Simple Circuit - But i am stuck

Ron H

Joined Apr 14, 2005
7,063
Hi There,

I have now switched it backwards so that the gate is to the resistor, the drain is to the relay and the source is still at ground.

Unfortunately its still not working, again over 10v measured across the relay but its not being thrown.

Many Thanks,

Scott
All you need to do is wire it exactly like the schematic, and use working parts.
 

Thread Starter

scollinguk

Joined Oct 9, 2010
34
Hi Ron,

I am very thankful for your help so far;

I have tried to follow the schematic and have posted my pictures of the circuit board itself so that hopefully someone as helpful as you can point out if I have done it incorrectly.

I have also replaced the MOSFETs already with new ones and have performed the tests advised and posted the results so that again someone helpful can advise if the results show that the components are faulty or not.

As i said in my first post, this is my first time having a go at creating my own circuits and I have learnt a lot from doing this but dont have the knowledge to fault find or cure faults myself.

I am sure its almost there and a little more help should see it finished and working, I hope that the help will be provided for this to happen rather then all the time everyone especially yourself has invested in helping me going to waste.

Many Thanks,

Scott
 

thatoneguy

Joined Feb 19, 2009
6,359
What are the voltages on the MOSFET, with the switch on and off?

Is the relay clicking when power is applied? It may be on continually for one reason or another. See if there is continuity between COM and NO (It appears you are using COM and NC for your output?)
 

Thread Starter

scollinguk

Joined Oct 9, 2010
34
Hi,

Here are the voltages:

M1 is on the right when looking at the bottom of the board and M2 is on the left.

When the switch is closed:
M1 between G+D is 8.2v and lowering slowly
M1 between G+S is 8.2v and lowering slowly
M1 between S+D is 0v

M2 between G+D is 8v and lowering
M2 between G+S is 8v and lowering
M2 between S+D is 0v

When the switch is open:
M1 between G+D is 10.3v
M1 between G+S is 0v
M1 between S+D is 10.4v

M2 between G+D is 9.7v
M2 between G+S is 0v
M2 between S+D is 10.3v

The relay doesnt click when the switch is closed. If i take the relay out of the board and directly connect a 9v battery to the coil it will click over. NC will be used for the pump power as the basis of the circuit is that when the switch is closed it will turn off the pump until the switch opens plus 10-15 seconds.

Cheers,
 
Last edited:

thatoneguy

Joined Feb 19, 2009
6,359
It sounds like you have a capacitor in the wrong spot.

Keep the black probe of the meter on ground, then post voltages for the following:
Red probe on:
M1 S,G,D
M2 S,G,D
Coil side 1
Coil side 2


I assume this is the circuit you have:
 

Thread Starter

scollinguk

Joined Oct 9, 2010
34
Hi,

Yes, thats the correct circuit.

With the switch closed:

M1 S is 0v
M1 G is -7.7v
M1 D is 0v
M2 S is 0v
M2 G is -7.2v
M2 D is 0v
Coil side 1 is -8.25v
Coil side 2 is 0v

With the switch open:

M1 S is 0v
M1 G is 0v
M1 D is -10.3v
M2 S is 0v
M2 G is 0v
M2 D is -10.3v
Coil side 1 is -10.3v
Coil side 2 is -10.3v


When i measured M1 S after i had closed the switch i initially got 0.2v which went up to 0.3v and then dropped to 0v. Not sure if this was power coming from the cap as the mulitmeter was then showing 0.3v when it wasnt connected to anything and i had to turn it off and then on again.

Cheers,
 

Ron H

Joined Apr 14, 2005
7,063
I can't make out parts of your layout, but it looks like the drains are connected to each other. It looks like the gates are connected by 470k. Am I reading it all wrong?
 

Thread Starter

scollinguk

Joined Oct 9, 2010
34
Yes, thats all correct.

thatoneguy advised to move M2 so that the drain and gate were reversed from the original positions and when changing it in LTSpice to reflect this the simulation showed the traces to be unaffected by the change.
 

Ron H

Joined Apr 14, 2005
7,063
Yes, thats all correct.

thatoneguy advised to move M2 so that the drain and gate were reversed from the original positions and when changing it in LTSpice to reflect this the simulation showed the traces to be unaffected by the change.
The drains are not connected on the schematic. I don't know what thatoneguy was intending, but I don't think he wanted to to change the circuit wiring.
As I said before, if you wire it exactly as the schematic shows, and all your parts are good, it will work.
The only change I might make is to add 10k in series with the gate of M2. This will make it less susceptible to static discharge during testing, without affecting performance. This is not necessary - just an enhancement.
 

thatoneguy

Joined Feb 19, 2009
6,359
Yes, thats all correct.

thatoneguy advised to move M2 so that the drain and gate were reversed from the original positions and when changing it in LTSpice to reflect this the simulation showed the traces to be unaffected by the change.

The coil was attached to the gate of M2 (or was it M1?), which is the change I said to correct. It appears that the method of correction added the new problem.
 

Ron H

Joined Apr 14, 2005
7,063
The coil was attached to the gate of M2 (or was it M1?), which is the change I said to correct. It appears that the method of correction added the new problem.
The coil was supposed to be connected to the gate of M2 (before I added the optional 10k resistor).
 

cybertronics

Joined Aug 29, 2009
46
Here's a suggestion why don't you build the circuit on a breadboard so every one can see how the parts are being wired together. I think your problem is you don't know where each of the parts pins should be connect to each other. some times it gets confusing when wiring to a PCB. try it on a breadboard first. and take your time with each when looking at the schematic that RON posted. and it would be much easier to trace your wiring mistakes.

Just a trought.
 
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