I am confused, too. Rather than worry about fusing, I think that the real problem is to reduce the stall torque.

Is that a Pacer? O-290 or 320? Great airplane...I have .5 in a PA16 (Clipper). Seriously thought about a Pacer as a first plane, but lust after a little more speed.

I don't disagree with your comment.

I would love someone to walk me through an appropriate current limiting circuit that is up to the task. Sadly, I am a beginner and referrals to example circuits leave me with more questions than answers. I am not asking anyone to "just give me the answer", but I would like a "walk through" on the(a) recommendation.

I know, I know, a conversation would be much better than a web forum for this, and I am up for that, too.

I read a funny quote on another web board (not electronics) the other day, "when someone new asks a question and the first replies are 'search is your friend', 'go read this', or 'geez, not this again', somewhere, a unicorn dies"

You gotta love the "old timers" on some of these boards (and all boards have them, and I hope I am one elsewhere) who tirelessly answer questions and keep the board alive.

Thanks.

 

O-320 Tri-Pacer converted to the PA20 tail-wheel configuration. I have completely rewired it and installed modern instruments and avionics. I have ~450hrs in it. I have flown it all over the mountain West, including Alaska, Canada and to OSH.

Put a 0.5Ω to 2Ω power resistor in series with your motor, and then measure its stall torque. The goal is try different values that will limit the torque to your max allowed level so nothing breaks. That resistor will get stinking hot if the stall persists, so detecting the temperature rise in the resistor may be required to shut the power off.

 

O-320 Tri-Pacer converted to the PA20 tail-wheel configuration. I have completely rewired it and installed modern instruments and avionics. I have ~450hrs in it. I have flown it all over the mountain West, including Alaska, Canada and to OSH.

Put a 0.5Ω to 2Ω power resistor in series with your motor, and then measure its stall torque. The goal is try different values that will limit the torque to your max allowed level so nothing breaks. That resistor will get stinking hot if the stall persists, so detecting the temperature rise in the resistor may be required to shut the power off.

Nice. Maybe I'll see you at OSH in 2015. We go every year since it is only about an hour away. Someday I'll have my own airplane and camp there for the week, although we are looking at a camper with some other folks for this year. Everyone tells me it is a whole different experience in the evenings.

I'll try the power resistor idea. It makes sense that putting it in series will affect the motor performance. Maybe with the low duty cycle and also proper fusing I can get the overall behavior we are looking for.

 

Realistically if there is not a intelligent drive that monitors current, a time delay fuse should be fitted that is somewhere in the peak torque current range, a little above the continuous stall torque range, as this is the continuous point a motor can operate without damage or destruction.
Max.

 

What about a current limiting circuit with an additional layer of protection by breaker/fuse? Would you consider that too complicated?

 

What about a current limiting circuit with an additional layer of protection by breaker/fuse? Would you consider that too complicated?

I would not consider that too complicated. Can you point me to an example and give me an explanation of the circuit? Use small words, single-syllable if possible, and type slowly.
If can be a generic example, but I'll likely ask for a little help on sizing various components within the circuit.

 

Check this link, there's a circuit labeled "Electronic Fuse" it's rather simple and you might find it comes in handy.
Problem is, I don't know if the circuit applies to inductive loads too... I'll leave that to the real experts in this forum to decide

 

Check this link, there's a circuit labeled "Electronic Fuse" it's rather simple and you might find it comes in handy.
Problem is, I don't know if the circuit applies to inductive loads too... I'll leave that to the real experts in this forum to decide

Nice link. Let me study that for a bit. It looks like R2 sees full current, so I better make that a beefy resistor. I should probably look for a beefy transistor for Q2, too.....

An earlier poster on here said something about putting the current limiter inside a diode bridge. So would I put that whole circuit on the + line inside the 4 diodes of a bridge?

It will be interesting to see how it behaves with the messy load that is a brush dc motor....

 

Nice link. Let me study that for a bit. It looks like R2 sees full current, so I better make that a beefy resistor. I should probably look for a beefy transistor for Q2, too.....

An earlier poster on here said something about putting the current limiter inside a diode bridge. So would I put that whole circuit on the + line inside the 4 diodes of a bridge?

It will be interesting to see how it behaves with the messy load that is a brush dc motor....

Yes, it is possible to place that circuit inside a 4-diode rectifier bridge so that it will always deal with positive voltage... BUT, as I said, I don't know how it would behave with an inductive load, plus all of the back EMF and commutation noise generated by you motor, as you said... but I think the circuit is worth considering as an option, with the addition of possible adjustments for your application...

 

Yes, it is possible to place that circuit inside a 4-diode rectifier bridge so that it will always deal with positive voltage... BUT, as I said, I don't know how it would behave with an inductive load, plus all of the back EMF and commutation noise generated by you motor, as you said... but I think the circuit is worth considering as an option, with the addition of possible adjustments for your application...

I agree. I think it will be an "interesting" test....

Thanks again.

(no unicorns die today

 

I agree. I think it will be an "interesting" test....
Thanks again.
(no unicorns die today )

It just occurred to me that we could start by adding a TVS diode to that circuit, for instance, to protect it from transients above 40V, which are definitely present in any 12V motor application

 

Sorry for the crappy drawing, but, like this?
I think I have the circuit oriented incorrectly inside the diode bridge.

 

Sorry for the crappy drawing, but, like this?
I think I have the circuit oriented incorrectly inside the diode bridge.

No... let me check and then I'll get back to you

 

More like this:

 

A

More like this:

Awesome. Thanks. You are a gentleman and a scholar.

 

Awesome. Thanks. You are a gentleman and a scholar.

Thanks for the compliments... but regarding analog electronics I'm more like an advanced-level amateur than a scholar...
And as for the gentleman part, you'd have to ask my wife... ha ha ha...

 

Here's an LTspice simulation of a current limiter inside a diode bridge. I used an NPN-PNP (Sziklai) pair instead of a Darlington pair to reduce the forward drop by one Vbe value.
It shows the current being limited to about 4A for both plus and minus 12V with a value of 0.15Ω for R2 (current limit ≅ 0.6V / R2).

 

Here's an LTspice simulation of a current limiter inside a diode bridge. I used an NPN-PNP (Sziklai) pair instead of a Darlington pair to reduce the forward drop by one Vbe value.
It shows the current being limited to about 4A for both plus and minus 12V with a value of 0.15Ω for R2 (current limit ≅ 0.6V / R2).

View attachment 77956

Great circuit... two things:

• Could you please share the .asc file with us? I'd like to experiment with it too
• Would you recommend adding a TVS to it, or would it be overkill?

 

Did I miss something? In the original post, Jhausch said that his motor is a DC gearMotor. Why the emphasis on the bidirectional current limiter? Even if he is reversing the DC gearmotor, a unidirectional current limiter (be it a simple resistor, or something else) can be placed in a part of the circuit where the current flows only one way?

 

Did I miss something? In the original post, Jhausch said that his motor is a DC gearMotor. Why the emphasis on the bidirectional current limiter? Even if he is reversing the DC gearmotor, a unidirectional current limiter (be it a simple resistor, or something else) can be placed in a part of the circuit where the current flows only one way?

The motor/actuator is operated in both directions by reversing v+ and 0v. Does that answer your question or am I missing something?