Need a little technical help with a motor controller circuit. (PWM)

[TheMadMonk]

Not sure if this is exactly the right place to ask this question. If not, mods please put it where it should be. Anyone else.

HELP!

I have an electric billy cart with a dead controller board. I constructed the kit circuit below to replace the dead board and it works well enough for the job except for one small problem. The throttle on the billy cart utilizes a hall effect device and not a pot.

The billy cart also has a handbrake with a switch that closes when applied. I use this with a resistor to discharge C2 and pull pin 4 (inhibit) high. This ensures a gentle start (as the capacitor slowly charges through the 100k resistor) if the brake is released with the throttle advanced.

The output range of the throttle is 950 mV to 4.1 V if V_REF (5V) is used as the supply. (Essentially dropping it in in place of the pot.)

4.1 V is sufficient to achieve the maximum 90% duty cycle of the chip, but 950 mV leaves me with a minimum duty cycle of about 21%, and of course a motor that won't stop.

I think an AA cell between the circuit ground and throttle ground might do the trick, but 1) I'm not sure what effect a negative voltage applied to the circuit might have; and 2) 6.5 Volts exceeds the recommended (though not the absolute) max supply voltage of the hall effect device. 6V & 8V respectively. And I'm not even positive about these figures as I can't get the actual device out to read the part number. (I'm using the datasheet for the UGN3503 as a reference.)

I'm also going to suffer from a significantly reduced top speed unless some of the resistors are changed. Which ones? And what should the new values be?

Can anyone confirm whether this would work, or suggest a different modification to the circuit that would allow me to use the hall effect throttle?

[Fredda Weinberg]

1. Why not a pot, when you'd need an amp for the semi-conductor anyhoo? n/t

[TheMadMonk]

3. Because I have to work with what I have.

The throttle is built into a ring which slides over a lengths of pipe, which in turn slips over a braided rope. It is actuated by a thumb tab The magnets inside the throttle (one either side of the hall device) are the physical end stops. Their just ain't anywhere to put a pot, short of a complete redesign and rebuilding of the hand grip. Putting one elsewhere brings up safety issues.

No amplification of the signal from the throttle appears necessary, I'm getting a 3.15 V variation in output between zero and full deflection. My problem is that at zero deflection (hands off) I'm getting a signal of 0.95 V when ideally I want 0 V. My other problem is that my analog knowledge is 20 years old, and woefully limited anyways, since I was always far more interested in logic circuitry.

[Fredda Weinberg]

5. I've controlled motors, but always had impedance to manage the

current at the extremes. Something to isolate the digital from the analog, even if the voltage didn't need converting. I appreciate the concept and it can work ... a volume control from an old radio, if there's no Radio Shack nearby?

[tech3149]

2. If I read the ckt correctly, changing the 47k to A1 inverting input

coming from the FET's should get you the slower motor speed. Changing the ratio between the divider resistors for A1 + should get you the upper speed you need. You could substitute a 25K trimpot to get an idea of values. Not sure on the hall effect throttle, never looked into them.

[Fredda Weinberg]

4. A trimpot ... indeed

[TheMadMonk]

6. So if I'm reading you correctly.

Replace the 20k and 2.2k resistors with a 25k trimpot and the two 47k resistors with a 100k. Adjust them to approximate the conditions of the circuit as is, (10% and 50% respectively) power up and fiddle. I suppose I should probably be using lower valued pots and putting guard resistors at either end, but I think I'll make do with some epoxy and a little slack at either end of the throttle.

Thanks for your help.

[tech3149]

7. sometimes the simplest solutions are best n/t

[TheMadMonk]

8. Problem solved.

Fiddling with the dividers was a disaster. A complete dogs breakfast. Every time I touched one, I'd throw the other completely out of kilter.

So I tried using the battery for a -1.5 V rail. (After a lot of swearing and burnt fingers while I put the resistors back in the board.)

Not a bloody thing. The reference regulator couldn't manage the extra current the hall sensor drew at the higher voltage AND charge the "soft start" caps at the same time. Would only work if I connected the hall sensor after the board stabilised.

Back down to Jaycar for a 7805. Bolted it back to back with the 7812 and soldered it's in leg to the out of the 7812. (Bit rude to put it on the 24v line and a convenient place to mount it anyway.)

Success. Finally. -ve 100 mV on the hall output. 100 mV - 22 V at the motor output.