Choosing transistors

Hi, I need guidance on choosing a transistor that when 5v is applied to the base 24v will flow from the collector to the emitter. I have many mosfets designed to handle 24v but I’m a newbie and I don’t want to mess with common ground and fry my Arduino YUN.
Thanks,
Ethan

Hi,

Volts don't flow, Amps (ie. current) flows.

Almost any common bjt or fet will do that for you. Question is, how much current? And do you need to switch on the high-side or the low-side? i.e. will the transistor have to go between 24V and the load, or between the load and ground? It would help if you can tell us what you're switching.

Paul

I am driving two motors with an arduino from a 24v gel battery. Im a little sketchy on amps and ohms and stuff.

ComputerNerd: Im a little sketchy on amps and ohms and stuff.

This site may help, and if you have some $$$ to spare you can't beat H&H.

ComputerNerd: I am driving two motors with an arduino from a 24v gel battery. Im a little sketchy on amps and ohms and stuff.

We will need more than "sketchy" if we are going to help. Do you need to control the speed of these motors? Do you need forward and reverse?

Do you have a multimeter? If so, measure what current flows when the motors are free-running. Also measure the current when the motors are under load, if you can. Even more importantly, what current flows when the motors are stalled? That will be the highest current and the transistors must be able to cope with that.

Also with motors, you must use "flyback" diodes, or the reverse voltage generated when the motors stop could indeed damage the rest of of the circiuit including your Arduino.

I do not need to control the speed or direction. I hope to use the bridge example or console pixel example to control the motors. The sticker on my battery says M24 SLD G and some other stuff but i assume that is more important. I don't have a multimeter but I should get one anyway. The numbers on the motors are part: 49943 Model: 5097-016 serial: 002189.

Is it one of these?

Electric_Statuce_5097-016_800_788.jpg

yep and i was mistaken, the battery is 12v. The motors are mounted on the wheelchair body where all the electronics will go. I will post a picture of the whole assembly.

ComputerNerd:
yep and i was mistaken, the battery is 12v. The motors are mounted on the wheelchair body where all the electronics will go. I will post a picture of the whole assembly.

If its a wheelchair, it is most likely a 24 volt system - generally two 12 volt SLA gel cell batteries wired in series.

This is not a small system - and with the level of knowledge you have indicated you have - you are not going to be able to build your own controller without either spending a lot of money on blowing up parts - or spending a lot of money on already built parts.

Wheelchair motors are kinda variable in current ratings - I’ve seen everything from really low running currents and low stall currents, to really high running and stall currents.

I’m not familiar with that particular motor or its specs - but you are definitely going to need to measure the current. I would not do this with a meter inline with the motor, as the motor may exceed the current measurement capabilities of your meter. Instead, you are going to want to measure voltage drop across a very small value, high-wattage resistor. Something around 0.1 ohm, maybe 10-20 watts would be ok.

Don’t even attempt to stall the shaft (you won’t be able to stop it), but if you have a way to put a large load on it while taking quick measurements, you might be able to get an idea of what its stall current will be. Note that when a motor is at “dead stop” - it is in a stall condition. So - at startup - a motor is “stalled” - and the maximum current it can draw will happen then. Your controller needs to be able to supply this current, if only for an instant. Furthermore, if you can measure the current at that instant, that will tell you the stall current.

Another way to measure the stall current would be to measure the winding resistance of the motor, then divide the voltage being used by that number to get the current. This would probably be the best way to measure the stall current, since it doesn’t require doing anything hazardous with the motors running.

Be aware that these motors can be hazardous - indeed, the whole wheelchair platform can be a danger, if you aren’t taking proper precautions. Note that most wheelchair controllers do not run the motors at 100%, because those motors can move a wheelchair quite fast at that setting. So - when experimenting with the motors - keep the wheels of the chair off the ground, and keep any loose clothing, hair, jewelry, etc - tied back or removed - because if that motor catches on it, it ain’t going to stop - and pain will result. Do this until you know you have complete control over the system (all hardware and all software). Even then, be sure to design in some form of a failsafe switching system (do this right at the beginning, btw - much easier then, than later) - to allow you to hit a switch (both on the platform and remotely) to kill all power to it.

So, how do you think I should drive the motors?

The suggestion appears to be - choose a number of simpler projects before you approach this one as you clearly do not at this point, have the necessary skills.

So - if you need this any time soon, just go buy the complete motor controller assembly that is sold for this particular unit.

I did complete everything but this for my robot. I took it apart, got an arduino, learned the programing, got the control program working, and setup a live webcam.

Nevertheless, this part (power electronics) is pretty much a "whole new ball game" and not for the faint-hearted, as you need to consider circuit isolation and adequate conductor gauge. You definitely need power FETs and not BJTs.

You almost certainly will want speed control and would likely be much better off to use a ready-built module for the purpose.

Oatley Electronics here in Australia, seems to have stocked such things in the past.