DMR Mosfets..

[Hopewell]

This isn't a 'what am best mosfet' thread, more a query about AB mosfets..

Now, I can see the advantages of AB in a mosfet, as well as thermal resetting fuses BUT, recently I am seeing people recommend that DMRs use mosfets without AB.

Can someone explain why this is?

I understand that AB puts more stress on the motor, but is there anything els eI am overlooking?

The reason I ask, is that I am going to get a MOSFET for my 400fps M14 project, and want to know the ins and outs before I order one!

[private parts]

Re: DMR Mosfets..

I have 1 in my DMR, I thought the AB prevents lock up as it completes each cycle and doesnt over run?

[North West Outpost]

Re: DMR Mosfets..

If you have a decent battery and High torque motor the gear box will not lock up as there will always be enough power/ torque to cycle the gear box. Also with neo magnetted motors and big springs you won't get over run. I have tried and tested them to high 20's with none or only a tiny ammount of over run. Not enough to cycle twice.

The down side with an AB fet is with constant fast single shots you will generate loads of heat. Basic fets control the power delivery, when you add the P fet into the system. They zap the motor with reverse pollarity to stop the motor dead in it's tracks. Unless you have a sophisicated fet it will generate heat.

[sj_asc]

Re: DMR Mosfets..

The soft braked FETs are better because they slow rather than give a dead short, ideally the residual momentum after cut off is dumped as heat but not where it is causing damage. The biggest issue with the traditional AB design is the damage that the brake does to the motor comms and brushes, the braking arc will nearly always be larger than the start up arc. If you can avoid using an AB Device all together then it makes life far easier for the motor. Unfortunately it often seems that people are led to believe they need these devices or they are beneficial in some way or another when
In reality they aren't needed for most setups, all they are doing is decreasing the lifespan of your motor parts.

[No1_sonuk]

Re: DMR Mosfets..

I have 1 in my DMR, I thought the AB prevents lock up as it completes each cycle and doesnt over run?

It'll reduce overrun, but won't guarantee each cycle completes. The only way to do that is active feedback of piston or sector gear position.

The down side with an AB fet is with constant fast single shots you will generate loads of heat. Basic fets control the power delivery, when you add the P fet into the system. They zap the motor with reverse pollarity to stop the motor dead in it's tracks.

Yes, and no. The P-channel MOSFET shorts out the motor making ti try to drive itself backwards. This happens because the moving motor will generate power when none is applied. The power it generates is opposite polarity to what it takes to drive it in that direction. The P-channel MOSFET creates a short circuit across the motor, making the motor attempt to drive itself in the other direction, causing it to slow down rapidly.

I have heard of MOSFET units that do actually supply reverse power, but they would need 4 MOSFETs to do that.