The relation between the pwm frequncy and the rpm of a dc motor

jpanhalt

Joined Jan 18, 2008
11,087
There are tomes of material on gate drivers. Probably not a "best" way. If it works, use it. Your application is not that demanding.
 

shortbus

Joined Sep 30, 2009
10,045
Not at all meant toward this TS alone, but have the rest of the participating members ever noticed that new members that use a picture of themselves as an avatar are the most resistant to taking advice to their questions/problems? Just an observance over time on the forum.
 

Audioguru again

Joined Oct 21, 2019
6,673
Aragon, in a previous post I said that the datasheet for the IRF520 says that with a Vgs of 4V it is at its maximum threshold voltage when it barely turns on only 0.25mA. It turns on fully when its Vgs is 10V that you do not have. I said to use an IRL520 that is fully turned on when its Vgs is 5V.
The IRF540 is exactly the same. You need an IRL540 instead that turns on fully when its Vgs is 5V. You need a Mosfet that is called "logic level".

Another problem with your -5V to +5V gate drive is that it takes a longer time to charge the gate-source capacitance from -5V to +5V than if the drive is 0V to +5V driving a logic level Mosfet.
 

Thread Starter

aragon1971

Joined Apr 7, 2008
141
Aragon, in a previous post I said that the datasheet for the IRF520 says that with a Vgs of 4V it is at its maximum threshold voltage when it barely turns on only 0.25mA. It turns on fully when its Vgs is 10V that you do not have. I said to use an IRL520 that is fully turned on when its Vgs is 5V.
The IRF540 is exactly the same. You need an IRL540 instead that turns on fully when its Vgs is 5V. You need a Mosfet that is called "logic level".

Another problem with your -5V to +5V gate drive is that it takes a longer time to charge the gate-source capacitance from -5V to +5V than if the drive is 0V to +5V driving a logic level Mosfet.
yes in find a LOGIC MOSFET IRLB8721 ALSO I CAN ADD AND OFFSET VOLTAGE (+5) AND THE PULSE IS BETWEEM 0 +10V
 

sagor

Joined Mar 10, 2019
903
this is a good choice for low voltage
View attachment 221587
I would use the IRL540 instead, it is logic level compatible. At Vgs of 4.0V, the RDSon is 0.11 ohms, and can handle over 20A at 100V. At Vgs 5.0V, RDSon is only 0.077 ohms.
The key is to use a logic level MOSFET. Many of the IRF540 series (520, 530, etc) also have the IRL series of logic level MOSFETs. Only issue however, is that maximum Vgs is +/-10V, not 20V like most other IRF5xx series. You have to make sure your gate voltage does not approach 10V or more.
 

Thread Starter

aragon1971

Joined Apr 7, 2008
141
I would use the IRL540 instead, it is logic level compatible. At Vgs of 4.0V, the RDSon is 0.11 ohms, and can handle over 20A at 100V. At Vgs 5.0V, RDSon is only 0.077 ohms.
The key is to use a logic level MOSFET. Many of the IRF540 series (520, 530, etc) also have the IRL series of logic level MOSFETs. Only issue however, is that maximum Vgs is +/-10V, not 20V like most other IRF5xx series. You have to make sure your gate voltage does not approach 10V or more.
and the IRLB8721PbF have Very Low RDS(on) at 4.5V VGS
 

DarthVolta

Joined Jan 27, 2015
521
In general, for MOSFETS, and JFETs, how constant is Rds, as you vary voltages, currents, and frequency ? So far when doing JFET calulations, I ignored Rds. I haven't tried MOSFET yet...still. That should be on my real short list of thing's to learn next.
 
It would be linear, as you imagine, if it were:
PWM pulse high: instant full current and thus full torque.
PWM pulse low: instant zero current and thus zero torque.

But just like things, you can't speed them up immediately, or slow them down to 0. You're going to feel very clearly that when you run into a wall, Inertia prevents that. Much as inductance prevents immediate shifts in current flow.

So in fact it's like: PWM pulse high: the current starts to flow before it's fully formed. Rising-time depends on inductance. As the current increases, the torque rises to a steady state.

PWM pulse low: the current begins to reduce and dissipates, e.g. The diode of the freewheel. That way, the engine functions as an alternator, producing a 'negative' torque, or rather, a torque in the opposite direction.
 

shortbus

Joined Sep 30, 2009
10,045
Up to 49 posts in this and the real reason for the problem could still be found in post #10. A BLDC fan doesn't like it's power supply pulsed, it's looking for a steady DC voltage. The TS has never answered to what kind of fan he has, but any that have been made/sold for quite a while now are BLDC ball bearing fans.
 

Thread Starter

aragon1971

Joined Apr 7, 2008
141
i run again the lab with IRLB8721PbF and a normal dc motor not (brusless)
according to data sheet the IRLB8721PbF is full open with voltage 4.5 V The rpm decrase with increase of frewuincy
 

shortbus

Joined Sep 30, 2009
10,045
a normal dc motor not (brusless)
This is a different motor? Or the same one? If it is the same one, how do you know it's not brushless? Just because it has only 2 wires to it doesn't mean it can't be brushless. Show a picture, a link or a part number of your motor.
 

Thread Starter

aragon1971

Joined Apr 7, 2008
141
This is a different motor? Or the same one? If it is the same one, how do you know it's not brushless? Just because it has only 2 wires to it doesn't mean it can't be brushless. Show a picture, a link or a part number of your motor.
i have a video in my last post and you can see the mootor

1605197008646.png
 
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