Diode Question - Page 1
Forum section: electronics / beginners

author: vismund624
author_rank: Newbie
timestamp: December 04, 2014, 05:09:50 am
content: Hello all,I feel like I must be doing something wrong. I have a circuit I am using that has an input and an output, to prevent people from accidentally supplying voltage from the output I put a diode in line. When using a thru hole diode this worked fine, but I switched to a SMD package as I am moving forward and it is passing voltage in both directions.I supply 14.4Vs as this is a typical real world value for my situation and I would like this diode to be able to handle loads of up to 3 amps (Which is a little bit of overkill.)http://www.mouser.com/ProductDetail/Panasonic/DB2440400L/?qs=sGAEpiMZZMtoHjESLttvkgPMEZYQGTCW58p%2fpfvUTc4%3dThat is the diode I am using. Note I have only checked this by putting the power supply and checking voltage with a multimeter, I have not put a load on this yet as i didn't think it would make much of a difference.I checked the board is not shorted out and the diode is in in the correct orientation, is this just the wrong type of diode for purpose?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 04, 2014, 05:16:23 am
content: A diode can certainly pass voltage in reverse, it just can't pass a significant amount of current. The data sheet for your diode says that it can pass 50uA at 40v reverse voltage. This is probably enough to drive a multimeter, which is how you're measuring it. Whether or not it will cause a problem with your circuit depends on your circuit.

author: c4757p
author_rank: Super Contributor
timestamp: December 04, 2014, 05:16:31 am
content: Diodes don't "block voltage", they present a high impedance in the reverse direction which makes it difficult for current to flow. It stillcanflow, just notwell. Except - your voltmeter barely requires any current at all, so even the tiny trickle that can pass through the diode is enough to be measured.Perhaps the through-hole diode had lower leakage. Some Schottky diodes are really leaky.Try adding a small load (a 10k resistor or so) permanently to the output and see what that does.

author: Falcon69
author_rank: Super Contributor
timestamp: December 04, 2014, 07:11:55 am
content: It sounds like what you are trying to do is prevent people from hooking up the supplies in reverse polarity. You need a Reverse Polarity protection circuit, comprised of a P-Channel Mosfet, Diode, and a Resistor.Check this video, It is a good source to start.

author: Seekonk
author_rank: Super Contributor
timestamp: December 04, 2014, 10:04:45 am
content: There is always a chance you did something wrong....overheated the diode soldering or left a whisker shorting out the pads. A simple resistance check is the first step. Most meters will show the normal forward voltage of 550mv in ohms.

author: vismund624
author_rank: Newbie
timestamp: December 04, 2014, 01:45:27 pm
content: Thanks everyone, this is what I was hoping. I will double check with a small load and if it works 50ua leakage is fine for me.Quote from: Falcon69 on December 04, 2014, 07:11:55 amThank you Falcon this is a great resource I will probably implement this on the input of my device but I am not protecting reverse polarity I am just trying to stop people from accidentally supplying voltage in 2 spots as one of my output's connectors is also commonly used as an input on other devices.