Kiwi Bruv

I built the circuit out of bits and bobs so now I need to buy a new amp I might as well build it out of an LM324 quad. Will this allow me to have the pwm at 20kHz? Where do I look in the datasheet to show me this? Circuit I have in mind is at www.cpemma.co.uk/pwm_erg.html

OK, I'll change the op amp. The pnp is to make the 555 timing cap charge linearly instead of exponentially so it gives a sawtooth output.

Thnaks Audioguru, one more question if you please. I am finding that up to about 15kHz the full 0 to 100% duty cycle is covered by the full range of the 10K pot. As expected the 555 puts out 4 to 8V so the pot divider at the comparator is designed to duplicate that range. However at 25kHz the duty cycle is behaving strangely. 100% duty is at about 60% of the pot range ie 100% duty has shifted from 4V on the pot to maybe 6V and the range on the pot from 100% to 60% duty seems sharp or compressed, but after 50% it's smooth again. It's like the 555 is only putting out 6 to 8V at higher freq. I don't have a scope so can't see it. I wanted to operate the drive out of audible range so would liek to get it to 20Khz. Can you see why this might occur?

Found it, wired the mosfet and driver trans wrongly. Both working now though! Sorry for the bother.

Audio guru, I don't think I explained properly. When the circuit powers up the bulb comes on fully. The signal is oscillating at the gate at 50% duty cycle but the mosfet isn't switching. I can remove the drive trans completely (source gate res remains in place) and the load remains on. So I thought it was the mosfet. I built the straight 555 astable to test the mosfet and as described the mosfet is switching at 50% duty in that circuit. Many thanks for your help

I have built a simple PWM drive with a 555 and an op amp. The op amp drives a trans which drives a mosfet. However the mosfet seems to be latching. The mosfet is conducting, with no gate signal present, a test bulb lights brightly. When I apply the PWM to the gate the bulb dims as it should. I'm using an IRF 530. I simplified the circuit just driving straight from a 555, again it works fine until the gate signal is removed when it lights the bulb brightly again. PS is a 12V batt. I put a 91 ohm resistor in the gate line and a 100 microF cap across the power supply, but it made no difference. Can someone help me fix the problem?

Thanks. I've done alot of reading over the last week and now have a much clearer understanding of DC motors. I know I don't need a micrprocessor for the project but it just makes it more flexible and interesting for me. I am going to start with a motor & on off switch, then a PWM control and then see if I want to/ need to refine it further.

Thanks Guru, the chain tension switch is a great idea. I am beginning to think an electic assist bike is just a name for an underpowered electric bike. If you think of the rider as 100W motor. When the other motor is driving at a set speed and the 100W motor kicks in and takes some of the load the other motor would slow down an amount exactly equal to the load taken up by the 100W motor for the same speed. So it's just a balancing act around the set point of the other motor. The assist level is really the same, it just happens at different speeds. I wonder though how some commercial bikes advertise a range of assist levels 0.5 assist (regen or charging the batteries) and 1 assist up to 4 assist, these being equivalent to extra riders. I think these must be tied to speed.

Alun, I've been thinking about this and looked on the 4QD site for some explanations but I still don't understand how the voltage and current can be controlled seperately. PWM will control the motor speed by applying bursts of full voltage (and current). Current limiting seems to work by turning down the PWM and therefore the speed. So current limiting will also reduce the speed won't it? I can see how I can use my microcontroller to do all this but don't get the principle. Any comments or references gratefully received.

Thanks Alun, Suprisingly the mechanical part is quite straight forward. There are a lot of scooter and bike parts available now. I have welded a cog in place of the 1st gear on the back wheel. The motro drives this cog and the rider has the other 12 gears to choose from. The motor then sits under the riders seat, fixed to the two struts. The chain and cogs are standard electric scooter parts. I have an 11.65 to 1 ratio so if the motor is doing 2000 rpm the wheel is doing 171 rpm or about 20 km/h I know my concept isn't quite right. Ideally I need to measure torque but this will get too complicated/expensive I think. Also this is an assist machine, so I can keep the motor and drive chain relatively small. I can't quite figure out what "assist" means electrically. If you imagine riding along and someone starts pushing you gently, you take some pressure off the pedals and maintain speed, this is the effect I want. So after your clear help I'm thinking the rider determines the speed set point by pedalling at the appropriate speed, the processor then controls the motor to this new speed and also controls the current applied depending on the level of assist selected (might be 3 or 4 levels) I know how to build a PWM speed controller but not how to control the current through it. Do you have any references? Many thanks

Hi, I am building an electric assist bike and need some help with the concept as I don't know much about motors. It will be a 300W 24V DC motor, driven by PWM with a microcontroller helping. I want the level of assist to be dependant on the rider - not have it pre-set by setting the motor speed. The rider selects an assist level which remains fixed but proportional to the riders effort. The motor drives a seperate cog to the rider (ie there are two chains). ie At 50% assist the motor will attemp to drive the bike at 30 rpm if the rider is wanting to maintain 60 rpm. If the rider changes up a gear and is going at 70rpm the motor will adjust to supplying 35 rpm of the effort. If the rider freewheels the motor turns off. It's speed is set by the rider pedalling at a set point At 66% assist motor drives bike at 40 rpm if rider goes at 60 rpm At 90% asssit the motor drives bike at 54rpm if rider goes 60 rpm At 10% assist the motor drives bike at 6 rpm if rider goes 60 rpm (In practice there will be a high and low limits the motor won't be permitted to exceed ) Imagine the rider maintaining 60 rpm in 50 % assist, motor is driving bike at 30 rpm. As the rider approaches a junction they slow down but keep pedalling. As they slow or change gear the motor slows proportionally - at 40 rpm, motor drives bike 20 rpm, at 20 rpm motor drives bike at 10 rpm until the low rpm cut out stops the motor assist or the rider stops pedalling. Same in reverse when pulling away. So the motor will be "over driven". My PWM circuit will be tuned to drive the motor at a ratio less than the motor is actually going. The rider will be driving the bike and the motor. Does this sound OK, can I damage anything? Will the motor act as a brake? All comments welcome.

OK, thankyou

I am building an electric bike. I am going to build a PWM circuit using a 555 chip to generate the PWM signal and and MOSFET to switch the motor power supply. I will drive it using a variable resistor initially and later a Picaxe microcontroller so I can do some fancy stuff. My motor is 300W 24V. What is the best frequency to drive the motor at and why? How do I size the flywheel diode? How do I decide the size of cap to put across the motor terminals?

Thanks. I'm now using an LF353 at 12V, just because I had it available. I have 13mV to 1300 mV op amp output. It also seems to work OK down to 9V. I doubled the transmitter power to 180mA and this made a big difference in stability. Output now starts at 9mV at 200mm from the wire and goes up to 1700 mV on the wire very smoothly. My microcontroler analogue input will resolve to 5mV so I think it's in a usable state now. I think I have it in a position to use with my robot. I will build the rectifier circuit tonight and try your other suggestions. Many thanks with all the help. This is my first analogue circuit and it's been a challenge! I've learned a lot and now it's working I can play with it to learn more.

Thanks guru. I see how the rectifier would work and I can use that as an input directly to my microcontroller. I have reduced the power supply voltage to 6v , but I can't get my multimeter to show a voltage change as the sensor approaches the wire - but the audio output says different? I've attached my detector circuit. I have not built the meter part, my circuit finishes at the earphones connection.