555 Timer from Bread Board to Perf Board Problems

Thread Starter

Rittter

Joined Dec 5, 2015
60
I just can not make these timers work after soldering. They work perfectly on a bread board but once soldered, the timing is erratic. I use a socket for the timer so I don't over heat the timer. I inserted both the LMC555 and the LM555 with identical results. I check each solder line with the meters diode setting. If the buzzer even stutters a little, I unsolder and re-solder the area. I check neighboring solder lines to make sure there is no cross connections.

I am using Bourns #3296-3/8" square trimpots for R1 and R2. I set the resistance before soldering, and leave the prongs long to allow measurements after but once soldered, I can't get the same resistance readings. The readings I get pretty much match the pulse rate I get which is incorrect. I removed the pots and replaced them with fixed resistors with the same results. Even the soldered diode from R1 to C1 was giving a reading of 18K ohms regardless of which side the + and - probes were positioned on it, (how is that even possible ?).

For testing purposes, I have an LED at the discharge of both transistors for visual confirmation of the pulse rate. The pulse rate does not have to be exact but it has to be in the same ball park. Any input would be greatly appreciated .
 

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AlbertHall

Joined Jun 4, 2014
12,338
I don't know about the '555 but that MOSFET circuit won't work if the voltage across the coil is supposed to get above 10V as there will then be no voltage between gate and source. Connect the coil between the drain and the supply and connect the source to ground. You will also need a diode across the coil to catch the back emf when the MOSFET switches off.
 

dl324

Joined Mar 30, 2015
16,788
I just can not make these timers work after soldering. They work perfectly on a bread board but once soldered, the timing is erratic. I use a socket for the timer so I don't over heat the timer.
If you think it's an assembly problem, you're going to need to post well focused pictures of your soldering.
 

Thread Starter

Rittter

Joined Dec 5, 2015
60
I don't know about the '555 but that MOSFET circuit won't work if the voltage across the coil is supposed to get above 10V as there will then be no voltage between gate and source. Connect the coil between the drain and the supply and connect the source to ground. You will also need a diode across the coil to catch the back emf when the MOSFET switches off.
"if the voltage across the coil is suppose to get above 10V" ? According to the data sheet, if Vgs=10v, Id=20A, Vds=400v. Please explain and educate me or refer me to something that would explain this. I've been wrestling with your suggestion for a while. The coil is an elongated wire. I've thought about putting in between the drain and supply, but figured that the coil has minimum resistance and the source is basically seeing ground through a longer wire. In addition I don't have to worry about kick back. What change would there be having the coil between the supply and drain ?
Thank you for your input.
 

AlbertHall

Joined Jun 4, 2014
12,338
Do you expect voltages of more than 10V across the coil/wire, like anywhere near the 400V the drain is fed with?
Just picture the voltages on the MOSFET when it turns on. The gate will be at 11.3V. If the voltage on the source rises to 11.3V then the gate to source voltage will be zero and the MOSFET will be off again.
 

Thread Starter

Rittter

Joined Dec 5, 2015
60
Do you expect voltages of more than 10V across the coil/wire, like anywhere near the 400V the drain is fed with?
Just picture the voltages on the MOSFET when it turns on. The gate will be at 11.3V. If the voltage on the source rises to 11.3V then the gate to source voltage will be zero and the MOSFET will be off again.
Yes I do expect all of 400V to be discharged from the drain to source (laughing at myself as I write this). The data sheet as I stated above, requires 10v Vgs. In the graph on the data sheet of Vishay; Fig. 6 indicates I may need around 15v Vgs. for 400v. My interpretation of what you are saying is I need 400v at the gate to get 400v out, at which point it will shut off. If this is the case, what is the data sheet talking about ?
 

AlbertHall

Joined Jun 4, 2014
12,338
In the graph on the data sheet of Vishay; Fig. 6 indicates I may need around 15v Vgs.
Vgs means the voltage between the gate and the source. If the source is at 400V then the gate must be at 415V to turn on the MOSFET. That is why this kind of circuit is much easier if the source is connected to ground.
 

Thread Starter

Rittter

Joined Dec 5, 2015
60
Vgs means the voltage between the gate and the source. If the source is at 400V then the gate must be at 415V to turn on the MOSFET. That is why this kind of circuit is much easier if the source is connected to ground.
Thank you for that ! All the reading that I have done on MOSFETs, not one has stated this. I guess the next question is, how does one put 400v on the gate without damaging the rest of the circuit ???
 

AlbertHall

Joined Jun 4, 2014
12,338
Thank you for that ! All the reading that I have done on MOSFETs, not one has stated this. I guess the next question is, how does one put 400v on the gate without damaging the rest of the circuit ???
With great difficulty.
With the source grounded, the gate voltage can be a much more sensible 10V to 15V to turn on the MOSFET, then the drain can be any voltage up to its maximum drain to source voltage (but leave some margin to avoid accidental smoke release!).
 

Thread Starter

Rittter

Joined Dec 5, 2015
60
With great difficulty.
With the source grounded, the gate voltage can be a much more sensible 10V to 15V to turn on the MOSFET, then the drain can be any voltage up to its maximum drain to source voltage (but leave some margin to avoid accidental smoke release!).
Thank you for the humor of the day. Accidental smoke release is my middle name. Funny you should mention that, but I try parts of circuits before I commit and the MOSFET did trigger with 11v when the coil was connected to the drain. That was the good news, but I did not put in the diode and the kick back smoked the charging circuit. In case anyone is wondering, kick back is for real.

One last question. If I understand you correctly, if I connect the coil to the drain, 11v will trigger the MOSFET, all is good. If connected to the source, I would need 400v at the drain. What makes the difference ? A coil is a longer wire from the source to drain. I think this is like the thermos question.
 
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