Function Generator vs. Pulse Generator - Page 1
Forum section: electronics / testgear

author: larsdenmark
author_rank: Regular Contributor
timestamp: September 26, 2014, 11:38:18 am
content: I have started to look into motor control using PWM. I'm just using small hobby motors and in order to easily set something up I would like to use a function generator or pulse generator (as opposed to rewiring and programming an Arduino).Can both a function generator and a pulse generator be used for this? Which is most suitable? I would like to be able to test duty cycles from 20% to 100%.How well protected are function and pulse generators? Can I generally drive a small hobby motor (e.g. 6V, 1A) directly from the output?In case anyone is wondering: I would normally drive the motor using an optoisolator and a MOSFET, but it would be nice to be able to drive things directly - if that doesn't destroy anything.

author: jpb
author_rank: Super Contributor
timestamp: September 26, 2014, 12:22:36 pm
content: Quote from: larsdenmark on September 26, 2014, 11:38:18 amMost general purpose function generators have 50 ohms internal resistance and can drive up to around 20V into high impedance at low frequency. So if your motor looks like 6 ohms (6V and 1A) then the maximum current will be around 20V/(56 ohms) which is only around 350 mA and it might not be happy with that. So I guess you'd need some sort of buffer like a power FET.Otherwise generating PWM from function generators is something that they are happy to do in the duty cycle range that you mention (20% to 80%) some of them have problems when the duty cycles become more extreme.You're not looking for particularly fast rise and fall times and you don't seem to need multiple channels so you don't particularly need a pulse generator and signal generators with lots of memory might allow you to try varying speed and so on. It all depends on your budget.

author: oldway
author_rank: Super Contributor
timestamp: September 26, 2014, 12:51:56 pm
content: In my opinion, you do not need nor a function generator nor a pulse generator to do what you want to do.A simple circuit with TL494 and 2 potentiometers (one to adjust the frequency and the other for the pulse width) do what you want at very low cost and without the risk of damaging an expensive test equipment.

author: larsdenmark
author_rank: Regular Contributor
timestamp: September 26, 2014, 06:23:33 pm
content: Quote from: oldway on September 26, 2014, 12:51:56 pmI haven't used a TL494 before, but that should be easy enough to do some experiments with (and it's available on taydaelectronics).Maybe I'll wait a little longer before I get a function generator.

author: electronic_eel
author_rank: Regular Contributor
timestamp: September 26, 2014, 08:34:27 pm
content: I suggest a SG3525 instead of the TL494. The TL494 is more common, but the SG3525 has true push pull outputs that the TL494 lacks. So the SG3525 is better suited to create a variable PWM / Freq. generator.

author: oldway
author_rank: Super Contributor
timestamp: September 26, 2014, 08:59:35 pm
content: QuoteNot really a problem: using the 6 buffers of a CD4049 or CD4050 in parallel, you can sink and source far more than 60mA, enough to drive the gate of a Mosfet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 26, 2014, 10:46:18 pm
content: TL598 is the totem pole equivalent to TL494.

author: oldway
author_rank: Super Contributor
timestamp: September 27, 2014, 07:53:13 am
content: No need to buy a TL494. You find it for free in every old PC power supply.Have a look on preregulator switched power supply of Tektronix 2235 / 2236 scopes.You will see that totem pole is not needed.(NB: TL594 = precision TL494 with same pinout)