PCB layout problem??

[johnl69]

Could someone please take a look at the pcb layout in the pic below and tell what if anything is wrong with it. I have already built this prodject once but on 2 seperate pcb's (1 for the powersupply and 1 for the control circuit) and it worked fine now that ive stuck it all on 1 pcb im getting problems, first LED 3 is on permanetly even though it should only be on when SW3 is close it is also getting 7.2 volts when the circuit is run on 6 also LED2/1 glow dimmly when they should be off they are receiving 1.3v when it should be 0v,

I been trying to figure this out for the last 4 hours and am tearing my hair out.

thanks in advance

john

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PS this is the transformer im using if its any help **broken link removed**

 

[GraveYard_Killer]

as much as possible dont do sharp turns on PCB (90 degree turn)

 

[Boncuk]

Hi John,

both, the schematic and the layout are barely readable. Please post again disabling the dotted grid.

Boncuk

 

[mvs sarma]

Could someone please take a look at the pcb layout in the pic below and tell what if anything is wrong with it. I have already built this prodject once but on 2 seperate pcb's (1 for the powersupply and 1 for the control circuit) and it worked fine now that ive stuck it all on 1 pcb im getting problems, first LED 3 is on permantly even though it should only be on when SW3 is close it is also getting 7.2 volts when the circuit is run on 6 also LED2/1 glow dimmly when they should be off they are receiving 1.3v when it should be 0v,

I been trying to figure this out for the last 4 hours and am tearing my hair out.

thanks in advance

john

............ /QUOTE]

1. Check what brand you have used for 2N7000 as there are many pin-out available. see their make, and cross check from relevant data sheet.

2. The Transformer is said to be 10:1 ratio. thus secondary would be 24AC and 34V DC At this voltage level 1K series resistor is low for the LEDs. Red LED drops around 1.4V across it.34-1.4=32.6V . This 32mA current flows thro each LED- I fear it is too much a current. you may limit it to 8to 10mA irrespective of color, by changing R2, R3, R4 to say 3k3 each.

3. If Neutral can be ensured by use of 3 pin plug at mains voltage, it would be better to direct the neutral wire and use 2 contacts in parallel on phase lead. this would ensure good contact when relay operates.
Also perhaps you need a spark quench in the form of R-C series across the contacts -- otherwise the relay contact would be short lived.

4. At 34V supply line, 1N4148 is inadequate for snubbing the back emf across relay coil. perhaps you may choose BA159 or any other such fast recovery diode of higher voltage.

 

[johnl69]

Hi John,

both, the schematic and the layout are barely readable. Please post again disabling the dotted grid.

Boncuk

**broken link removed**

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[3v0]

Did you use PCB layout program with schematic capture? That would ensure that the PCB was the same as the schematic. Or are you asking for the PCB to be verified against the schematic?

 

[johnl69]

Did you use PCB layout program with schematic capture? That would ensure that the PCB was the same as the schematic. Or are you asking for the PCB to be verified against the schematic?

I used pcb wizard to make the layout from the schematic but am getting the problems I stated in the original post, I am not sure if the pcb was generated wrong or not.

 

[johnl69]

1. Check what brand you have used for 2N7000 as there are many pin-out available. see their make, and cross check from relevant data sheet.

2. The Transformer is said to be 10:1 ratio. thus secondary would be 24AC and 34V DC At this voltage level 1K series resistor is low for the LEDs. Red LED drops around 1.4V across it.34-1.4=32.6V . This 32mA current flows thro each LED- I fear it is too much a current. you may limit it to 8to 10mA irrespective of color, by changing R2, R3, R4 to say 3k3 each.

3. If Neutral can be ensured by use of 3 pin plug at mains voltage, it would be better to direct the neutral wire and use 2 contacts in parallel on phase lead. this would ensure good contact when relay operates.
Also perhaps you need a spark quench in the form of R-C series across the contacts -- otherwise the relay contact would be short lived.

4. At 34V supply line, 1N4148 is inadequate for snubbing the back emf across relay coil. perhaps you may choose BA159 or any other such fast recovery diode of higher voltage.

Sorry only just noticed this reply,

These are the 2n7000 **broken link removed** Ive been over the data sheet and there are connected in the correct configuration,

As for the transformer I always forget to delete the 10:1 ratio sign, the transformer im using is the 6v version of **broken link removed**

and this is the relay **broken link removed**

Ive double checked all the connections on the pcb and they all connect to where there supposed to be for the life of me I cant find out where the problem is. does it matter where on the board the components are placed?

john

 

[mvs sarma]

Are you sure that the components bear the symbol of ST on them
i ask because, the firm can sell any brand. but , if it is ST, the said datasheet is valid. please have a double check for the manufacturer's stamp on the FETs

 

[johnl69]

Are you sure that the components bear the symbol of ST on them
i ask because, the firm can sell any brand. but , if it is ST, the said datasheet is valid. please have a double check for the manufacturer's stamp on the FETs

Yeah its definately got the ST stamp on it, I take it the board is ok if it is then I must have done some thing wrong but I cant figure out what.

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[mvs sarma]

Yeah its definately got the ST stamp on it, I take it the board is ok if it is then I must have done some thing wrong but I cant figure out what.

Nice John, Thanks for the photo, though little out of focus though. the lay out appears to be OK with ref to schematic. Perhaps you may have to check the actual components for placement and values. Also see whether you have positioned the FETs as per the artwork indicated.

 

[johnl69]

Yep they all look ok to me

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and the back the power wire are just soldered there tempoary for testing

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[kchriste]

At a quick glance it all looks OK. It is possible that the FETs were damaged by ESD when you installed them. If LED3 is on, then the relay should be on also. Is it?

when SW3 is close it is also getting 7.2 volts when the circuit is run on 6

6VAC, when rectified and filtered, will give you around 7.1VDC. Depending on the line voltage and loading it is likely that the transformer is putting out a little more than it's nominal 6V rating, so 7.2Vdc is nothing to worry about.

 

[positronicle]

I would start by cleaning up your board a little more. After you get all the rosin off, then you would be better able to spot any possible "whiskers". It looks like a possible solder short across sw3(at q1), to me. As you probably already know, it doesn't take much of a visible connection to make all the difference in the world. good luck with your circuit.

 

[johnl69]

I stripped down the board completely, cleaned it up, checked the conitinuity of all the track and they are ok and there is no shorts, then I carefully resoldered new components onto the board checking for any stray solders as I went and it is still the same, apart from LED3 did not blow and is getting correct voltage, there is still a dim glow from LED2/1 when the circuit is not running(SW1/2 open SW3 closed) where would this voltage be coming from?

 

[mvs sarma]

I stripped down the board completely, cleaned it up, checked the conitinuity of all the track and they are ok and there is no shorts, then I carefully resoldered new components onto the board checking for any stray solders as I went and it is still the same, apart from LED3 did not blow and is getting correct voltage, there is still a dim glow from LED2/1 when the circuit is not running(SW1/2 open SW3 closed) where would this voltage be coming from?

John, where is basis of this design from? Because,it looks odd for me at least, generally we don't find such designs, where there is an LED in series to gate bias of Mos-fet. Generally gate of an FET is terminated and switching voltage is either low or high.

what is the circuit supposed to do as per your design goal, and try to indicate meaning of each LED? Why at all FETs were chosen in stead of BJTs? it will help us to analyse it more meaning fully to meet your need.

BTW, can you do one experiment. please remove the ac completely from this board. apply 12V battery, at the filter capacitor with proper polarity. clean with a sharp tool the gaps between the 3 pins of each FET thoroughly. let the relay even not have AC. apply the same 12V to relay contact and try to put a 12V lamp at the output with limited current say 300mA.

then test for your needs. the relay operation is indicated now ny the glow of bulb.

keep the Switches with short wires . let us analyse before we power up with AC.

 

[blueroomelectronics]

Good question(s) Sarma, I was wondering the same thing.

 

[johnl69]

John, where is basis of this design from?

The circuit is from here this should explain what I am doing https://www.electro-tech-online.com/threads/transistor-as-a-switch.37656/

Ive made a new board thinking that maybe I had done something wrong, this time I used a different program to generate the layout, this one works apart from the dim glow of the led's, I powered it up with a 9v supply and it does the same.

 

[mike50]

Can you measure the voltages (relative to ground) at all three pins of each of the FETs when SW3 is closed and SW1 and SW2 are open?

I think that will help identify where the problem is.

Mike

 

[johnl69]

Can you measure the voltages (relative to ground) at all three pins of each of the FETs when SW3 is closed and SW1 and SW2 are open?

I think that will help identify where the problem is.

Mike

Q1, D=0.00, G=7.40, S=0.00
Q2, D=0.00. G=0.00, S=0.00
Q3, D=0.00, G=8.50, S=0.00

This is when connected to a 9v battery (pp3)