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mkruger

protection diode on voltage regulators and relays
« on: August 12, 2004, 10:18:03 PM »
i don't know much about using a diode as protection when switching off and on relays and voltage regulators.

example: a voltage regulator that i'm using for 48V phantom power. do i just use a diode rated for a maximum voltage and amperage draw.

does the same apply to relays in power supplies? would i just use a diode rated for the output of that power supply?

the goal here is to protect a relay or VR from "popping" due to cap discharge. this seems to be a well known practice.

more specifically i want to use this voltage regulator for phantom power but im not sure what type and rated diode i need.

http://www.farnell.com/datasheets/13622.pdf


sismofyt

protection diode on voltage regulators and relays
« Reply #1 on: August 12, 2004, 10:21:39 PM »
Buy a hundred 1N4007 and use them for all those apps :wink:

mkruger

protection diode on voltage regulators and relays
« Reply #2 on: August 12, 2004, 10:26:45 PM »
simple enough :)

CJ

protection diode on voltage regulators and relays
« Reply #3 on: August 12, 2004, 10:45:11 PM »
For a relay snubber to protect your circuit from inductive kick spikes in the audio which happens when you disconnect power to a relay which has a pull in coil inside, the diode should have a reverse voltage that exceeds the voltage that the relay coils run on. This will usually be 6, 12, 24 or sometimes 48 volt relays. A 1N4148  is a common choice, (75 volt PIV-peak inverse voltage) as is the above mentioned 1N4007, which has a PIV of 1000 volts.
The 1N4148's are half the price of the 4007's.
   The diode is usually wired right to the relay. You must observe polarity. The cathode, or stripe on the diode, should go to the positive side of the pull in coil power pin. This is a reverse bias setup. You do not want to forward bias the diode, otherwise, it will conduct current and short out your pwr supply.
The spike induced in your supply line is opposite in polarity to your pwr supply, thus, the reverse bias. Some folks put a cap/resistor snubber in there as well.  

I am not sure about your reg. protection diode, so I will let someone else take that question.
cj
If I can't fix it, I can fix it so nobody else can!
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SonsOfThunder

protection diode on voltage regulators and relays
« Reply #4 on: August 13, 2004, 12:46:58 AM »
Lack of RP diode on a regulator can trash the regulator when power is turned off and capacitors discharge.

Go for twice the voltage of the supply and you'll be very safe.  Usually on 3 term regulators, a 1 Amp diode is recommended.  Check the app sheets at ti.com!

HTH!
Charlie
"The sow would rather have her ear than a purse." - PRR

Svart

protection diode on voltage regulators and relays
« Reply #5 on: August 13, 2004, 10:43:09 AM »
the snubber is usually put on the output side of the relay, it's used to lessen the current spike of the arcing contacts on make/break under highly inductive loads(powering a large coil like a transformer or motor), however is almost never used powering the coil of the relay since it's usually not a huge current draw..  

I assume you mean a zener to ground to short over voltages into the Vreg?  You need to know what the voltage going into the regulator is, and get a zener rated a volt or two more than the input to the Vreg, place this cathode(black stripe) to the input, anode to ground.  this will short anything over the zener voltage to ground saving your Vreg.  however in extreme overvoltage expect both the zener AND the Vreg to die.

 :thumb:
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occupation: General Electron Mayhem

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sismofyt

protection diode on voltage regulators and relays
« Reply #6 on: August 13, 2004, 10:56:13 AM »
I don't think he means a zener, it's the diode you need if the output cap is higher than 47yF so you don't kill the Vreg when you turn off your gear. Vregs don't like voltages on their output terminals :roll:

NewYorkDave

protection diode on voltage regulators and relays
« Reply #7 on: August 13, 2004, 11:00:06 AM »
Svart,

I'm not sure what you're thinking of, but a relay snubber is a reverse-biased diode across the coil. Its purpose is to shunt the flyback voltage when the current through the coil is cut off suddenly. Otherwise, the switching transistor (as an example) driving the coil would break down and smoke.

As for the zener acting as an overvoltage "crowbar" on a regulated supply line: that is done, but I think the original poster is asking about the reverse-biased diode you usually see across the regulator. This ensures that the voltage on the output side of the regulator does not exceed that on the input side (well, not by more than 0.6V, anyway  :wink: )

Svart

protection diode on voltage regulators and relays
« Reply #8 on: August 17, 2004, 07:37:33 AM »
sismofyt, ok i see what he was talking about now.

newyorkdave,  

The snubber is the resistor/cap across the output terminals of a high powered bridge(or relay) that shorts transients that would normally kill FETs and/or burn out the relay contacts with huge arcs.  I think you are thinking of the flyback diode across the coil, much like most power MOSFETs have from drain to source.

http://www.aromat.com/pcsd/ssrtec.pdf
a very informative document on the subject.

also, for  Vreg to work properly you need the input V to be at least 3 volts higher or else you will get weird effects especially at high draw from my experience.
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

 

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