Variable voltage dropper for speed control of tattoo machine

[Kastrosama]

Sweet, im going to go and get the parts, some proto board and see how it all works out
Thanks for all the imput guys! Ill have fun trying to put this all together now
Regarding the parts, anyone here run an online shop? would be one way to say thanks!

 

[Arouse1973]

Sorry for the delay. Here's a design with all the features we've discussed.

View attachment 15120

That looks good Kris. The base resistor I mentioned is for start up. Two factors I can see which may be ok. When the circuit is switched on with the 470uF discharged there will be approx. 1.2 Amps inrush through the base if the POT is set to zero. Also momentarily the motor will draw it's stall current and will look like a very low resistor this could be tens of milliamps I am not sure if the power Darlington can handle this. I guess the same applies if the motor stalls.
Just a thought.
Adam

 

[KrisBlueNZ]

That looks good Kris. The base resistor I mentioned is for start up. Two factors I can see which may be ok. When the circuit is switched on with the 470uF discharged there will be approx. 1.2 Amps inrush through the base if the POT is set to zero.

I don't know what you mean. Can you define the path this current will take?

Also momentarily the motor will draw it's stall current and will look like a very low resistor this could be tens of milliamps I am not sure if the power Darlington can handle this. I guess the same applies if the motor stalls.

That won't be a problem unless the battery can supply dozens of amps. And a resistor in series with the base wouldn't help unless a capacitor is added from the base to 0V to force the transistor to "soft-start" the motor.

 

[Arouse1973]

Hi Kris

As I said it might not be a problem. The transistor in question has a maximum base current of 0.2 A. When the circuit is first switched on in this scenario it's momentarily in common emitter mode, i.e. the base is very close to 0V or at 0V because of the cap and possibly the motors low initial resistance. Until the cap and motor have had time to settle you will get a large base current, this might be Ok but just thought I would mention it.

Cheers
Adam

 

[Arouse1973]

Here is the base current of a BCX38B with a 470uF cap on the emitter. A 0.5 R resistor from the supply simulating POT all the way up. It will be more than this if the POT goes lower.
Cheers
Adam

 

[KrisBlueNZ]

When the circuit is first switched on in this scenario it's momentarily in common emitter mode, i.e. the base is very close to 0V or at 0V because of the cap and possibly the motors low initial resistance. Until the cap and motor have had time to settle you will get a large base current

You mean the emitter is close to 0V?

Here is the base current of a BCX38B with a 470uF cap on the emitter. A 0.5 R resistor from the supply simulating POT all the way up. It will be more than this if the POT goes lower.

What internal resistance did you assume for the battery when you did that simulation?

 

[Arouse1973]

You mean the emitter is close to 0V?

What internal resistance did you assume for the battery when you did that simulation?

Yes emitter sorry. A PP3 battery has a very low resistance when new and could easily put out 5 Amps momentarily, It's less than the 0.5R I added. We are talking transients here not constant.
Cheers
Adam

BTW it's 4:42am over there. Have you woken up or not gone to bed yet

 

[KrisBlueNZ]

I really don't think there's a problem.

I'm awake at random times. Folks get immediate replies from me at all times of the day or night. I'm trying to make people wonder if I'm super-human

 

[davenn]

........I'm awake at random times. Folks get immediate replies from me at all times of the day or night. I'm trying to make people wonder if I'm super-human

just admit it ... you have LCD panels glued to the insides of your eyelids

D