Shotky Rectifier - what am I misunderstanding? - Page 1
Forum section: electronics / beginners

author: permal
author_rank: Regular Contributor
timestamp: October 15, 2017, 04:00:56 pm
content: Hi,So, I'm working on a circuit with a few outputs, as per the attached image. My intention is to be able to control a relay so I added a diod in reverse across the transistor to handle the back current from the coil when the relay is powered off.Not really knowing how high the voltage or current can get, I choose a diode that seemed overkill, aMBR0580-TP, capable of 80V, 5.5A peak.Now to the problem: Instead of getting no voltage measured between +5V and "DO1_GND_CTRL", I'm getting ~2.5V, despite the DO1 being grounded. Pulling DO1 high gives the expected 5V. So I desoldered the diode and am now getting the expected result, i.e. no voltage.Measuring the diode shows a 1.7M Ohm reverse resistance, which explains how I get a voltage measurement.From the above, my question is this: Have I chosen the wrong type of diode, and if so, what is the purpose of this kind and what should I choose instead?

author: IRFP460
author_rank: Contributor
timestamp: October 15, 2017, 04:03:39 pm
content: shotky diodes have a rather high reverse leakage current, that might have interfered with your measurement.Or the diode's just fucked.

author: GreggD
author_rank: Regular Contributor
timestamp: October 15, 2017, 04:16:12 pm
content: The diode should go across the relay.

author: permal
author_rank: Regular Contributor
timestamp: October 15, 2017, 04:54:25 pm
content: Quote from: GreggD on October 15, 2017, 04:16:12 pmHuh. Right you are. What was I thinking?

author: permal
author_rank: Regular Contributor
timestamp: October 15, 2017, 05:12:28 pm
content: Followup question: Does a diode in reverse across the transistor do any good (instead of across the relay which is the correct place as GreggD rightly pointed out)?

author: JohnPi
author_rank: Regular Contributor
timestamp: October 15, 2017, 05:31:33 pm
content: in the connection you have, the schottky will be damaged by the inductive kick from the relay. It will breakdown (like a zener) above 80 V. The schottky is not intended to be used this way and probably is damaged.Either use a zener (you don't need 80 V unless you specifically need to turn off the relay extremely quickly) -- 15-25 V is reasonable and will have a minor effect on the turnoff speed. Alternatively use a diode 'backwards' across the relay. Turnoff then will be a little slower (and quieter).

author: SeanB
author_rank: Super Contributor
timestamp: October 15, 2017, 05:33:45 pm
content: Yes, if it is a supressor diode 9 or zener diode) with a voltage higher than the maximum supply voltage and below the absolute maximum voltage the transistor can withstand. It then acts as a clamp diode, keeping the transistor from being damaged.

author: Gyro
author_rank: Super Contributor
timestamp: October 15, 2017, 05:34:00 pm
content: Quote from: permal on October 15, 2017, 05:12:28 pmIt will wouldprobablywork and would of course protect the C-E of the transistor from damage. The difference is in the size of the current loop you form - Instead of the diode directly clamping the back emf at the relay coil, putting the diode across the transistor shunts the current due to the back emf through the negative rail, through the supply, and back through the positive rail to the relay coil. A much bigger current loop.Depending on how good the supply rail decoupling is, it could cause anything from a mild circuit glitch to actual damage (unlikely apart from with sensitive electronics). The glitch could be enough to cause a microcontroller spurious reset for instance.

author: permal
author_rank: Regular Contributor
timestamp: October 15, 2017, 05:37:53 pm
content: Alright. Then I'll just not solder these diodes onto the PCB and will just make sure I put the diode across the external relay.

author: Ian.M
author_rank: Super Contributor
timestamp: October 15, 2017, 06:45:01 pm
content: OTOH the current is already flowing through the transistor at the moment of switchoff, so the current flowing in the much larger loop all the way back to the relay coil has to collapse if you put the diode across the relay. Moving that current over a centimetre or so to flow through a reverse biassed Zener across the transistor isn't going to cause a big problem as its a small change in the loop area. Fit Zeners that split the difference between your supply rail and the transistor Vceo rating, and can carry a pulse current equal to the relay coil current for long enough for it to decay to zero and call it good.

author: Gyro
author_rank: Super Contributor
timestamp: October 15, 2017, 06:55:21 pm
content: Actually, while we're on the subject, zeners also allow the relay to drop out quicker. Depending on the mechanics, this can improve the speed of contact separation.