mazur50

Hello I am trying to build a motor control with varable brake. and i was woundering if you can take a look and let me know if you see any problems.

Attached is the Diag of what i have so far. I would like to replace Q2 with a mosfet. becasue of size.

What this is a slotcar controler. I want to be able to set the throttle acceleration adjustment with a pot. I have all that working the problem i am having is with the brake circuit. somtimes when i set the pot to the no brake side it burns out. What i need is a pot that i can adjust to change to amount of short/resistance in Q2. so i can go from 0 brake to full brake. I really dont want to use a relay for full brake. as somtime you can fell the delay from a realy

thanks
for your help

Mike

Attached Images

Controller with Transistor Brake small copy.gif (3.2 KB, 27 views)

RadioRon

The darlington transistor Q2 may be burning out simply because there is nothing to limit the current through the base-emitter path when P2 is adjusted to it maximum. This is easily fixed by inserting a resistor in series with the brake switch or by putting an extra resistor from the hot side of P2 up to 12V. This latter idea is probably better, and I recommend a value of about 330 ohms. This would give you a maximum base current of about 36 mA or so, so your collector current should be able to go up to at least 3 amps or more which should be OK for braking some fairly hot motors.

I think that it is dangerous to have Q1 and Q2 wired that way without some way of insuring that they can't both be on at the same time, or you could get a nasty short. Same goes for that full power switch in series with Q2.

PS, use at least half watt rated resistors, or better to use 1 watt resistors.

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RadioRon

ljcox

I agree with RadioRon.
You could replace Q2 with a N chgannel MOSFET, but you would need to calculate the power dissipation.

It is a crude circuit. If it were mine, I would modify it so that Q1 is turned off when the brake is applied.

A simple way to do this would be to replace SW2 with a SPDT switch and connect in the +12 ~ 16 Volt line so it turns off the power to the accelerator and applies it to P2 for braking. But I know nothing about model car controllers, so I may be missing something.

I don't understand what you mean by "as somtime you can fell the delay from a realy"

Obviously realy is a typo and should be relay, but what does "fell the delay" mean?

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Len

Roff

I think "fell" should be "feel".

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Ron

mazur50

Sorry about the spelling i was typing from my phone sorry about that.

Q1 is turning of when q2 is on. also how to is calc the power disapation. i am new to mosfets. and how can i make the mosfet varable like i have q2 now.

Roff

How are you turning Q1 off when Q2 goes on?

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Ron

mazur50

q1 is not really controled by a pot it is a voltage divider wiper block when the wiper are is on bland 1 it is dead and that closes the switch for q2 when it is on band 2 then p1 starts.

ljcox

The power dissipation is the voltage across drain-source multiplied by the drain current.

Remove the transistor & insert the MOSFET such that the gate is where the base was, the drain where the collector was and the source where the emitter was.

Use antistatic precautions since MOSFETs are static sensitive.

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Len