CD4541B timer delay off - schematics review - Page 1
Forum section: electronics / beginners

author: Cvijo
author_rank: Contributor
timestamp: July 04, 2025, 11:19:24 am
content: Hi to everyone.I am still a beginner in this area, so if anyone could have a look at my circuit diagram and point out any problems or mistakes. Searching the internet, I have found numerous variations and different schematics, and I would appreciate any help.PCB is designed to delay the off timer depending on the dip switch resistors (1, 2, 3, 5, 10 minutes).This is for my boat. When someone turns on the shower pump, it will automatically turn off after a set period of time. The pump is on the high-side switch, so I am using a P-channel MOSFET with an N-channel to pull the gate to the ground when the timer is ON.CD4541 datasheet -https://www.ti.com/cn/lit/gpn/cd4541bThank you in advance.

author: ledtester
author_rank: Super Contributor
timestamp: July 04, 2025, 12:40:44 pm
content: It seems that you intend for only one DIP switch to be on at a time.Another way to do this is to use the DIP switches to put various capacitor values in parallel instead of resistors. That would enable combinations of the DIP switch settings to be useful.For instance, if the capacitor values were 1u, 2u, 4u, 8u and 16u, the five switches would allow you to select 32 different durations.In this case R_TC and R_S would be fixed values and that would help satisfy the datasheet design recommendation that R_S be approximately 2*R_TC.

author: Cvijo
author_rank: Contributor
timestamp: July 04, 2025, 12:49:31 pm
content: Quote from: ledtester on July 04, 2025, 12:40:44 pmYeah you are completely right, never crossed my mind to switch capacitors, not the resistors... i will have to check the values of capacitors to get the required durations. And yes, in my desig,n only one switch should be "ON" just for simplicity of use.Thank you for your feedback i will look into it to switch to capacitors

author: PGPG
author_rank: Frequent Contributor
timestamp: July 04, 2025, 02:59:16 pm
content: D1 looks being duplicated by diode in Q3 (not Shottky so if it cares..).I don't know what are the boats 12V supply parameters.Do over-voltages can happen there?If so then what voltage/current?Can they disrupt CD4541?Can they burn CD4541?Absolute Maximum Ratings says Supply max is 20V.

author: ledtester
author_rank: Super Contributor
timestamp: July 04, 2025, 03:05:43 pm
content: Just an fyi... in the datasheet the description of the "Digital Timer Application" says:QuoteSo the pump will run for the specified duration when you initially apply power.

author: Cvijo
author_rank: Contributor
timestamp: July 04, 2025, 06:38:45 pm
content: Quote from: ledtester on July 04, 2025, 03:05:43 pmI don't think this will be an issue. Why I need this timer is because the guests are using the shower, and when the water tank runs out of the water pump is still running, trying to create pressure, and it runs for hours until it burns out. We are changing like 2-3 pumps over one season, so this will be a preliminary solution until we think of something better.I have change the schematics like you suggested:

author: Cvijo
author_rank: Contributor
timestamp: July 04, 2025, 06:43:29 pm
content: Quote from: PGPG on July 04, 2025, 02:59:16 pmit is powered by normal battery 12V .. so it is 12-14V .. so i think everything is within the limits chip, both mosfets (even gate to source voltage is rated +-20V), etc.Regarding the shotky diode, i know mosfet has it by default but whenever i see any schematichs with inductive load everyone saying put a diode to protect from motor spikes when it turns off

author: PGPG
author_rank: Frequent Contributor
timestamp: July 04, 2025, 10:18:19 pm
content: Quote from: Cvijo on July 04, 2025, 06:38:45 pmThe idea of switching capacitors instead of resistors is that you can simply sum times. If you will use capacitors to set 1.5min, 3min and 6min than with only 3 switches you will be able to set:1.5 / 3 / 4.5 / 6 / 7.5 / 9 / 10.5 minutes time delay.Quote from: Cvijo on July 04, 2025, 06:43:29 pmHave you noticed that inductive spike limiting diodes are installed paralel to inductive load?

author: Cvijo
author_rank: Contributor
timestamp: July 07, 2025, 08:39:00 am
content: Quote from: PGPG on July 04, 2025, 10:18:19 pmThank you for helping me out .. -i will change to 3 condensators like you suggested, should be good enough.So i have put my diode D2 wrong, should be from drain (since it p-channel) to ground or?I wasnt sure so i was asking you know who (ChatCPT) and he pointed out that on high side switch kathode should go to +12 and anode to drain.Do i need some other kind of protection from the pump and i read that it should be close to the load, but in my case, this is not possible, or can you give me some idea what should i search for or any info how to protect the mosfet. I read a lot of articles and questions people saying they mosfet got fried but most of the answers are so different and i don't know what to trust anymore.Thank you very much, it means a lot from you guys helping out !

author: PGPG
author_rank: Frequent Contributor
timestamp: July 07, 2025, 12:22:25 pm
content: Quote from: Cvijo on July 07, 2025, 08:39:00 amYes. It should be from Drain to GND (pointing with its arrow from GND to Drain).The info that it should be from Drain to +12 is true for low-side switch when inductive load is connected between +12 and NMOS drain.I am using fully protected NMOS-es (search for OmniFET or InteliFET). I was not using fully protected high side switches but they probably exists.Fully protected = overcurrent, overvoltage, overtemperature.Diode is probably enough protection againstdamageof your system caused by pump but fast switching high currents can propagate by air to the sensitive, high impedance CD4541 (it is CMOS) inputs. The effect can be for example: timer works correctly, but when it switches pump off the pulse generated when switched off restarts the timer and so onBut your circuit is not protected (what I mentioned in my first post) against any overvoltages that can happen at 12V supply. I don't know the standard used for car 12V supply but may be even a 400V Surge pulses can be specified there.I know nothing about bout 12V installation. Do there are other sources of pulses. What happens to your circuit during storm strike nearby. At land in such case you should expect several kV pulses between ground systems of different buildings and up to 50V pulses between ground points in the same building. What happens in the boat if through the water it is standing in strike 100kA current is flowing (or better from boat mast to the water).When I was in EMC lab testing 12V powered device they connected to 12V supply 1kV Surge generator through (brick size) 40 ohm resistor. In standard the big capacitor in serie should also be used, but they just hadn't one (it was 20+ years ago and it was not certified lab).Since then I design all my inputs and 12V supply to withstand 25A (1kV/40) 50us pulses.Some time later direct strike happened to the skyscraper our access control system was just installed. We got mail from installer that everything with long cables there is damaged (alarm systems, telephone exchanges, Ethernet routers, hubs, switches, cards in PCs) but only our system is working. Now all other installers will earn money and he nothingIf it were my design I would certainly used at 12V at least 600W 18 transil. 18V transil I mean such that can work with 15V and starts conducting at 18V (I use P6SMB18CA or SM6T18CA). With transils be careful as in other series the same transil have in name 15 and not 18 (some series are specified by working voltage and some by breaking voltage).If 25A pulse goes through such transil you get about 25V at it (see in datasheets).But 25V is too much for CD4541. The best would be to use there voltage regulator (giving for example 9V) that can accept 25V at input. But the output transistor should connect 12V to pump and not 9V and the output should be designed such than in the case you have 25V at 12V supply but still 9V at the CD4541 nothing wrong happens to the output circuitry.It would be easier (with that output) if the pump could be switched by low side switch. Do the pump really have to have one its end connected to GND?

author: Cvijo
author_rank: Contributor
timestamp: July 07, 2025, 03:36:43 pm
content: Quote from: PGPG on July 07, 2025, 12:22:25 pmwow, this was really detailed and very helpfull to learn ..at least some part that i do understand (sorry you didnt earn money because your installation only was alive) , thank you so very much for your effort and trying to help you have no idea what that means to us begginers and googling and searching and asking AI (like i did and spent 1 hour trying to tell him i think he is wrong) .. thank you.Supply will probably never go above 14.4V since it is powered by noramal car battery (120 Ah) and regarding thunder strike, yeah i will have to do more research about it .. or would just TVS diode from ground to 12V protect it (https://lcsc.com/product-detail/ESD-and-Surge-Protection-TVS-ESD_GOODWORK-SMF16A_C2943844.html?s_z=n_C2943844) i already have that one or should i use your suggetsion (SM6T18CA). Should clamping voltage be lower than 20V since CD4541 is up to 20V supply ?Unfortunately, all boat installations are done like that .. all negatives are at some common bus bar below deck and all positive lines go to the console and switches. Rewiring on one boat it would be ok, but this will be installed on all rental boats (those are small boats) and often changed every year so high-side switch is something i would definitely keep as mandatory for now.

author: PGPG
author_rank: Frequent Contributor
timestamp: July 07, 2025, 05:03:31 pm
content: I agree that directly at accu you will not have higher voltage than 14.4V. But voltage at one end of 5m wire can be different than the voltage at its other end when fast, high current pulse is going through it.Because of 14.4V you need transil with Vmax 15V at least. Such transil have Vclamp about 25V. Sorry can't want be other.600W is stronger=better than 200W (from your datasheet the Vmax 15V diode can accept 8A pulse (while 600W one can accept 25A).Transil located directly at PCB protects everything locally - even at the other wire ends you will get pulses of higher voltage here they will be limited.I all the time don't know if you really should be care about overvoltages or not. But as you write that others had their MOSFETs died than probably yes.Check your CD4541 current consumption when working (it can be low, depending on resistor values used in generator).May be you can supply it from your 12V via resistor (for example 100 ohm) and big capacitor (for example 470uF) and this RC will be filtering pulses that can exists at 12V supply.

author: ledtester
author_rank: Super Contributor
timestamp: July 07, 2025, 05:04:51 pm
content: Quote from: PGPG on July 07, 2025, 12:22:25 pmWhat would be the advantages of low-side switching in this case? Is it the ability to use a fully-protected NMOS device?

author: PGPG
author_rank: Frequent Contributor
timestamp: July 07, 2025, 06:38:45 pm
content: Quote from: ledtester on July 07, 2025, 05:04:51 pm1. I have written this note while writing about assumed by me up to 25V at transil at 12V. I didn't searched PMOS from schematic parameters, but many have Vgs limited to 20V so you should protect it against overvoltage while not reducing Vgs in normal use (I don't know the pump current, but assume can be high and higher Vgs - smaller Rds).2. When you need not to transfer your signal from being GND referenced to being 12V referenced circuit is simpler (Q4 not needed).3. NMOS-es are generally stronger (have lower Rds) then PMOS-es.4. And yes. Fully protected NMOSes I just know, but about high side switchers I have only heared.My devices are supplied by 20..200mA. 1A is for me high current.

author: Cvijo
author_rank: Contributor
timestamp: July 07, 2025, 09:58:34 pm
content: Quote from: PGPG on July 07, 2025, 06:38:45 pmThis will be 3-5A current, PMOS is 120P03 (2.5 mOhma Rds, 120A continuous current, so no heat sink required) -https://lcsc.com/datasheet/lcsc_datasheet_2410122037_HL-120P03_C20071222.pdfI have already used this MOSFET with 20A current and a switching frequency for LED dimming, and no heat or any problems.

author: Zero999
author_rank: Super Contributor
timestamp: July 08, 2025, 09:41:19 am
content: Why are you using two MOSFETs?Connect pin 9 to 0V, rather than +V to invert the output and Q3's gate directly to pin 8.

author: Cvijo
author_rank: Contributor
timestamp: July 08, 2025, 11:37:46 am
content: Quote from: Zero999 on July 08, 2025, 09:41:19 amI was at the begining when using NE555 and first design with CD4541B was without but after googleing found that this chip has very low sink current and output current too so i put nmos on the gate. Probably shouldnt have and would work directly with output because i am bot using it with switching freq. Since i wasnt sure i put NMOS. And another reason is that in datasheet it say whenyou power it on output will be high for the duration of one-shoot and i need it to run from the power up. Thank you for the advice, appriciate it.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 08, 2025, 12:48:20 pm
content: To stop random states occuring at switch on, a power-on reset is a good idea. Tie pin 5 to ground not VddQ4 could be replaced with a cheaper npn such as an 293904 or mmbt3904.

author: Cvijo
author_rank: Contributor
timestamp: July 08, 2025, 02:36:44 pm
content: Quote from: Terry Bites on July 08, 2025, 12:48:20 pmI was thinking the same will pin 5 to the ground set power on not to se output high so i made branch with 0 ohm resistor one to ground one to 12V to test it but now when you confirmed it this sounds goodQuote from: Terry Bites on July 08, 2025, 12:48:20 pmthis AO3400C is 0.08 € per piece so i didn't bother searching for cheper but will keep in mind to switch it later if i will order new batch, thank you for your help