safely around the reg.

9th April 2006, 04:13 PM	(permalink)
Russlk	if the input in shorted to ground, the charge on the output capacitor could damage the 7805. The diode discharges the output cap. __________________ see my website: www.geocities.com/russlk

9th April 2006, 04:16 PM	(permalink)
William At MyBlueRoom	Thanks, I'll have to consult the datasheets to see if I require it. Are only 78xx regulators affected? __________________ Bill Home of the Firefly PIC Tutor Inchworm ICD2 http://www.blueroomelectronics.com

9th April 2006, 04:21 PM	(permalink)
justDIY	all linear regulators are affected, in varying degrees ... the datasheet is the best source for specific information

9th April 2006, 07:44 PM	(permalink)
hyedenny	but.... isnt that diode backwards???

9th April 2006, 07:58 PM	(permalink)
mramos1	If you flip the diode, lots of power will go to the circuit around the regulator. The way it is now, voltage on the output will run back to the input (with a diode drop) and keep the regulator happy. You probably looked at it like I did the first time, I do everything left to right. Power is on the right.

9th April 2006, 08:22 PM	(permalink)
hyedenny	oh whoops - I DID look at it backwards.... and they dont even drive on the wrong side of the road in Canada!

9th April 2006, 09:11 PM	(permalink)
mramos1	Well we know he did not mirror it for a PCB my mistake because the text is still correct. I stared at it. I went, OK power on the right. Do it that way. You can do it. Only after I knew what the diode was for. So he took the schematic from people that drive on the wrong side of the road... Or draws backwards. Hey I learned something as I have never used a diode to feed the power back to the regulator (never had a big cap on that side I guess). I have used diodes to keep the user from attaching the power backwards many times.. So it all worked out. So, what side do they drive on in Australia, just wondering? In Canada, it is on the "RIGHT" side..

9th April 2006, 10:59 PM	(permalink)
DigiTan	I've got a question. Why does the diode go from to the output to input? Couldn't you just have the diode in series with the output to block reverse current and drop more voltage than the regulator in this scenario? Also, if the diode in the diagram allowed too much leakage current, couldn't that cause a positive feedback loop and potentially make the regulator loose stability? __________________ The most-hated young engineer in America. * AVR Projects * Robot War: TI * About