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LM675 getting very hot when I switch motors

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steadicam12

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Hello all newbie here...just want to say I really enjoy reading these posts and I have learned a great deal...however I do have a question but don't have a lot of info...

OK I have a analog LM675 op-amp circuit that is used to drive a DC motor positional with a potentiometer...all is well with the circuit or so I think it drives a 6v rated Faulhaber motor and does just fine...so I went and bought a brushed dc motor from servo city rated 3-12v and it works fine the only problem and its not a good problem is that the LM675's are getting extremely hot and that just when the motor is connected and not being moved via the "pot" and usually the .22uf tantalum cap blows!!!??? Here are some details on the motors: Faulhaber motor draws when being moved via the pot about.2 a and the servo city motor is about the same draw .19 a...any and all help is appreciated. Again I am a noob and know some but not a lot. I have attached a jpg on what the circuit is based on since I do not have a proper schematic::::





Thanks

Erik
 

Vbase

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If the tantalum blows it is clear indication that in that position of the pot the LM675 oscillate freely. and gets hot. To stop that try 0.1uF across each of the 1M resistors.
 

Audioguru

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Maybe this is another example of a solderless breadboard oscillator. Use a compact layout on a pcb instead.
 

steadicam12

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Vbase thanks I'll try that...I know there is a 4.7 across a 100k resistor it's not a 1M...audioguru thanks for suggestion but it's in a very small board already...I kinda inherited this...it's been working nicely with the faulhaber motor as I've said...it just started doing this with a new servo city motor...it operates as it should but it get hot and then the tantalum goes poof!!??...thanks guys I appreciate the time and if you can think of anything else to try please let me know...but I will try maybe going higher than 4.7uf or should I try higher resistor value???
 

chuckey

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You cannot use a tantalum capacitor in these positions as they can become reverse biased, due to back EMF from the motor, it would be best to have one network across the motor. Use a non polarised type in either case.
Frank
 
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steadicam12

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Chucked thanks...what type and value should be used??
 

steadicam12

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Chuckey hey what value and type should I go with??? Thanks for your help.
 

steadicam12

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Schmitt hi thanks I tried it still getting hot??? anything else I can try??
 

SunnySkyguy

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Is it oscillating with the brush motor?

It may be that it is an underdamped oscillation near 75kHz with 5A current limit on ripple which can blow most caps except Polyester or Polyurethane film with low ESR. Since a 1R resistor is also in series to current limit these instabilities, the CAP ESR must be much less than 1R. THe driver is much lower impedance. THis design was intended for a 4~8 Ohm speaker. The Faulhaber is likely higher impedance with a DCR in the range of 10 Ohms.

THe Brush motor is likely much lower DCR or DC series resistance which is causing a back EMF step current impulse voltage that triggers an undamped oscillation that blows the cap.

Of course the cap must not be any polarized cap. as polarized caps can only handle -10% of rated voltage.

So the solution may be to reduce the 1R to a value less than the DCR of the brush motor AND USE A NON-POLARIZED low ESR CAP.

Pls advise what DCR readings you get and step response on scope.
 

Vbase

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Schmitt hi thanks I tried it still getting hot??? anything else I can try??
The reason for the amps are getting hot is oscillations caused by instability of the circuit. The 0.22uF blow because of the oscillations, changing the caps to another type may stop them blowing but it wont stop the amp from getting hot. You are only curing the symptoms.
To stop oscillations you can try 3 things:
1. Remove the two caps 4.7uF across the 100K and check the results.
2. Connect 0.1uF between inputs of each LM675 and check the results.
3. Remove the 0.1uF caps from the inputs, connect 0.1uf across the motor.
 

steadicam12

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Sunny thanks for your very informative response...I do believe its oscillating on both motors but it's fine with the faulhaber...I don't have a scope so...you say the 1R I assume your speaking to the 1M resistor...just a bit more info those 1M resistors are actually 100k with a 4.7 ceramic cap...as far as DCR I have measured both motors with a MM not connected...and I get 2-3 ohms on the faulhaber and about 10 ohms on the motor that gets the circuit hot...I don't have a schematic I am trying to get one drawn here by looking at the board and I will post it so you can see what's going on.

- - - Updated - - -

Vbase again I thank you for your time!!! I will try all the options you suggest...I am trying to get schematic drawn up to show more detail on this circuit!!
 

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