ULN2803 as driver for 2.3A stepper motor

[uplb]

Please pardon my ignorance.

ULN2803 have an output of only 0.5A. However, I have a stepper motor rated at 2.3A (2.31volts).

Is it possible to connect five ULN2803 in parallel to have an output of 2.5A? If so how would I do this?

Thank you.

[irving2008]

Yes, in theory you can just parallel up inputs and outputs but remember that each output will dissipate heat (1.5v saturation voltage at 0.5A = .75W) The thermal characteristics are 55degC per W junction to ambient so the maximum total package dissipation at 25degC ambient and junction at 125degC is just under 2W, so you cannot parallel more than 2 outputs in the same package at full current.

You could use individual high-power darlington transistors such as TIP121 and even then they'd need a decent heatsink.

Your best bet for these sorts of power requirements are MOSFETs as they have extremely low power dissipation when switched on. Without knowing more about why you want to drive the stepper motor directly rather than use a commercial stepper driver board (or one of the many existing DIY designs found here) its hard to advise further.

[uplb]

Thanks for replying. I can not use existing DIY driver designs or buy commercial drivers since I live in the Philippines and shipping costs and duties are quite prohibitive. I only cheap driver I that is available here locally is the ULN2803.

Again, another stupid question (sorry), is it possible to just dip the whole ULN2803 driver in mineral oil to dissipate the heat (please don't laugh)?

[irving2008]

Originally Posted by uplb Thanks for replying. I can not use existing DIY driver designs or buy commercial drivers since I live in the Philippines and shipping costs and duties are quite prohibitive. I only cheap driver I that is available here locally is the ULN2803. Again, another stupid question (sorry), is it possible to just dip the whole ULN2803 driver in mineral oil to dissipate the heat (please don't laugh)?

why should I laugh, its a lateral thought to solving the question and oil cooler baths are used on big CPUs. Unfortunately it wont help. Although the thermal resistance of mineral oil is 1/10 that of air (5degC/W against 50degC/W) it only increases the maximum power dissipation to 2.1W from 1.5W as the limiting factor is the thermal resistance of chip to case.

I'm not sure I understand where the difficulty in getting parts is. A quick Google reveals Farnell Philippines, and while they may not have everything that Farnell UK has (although they claim to be able to ship anything from out of US, UK or Singapore) they do seem to have most stuff. They list, by way of example, a L297/1 stepper controller at 934php, or roughly £11 sterling (against £7 in UK) and an L298 driver (4A capable) at 498php (roughly £6, about £4 here).

Your ULN2803 is 60php from Farnell while an IRFZ40PBF MOSFET is 70php (which is cheaper than the Farnell UK price!).

All of these prices are ex-VAT (12%) but delivered in Philippines. Delivery is free over 2500php, which is easily achieved with parts for a 3-axis driver.

[Mariss Freimanis]

Think twice about connecting the devices in parallel. Bipolar transistors have a negative temperature coefficient. That means gain increases with temperature and saturation voltage (collector to emitter) decreases with temperature.

Why does this matter? One device out of the 5 will have a slightly lower saturation voltage. It will conduct slightly more than 20% of the total current. This will cause it to get warmer than the other 4 devices. The extra temperature will decrease its saturation voltage even more, causing it to carry an even higher current, causing it to warm up even more,.... This circle continues until the part fails. It's called current hogging or thermal runaway and it occurs rapidly.

MOSFETs have a positive temperature coefficient. The hotter a MOSFET gets, the less current it will carry. This makes MOSFETs safe when they are connected in parallel; they share a load evenly.

Mariss

[uplb]

Thanks Irving2008 and Mariss.

Wow. Never thought that we have Farnell Philippines.

Thanks for mentioning the futility of my proposal. I think I will just stick with the L297/L298 driver.

[cjgaus]

Just curious to what software ya used to run this cnc board?