I will be driving a few lamps, just a small 12v 30 to 100milli-amp max. I had thought of using the same Logic level fets I am using for my H-Bridge as the drivers, they are IRLML2502 N Channel. If I use a 10K resistor on the drian tied to the plus rail I should be ok right ?

My thinking behind using Fets is once again like the H-Bridge, don't need to use any pull-up/down resistors on the gate so it makes everything simple.

the PIC is 16F seriers running at 5V. the Drian of the Fet connects to a 12v rail that also supplies the H-Bridge. Everything has a common ground.

 

Hi....If you are using the FET's as source followers I.E. drain to +ve, I think you will need P chan FET's....Also if using MOSFET's driven by a PIC, you it is a good idea to use a pull-down resistor 22K to 100K from the gate to ground..... The reason for this is that whilst the PIC is booting up, the outputs will be in a high Z state and this would leave the gates floating, during this time the FET's could do all sorts of things......Daniel.

 

You could consider a ULN2003A - 18 pin IC containing 8 darlington buffers with input resistors built in so it can be connected directly to the PIC.

Output capabilty is 50V max, 500mA per channel max, 2.5A total max, hence 8 lamps of 100mA should be no problem.

Very little wiring / soldering required and they are nice and cheap.

They also have built-in freewheeling diodes to allow them to drive inductive loads!

If you need details you can look it up on the texas instruments web site:-
http://focus.ti.com/docs/prod/folders/print/uln2003a.html

Steve.

 

Thanks, OK I will look at P Channel, yes I had the drain connected though a 10K resistor going to V+. So how do I work out weather to use 10k or 100k for the pull down resistors. I guess I should also put these on the gates of my Mosfets for the H-bridge as well since the PIC does funny things on startup. I don't really want a high and low side fet on the same side of the bridge turning on and creating a short to ground.

 

I agree with Steve about the darlington array. I have used them before to drive some relays. It is simple and your part count stays low. If all 8 are driving 100mA it might get a bit warm unless you have plenty of copper plane connected to the ground pin. I am assuming you want the smd version.

 

I guess I should also put these on the gates of my Mosfets for the H-bridge as well since the PIC does funny things on startup.

The P-ch will have pull-ups on the gate while the N-Ch will have pull-downs-- assuming you want them to be off when power is applied.

I don't really want a high and low side fet on the same side of the bridge turning on and creating a short to ground.

How are you preventing this "shoot-through"? Are you using software to generate some dead time?

John

 

I don't mean to hijack this post but the problem I am experiencing is being discussed here so I hope its okay.

I am using a pic controller powered by a 9v battery via a 5volt regulator, it controls a solenoid which is switched with a MOSFET. When the system powers up it momentarily fires the solenoid, but only if the circuit has been powered down for a little while. If I turn it off then on right away its fine. I started out with a 10k pull down resistor to ground on the gate and it seemed to be worse so I changed it to a 1k resistor and it helped. Does anyone have any ideas that could stop this, is it more of a power supply issue maybe?

 

@chris101
Could you post a schematic of your circuit and set-up?

 

Just a few thoughts since I have also experienced apparently inconsistent operation before.

Presumably you are powering the system down for long enough to force a cold restart by the PIC, it is amazing just how long a decoupling or reservoir capacitor can keep the processor powered up for.

If the processor is not resetting properly on subsrequent occasions, it could be a software glitch that pulses the output port during your initialisation code but then resets it later on during the startup sequence.

e.g. as far as I know the PORTB register value is undefined at boot so if you set a bit to output mode by setting its corresponding bit to 0 in TRISB before clearing its bit (or setting if your output circuit uses inverse logic) in PORTB you could get a glitch at the output.

If not done already you could clear the Power-up Timer Enable bit in the configuration register to enable the timer and allow power to stabilise before the PIC starts executing code.

Steve.

 

OK, I will explain what I have done so far. I don't have a schmatic. I have been doing everything on a bread board.

OK, so far i have a H-bridge, it uses IRLML6402 P Channel Fets for the high side, and IRLML2502 N Channel for the low side. (These are logic level Fets so I thought I could drive them directly from the PIC I/O). I was also using some other N Channel FET's as drivers to power some small 12v lamps, (lamps draw about 50milli amps on average). I was going to connect the gate of the N channel to the PCI I/O, the Source directly to ground, the drian to 12+ via a 10K resistor, and the lamp between the Drian and resistor. But someone suggest I use some BCR108 transistors to drive the lamps as the are cheaper and also easy to use.

So far the H-bridge has no pullup pulldown resistors at all. How do I work out the vaule for them, the H-Bridge is 12v, the PIC is 5v. You suggest in this post to use between 10k and 100k, how does one workout the correct value?

 

What is SHOOT THOUGH NanoVate ?

 

Shoot through is the condition when both the MOSFETs on the same side are on so there is a "short-circuit" from the supply straight to ground instead of through the motor.

 

Nanovate, I am using hte PIC16F685. This is one of the ECCP verions, it has PWM on 4 of its I/O, and it will not allow shot though. It can also brake the motor. I would say its somehow hardcoded into the PIC to not allow this condition. Also, Lighting man or Nanovate I tested my circuit with the H-bridge, using Lightingmans suggestion (between 10k and 100K) and putting in 10k pullup and pulldown resistors. Everything worked correctly. BUT, I would like to know what is the MATH to work this out properly. I feel there is a big difference between 10K and 100K. If the PIC is set at 5V, the I/O will be 5V. The PIC has a rating of up to 25milli-amps per I/O. (I think it stated 90milli-amp for one I/O port. Anyhow, if the bridge is 12v what is the MATH to work out the correct resistor vaule.

 

This is one of the ECCP verions, it has PWM on 4 of its I/O, and it will not allow shot though

It will allow shoot through if you do not make sure your software doesn't turn on both MOSFETs at the same time. N-CH and P-CH MOSFETs have different turn-on and turn off times -- N-CH are usually faster. Also if you look at the datasheet you'll notice that an individual MOSFET will have different turn-on and turn-off times -- turn-off being slower in most cases.