How to combat voltage drop due to resistance of PTC fuse  - Page 1
Forum section: electronics / beginners

author: MMMarco
author_rank: Regular Contributor
timestamp: June 12, 2023, 07:59:49 pm
content: I have a very simple circuit, basically two mosfets that switch a relay on/off:Relevant Parts:- F1 is a PTC fuse (Littlefuse 0805L035YR) rated at 350 mA.- Relays are IM01GR from TE-Connectivity.- Voltage Regulator is AP7361EA-33E-13 from Diodes Inc.- MOSFETs are SI2302CDS-T1-BE3 from Vishay.Now when the relays are switching I'm measuring a not-so-beautiful voltage dip on the 3.3V rail:(AC Coupled Signal)I tried to anticipate the current draw with the capacitors C3 and C4, but that doesn't work as one can see.What I tried:Changing C3, C4 to 10uF -> doesn't make a differenceAdding Capacitor after F1: 100nF, 10uF, 47uF (electrolytic), 1000uF (electrolytic) -> doesn't make a differenceChanging the fuse to a solder bridge removes the problem completely.So it seems that the resistance of the fuse is too high, causing a voltage dropWhat can I do (instead of replacing the fuse with something else) to stabilize the 3.3V rail?

author: floobydust
author_rank: Super Contributor
timestamp: June 12, 2023, 08:08:48 pm
content: Why are you putting a fuse there? I see none at the input for the AP7631 which is important for coverage there. It's an LDO so input and output current are the same, you could move the PTC to the input side?

author: MMMarco
author_rank: Regular Contributor
timestamp: June 12, 2023, 08:11:45 pm
content: Quote from: floobydust on June 12, 2023, 08:08:48 pmBecause I want two different supplies, protected by two different fuses (350mA for Relays and ESP, 50mA for MCU).

author: Manul
author_rank: Super Contributor
timestamp: June 12, 2023, 08:19:21 pm
content: If you want separate fuses, you could use one 350mA "master fuse" before linear regulator and 50mA for the MCU after the regulator. Plus if you care about voltage dip on the relays, why you add extra 10 Ohm resistors in series? These resistors might drop even more voltage than the PTC.

author: MMMarco
author_rank: Regular Contributor
timestamp: June 12, 2023, 08:21:45 pm
content: Quote from: Manul on June 12, 2023, 08:19:21 pmThat's an idea, but that just avoids the question at hand: how can I bypass the relay coils? There has to be a way I'm sure.The 10 Ohms resistors are there because the relay coil is rated at 3V, not 3.3V.

author: langwadt
author_rank: Super Contributor
timestamp: June 12, 2023, 08:31:38 pm
content: Quote from: MMMarco on June 12, 2023, 08:11:45 pmprotected against what?

author: Manul
author_rank: Super Contributor
timestamp: June 12, 2023, 08:32:28 pm
content: Quote from: MMMarco on June 12, 2023, 08:21:45 pmSo the relay uses 30mA and resistor drops exactly 0.3V? These resistor are unnecessary in my opinion, there is no problem in overdriving 3V relay with 3.3V (10%). Another idea would be simply to choose a mosfet with a few Ohms Rdson.Quote from: MMMarco on June 12, 2023, 08:21:45 pmAnd this part I don't understand. What excatly you mean by bypassing the relay coils? Furthermore I don't understand why you even care for the slight voltage dip on the relay rail? You don't have that dip on the MCU rail, right?

author: MMMarco
author_rank: Regular Contributor
timestamp: June 12, 2023, 08:41:09 pm
content: Quote from: Manul on June 12, 2023, 08:32:28 pmI mean ... I guess? I don't see how this contributes to the problem at all.Quote from: Manul on June 12, 2023, 08:32:28 pmWhat I mean is provide bypassing so that the voltage rail does not have to drop.I would need to remove the solder bridge to make sure, but I'm pretty sure it's not affecting the MCU rail because the dip is only at one end of the fuse F1.Nope, with and without fuse the MCU line gets affected too:The thing is, I have another device, an ESP8266 connected to the 3.3V rail that is "secured" by the 350mA fuse: so it's not only the relays that are connected to it.

author: Manul
author_rank: Super Contributor
timestamp: June 12, 2023, 09:23:01 pm
content: Does not look so bad. Can you scope the voltage at the input pin of the regulator (at C1)?By the way, have measured the current on that relay rail?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 12, 2023, 09:24:19 pm
content: Quote from: MMMarco on June 12, 2023, 08:11:45 pmWhy?What could your circuit usefully do if one of the supplies trips the fuse. A fault that would trip teh 350mA fuse would most likely make the regulator drop out , killing thr supply anyway

author: MMMarco
author_rank: Regular Contributor
timestamp: June 12, 2023, 09:33:10 pm
content: Quote from: mikeselectricstuff on June 12, 2023, 09:24:19 pmIt's just that I'm a goof and that's the top reason for having a fuseI'm sorry - I'm an idiot sometimes I KNOWIt seems that these fuses do more harm than good 😅 but I'm still interested from a learning stand point.I mean, I replaced the fuse with an 1 Ohm Resistor and got the same behaviour.It's not that Iabsolutelyneed the fuses (see reason above), I just want to take this as a valuable learning experience and I would like to know what I can do to prevent voltage drops.I just spent a good 20 minutes tracking a short circuit down, found it here (soldermask peeled off):Should've ordered 0 Ohm resistors insteadWasn't a good idea to repeatedly soldering/desoldering the fuse and replacing it with a solder jumper

author: MMMarco
author_rank: Regular Contributor
timestamp: June 12, 2023, 09:37:28 pm
content: Quote from: Manul on June 12, 2023, 09:23:01 pmI will have to measure tomorrow, I just trashed my current prototype board in order to find that stupid short circuit

author: Manul
author_rank: Super Contributor
timestamp: June 12, 2023, 09:58:11 pm
content: At some moment you will reach the limit of the LDO regulation capabilities. You can't expect to have rock solid voltage level while there are current transients. Big capacitors help, but if you start nitpicking, you will still see some glitches. If not 50mV, then 20mV, 10mV. You need to decide when it is good enough.Maybe relay current and MCU current paths go through same long trace before separating? If you want best separation, you should split these paths as close as possible to the LDO. Literally from it's output pin. You might consider separating the ground paths too, but that's an overkill in my opinion.Also ground bounce is of concern and might add to the effect. It is not clear if you are probing correctly and where you connect the ground lead. And if there is no ground loop. If you try to scope 10's of mV it becomes very important how you probe.There could also be something funny going on on the input of LDO, it is not clear how much drop is experienced there. If the input is dipping, it further affects the LDO regulation capabilities.

author: srb1954
author_rank: Super Contributor
timestamp: June 13, 2023, 07:12:24 am
content: Quote from: MMMarco on June 12, 2023, 08:41:09 pmA better design strategy would be to choose some higher voltage relays and run them off the unregulated supply into the LDO rather than from the regulated O/P. Relays are tolerant of reasonable variation of their operating voltage and generally function perfectly OK from an unregulated supply. As a bonus you will have less current drain from your overall circuit and less power dissipation in your LDO.You could then put your 350mA PTC on the input side of the LDO and protect it as well as the relays against faults. The variation in voltage drop across the PTC as the relays operate will be regulated out by the LDO so you will get a nice clean 3V3 supply for your MCU.

author: MMMarco
author_rank: Regular Contributor
timestamp: June 19, 2023, 09:50:51 pm
content: Thank you all for your help!I took your advice and re-arranged my circuit to have the following improvements:Use unregulated power supply for the Relays (I don't know what I was thinking, regulating the Relay PSU)Use two separate voltage regulators for the MCU and ESP8266 (Load)Use one PTC-Fuse at the input of the whole circuit, instead of the outputs of the LDOs.Also, I was able to track down another cause for the voltage drop: I stupidly put bypass caps AFTER the relay switch, not before.This means, whenever the relay closed, a great amount of current would flow to charge those capacitors.Putting them on the other side of the switch (i.e. pre-charging them) helped a lot!Old way (using big cap AFTER switch):New way (using big cap BEFORE switch):