why Output voltage of a speed control circuit is the same as input voltage

Thread Starter

mojtaba491

Joined May 16, 2019
7
i have a DIESEL Angle Grinder and Output voltage of speed control circuit is the same as input voltage.
pictures of circuit attached.
 

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AlbertHall

Joined Jun 4, 2014
9,826
If the voltages are the same without the motor connected (this may be an expected condition) then measure them with the motor connected.
If the voltages are the same with the motor connected then probably the triac/thyristor (three legged black thing) is short circuit. It may be other things but that's the first thing to check.
 

Thread Starter

mojtaba491

Joined May 16, 2019
7
If the voltages are the same without the motor connected (this may be an expected condition) then measure them with the motor connected.
If the voltages are the same with the motor connected then probably the triac/thyristor (three legged black thing) is short circuit. It may be other things but that's the first thing to check.
Thanks.
I tested it with motor but the voltages are the same.
if problem is for damaged triac, would you please tell me what happend when the triac is damaged and why input and output voltages are the same.

In Z2.jpg picture I see a component pin coming thru board that does not look soldered properly.



Regards, Dana.
which component pin ?
 

AlbertHall

Joined Jun 4, 2014
9,826
if problem is for damaged triac, would you please tell me what happend when the triac is damaged and why input and output voltages are the same.
The triac works like a switch. It is now (maybe) short circuit - the switch is permanently on.
Do you have a meter, can you disconnect it from the mains and measure the resistance between the legs of the triac?
https://www.electronics-tutorials.ws/power/triac.html
 

Thread Starter

mojtaba491

Joined May 16, 2019
7
The triac works like a switch. It is now (maybe) short circuit - the switch is permanently on.
Do you have a meter, can you disconnect it from the mains and measure the resistance between the legs of the triac?
https://www.electronics-tutorials.ws/power/triac.html
i disconnect the triac and connect it to digital multimeter.
1- resistance between leg number 1 and leg number 2 is O.L
2-in situation 1,when i connect leg number 3 (gate) to leg number 2, resistance between leg number 1 and leg number 2 is 500 ohm. but when i disconnect leg number 3 (gate) from leg number 2, the resistance is O.L.
 

AlbertHall

Joined Jun 4, 2014
9,826
That's a pretty basic test but it looks like the triac is OK.
Next you could check the values of the resistors on the board including the variable one.
Also if your meter has a capacitance range, check the little brown capacitor value.
 

Thread Starter

mojtaba491

Joined May 16, 2019
7
That's a pretty basic test but it looks like the triac is OK.
Next you could check the values of the resistors on the board including the variable one.
Also if your meter has a capacitance range, check the little brown capacitor value.
I measured the values by multimeter.
please look at the attachment. (also contain circuit)
 

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AlbertHall

Joined Jun 4, 2014
9,826
So all components are working just fine but the speed control isn't o_O
I think my next move would be to fit a new triac I case it is OK at low voltages but breaks down at higher voltages.
 

AlbertHall

Joined Jun 4, 2014
9,826
In that case I'm stumped. Fault finding on this circuit is difficult and dangerous because it is connected directly to the mains.
Last throw of the dice: Get a magnifying glass and carefully inspect the track side of the board for any bad joints, small flecks of solder that may be shorting two tracks, and finally for any sign of a crack on one of the tracks.
 

Thread Starter

mojtaba491

Joined May 16, 2019
7
I checked the soldered joints with magnifying glass. also i test them with multimeter but no fault found.
attachment is a board picture with high quality.
 

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AlbertHall

Joined Jun 4, 2014
9,826
I checked the soldered joints with magnifying glass. also i test them with multimeter but no fault found.
attachment is a board picture with high quality.
The joints with red arrows look suspicious and the track with yellow arrow possibly has a crack but it might just be a scratch on the solder resist.
You should really get some isopropyl alcohol (aka rubbing alcohol) and give the board a good brush with that to remove the flux from the board. This makes it much easier to see what's underneath and also make the board more reliable.
Capture.jpg
 

MisterBill2

Joined Jan 23, 2018
6,025
I am guessing now that the complaint is that the control was not able to vary the motor speed. I did not see that stated at all. AND the voltage that shows on a meter may be fairly constant because the meter would be reading the peak voltage.
Given the things that you have already checked and found to be OK, the item left is the variable control resistor. Checking that it varies the resistance as it is turned is the one thing that I did not see suggested.
 
Last edited:

AlbertHall

Joined Jun 4, 2014
9,826
Given the things that you have already checked and found to be OK, the item left is the variable control resistor. Checking that it varies the resistance as it is turned is the one thing that I did not see suggested.
It was tested. Results in the attachment to post #8.
 

MisterBill2

Joined Jan 23, 2018
6,025
It was tested. Results in the attachment to post #8.
The values of that variable resistor are not reasonable. Normally a variable resistor should be able to be set to a quite low value of resistance, but I think that the complaint is that the thing runs at full speed no matter what. Are you using it with the original motor that it was controlling?
 

MisterBill2

Joined Jan 23, 2018
6,025
I studied my reference materials and conclude that there is an error in the drawing on post #8. One or the other of the resistors should be connected to the gate side of the diac, instead of both resistors being in parallel. AND, if there really is no pull-down resistor connected to the gate, that may be why the control is full on all of the time. In addition, the values of the two resistors seem to be quite high, more than would be expected.
One more possibility is that the switch to operate the grinder has three positions, OFF, variable speed, and full speed with bypass. And if the connection is not right then the only speed is full speed. So if the trigger switch has three terminals that might be the source of the problem.
 

Thread Starter

mojtaba491

Joined May 16, 2019
7
I studied my reference materials and conclude that there is an error in the drawing on post #8. One or the other of the resistors should be connected to the gate side of the diac, instead of both resistors being in parallel. AND, if there really is no pull-down resistor connected to the gate, that may be why the control is full on all of the time. In addition, the values of the two resistors seem to be quite high, more than would be expected.
One more possibility is that the switch to operate the grinder has three positions, OFF, variable speed, and full speed with bypass. And if the connection is not right then the only speed is full speed. So if the trigger switch has three terminals that might be the source of the problem.
would you please draw a correct circuit
 
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