Missed the fact that the traces were generated using a 2 ohm resistive load ... would be different with inductance included.
Ah, I thought that was a general recommendation. I haven't used MOVs before, would you just put it across the load like the diode? And what specs would apply in this case?
Yes, directly across the load is best. I've used them successfully for many applications - mainly transformers in AC circuits. Since a transformer and motor are closely related ... hence my suggestion. For sizing, the first spec. I look at is the working voltage. In a 120VAC circuit, I use MOVs rated for 150VAC continuous, the clamping voltage is higher. Then I look at the surge current rating and energy rating.
If the 12V supply is regulated, then any MOV with a continuous DC voltage a few volts higher than this would be OK. They're even simpler to use than a diode because polarity doesn't matter.
So as a working recommendation, the one you mentioned (B72210S1140K551) with:
Varistor Voltage (Min) 19.8V
Varistor Voltage (Typ) 22V
Varistor Voltage (Max) 24.2V
Current - Surge 500A
... would probably be OK for a 12V motor?
But what does this mean?
Maximum DC Volts 16VDC
Is that basically saying that it is basically open circuit up to 16V but then conducts (up to 500A) starting at 19.8V?
Yes, that's how I understand it ... it works like back to back 16V zener diodes with a bit of series resistance. There are some detailed graphs somewhere SIOV metal oxide varistors
back to back zener diodes
See Transorbs:
Aren't the tradeoffs being discussed exactly why many manufacturers offer MOSFET gate driver ICs? Maybe something to add at some point is how performance characteristics change when using one of those, though choosing one that is representative of driver ICs as a whole might be difficult.
Would a lower value for R2 help here? Any suggested values?
Have you tried using a pot to vary the resistance to see if you can find a "sweet spot" for the resistance?
Also have you tried reducing the value of R1 some?
This might be a bit confusing now, zoomkat, as he's modified the schematic to reflect changes made. Originally, both R1 and R2 were 10k.
LarryD:
See Transorbs:
Transient-voltage-suppression diode - Wikipedia
Well, the interesting thing to come out of this discussion has been the transient suppression. Like many people I have read all over the place to "put a diode over an inductive load" but now there are lots of suggestions about types of suitable diodes, MOVs and now Transient-voltage-suppression diodes.
http://zone.ni.com/reference/en-XX/help/375472A-01/switch/inductive_load/
From the above page:
On a side note: 1N4001 is a bit slow for this application. I usually see 1N4148
I initially had a 1N4148 however:
Grumpy_Mike:
I would also use a 1N4001 diode, that one is just a signal diode.
So it seems that agreement on the diode type is hard to reach.
Other pages mention how useful the diode is without specifying how to choose one:
zoomkat:
Have you tried using a pot to vary the resistance to see if you can find a "sweet spot" for the resistance?
I am trying to have a mathematical basis for choosing values, so that if someone has a different size load (eg. 500 mA or 5A) they have a way of calculating what ought to work, without just using trial and error.
If I had an o-scope I'd use pots so I could make adjustments and see what is going on real time. I'd try driving the 2N3904 base with maybe a 250 ohm resistor to ensure that the voltage drop across the 2N3904 collector/emitter is as low as possible. This would help keep the PNP MOSFET gate at its lowest possible voltage.
I am trying to have a mathematical basis for choosing values,
I hope you are good with calculus and differential equations. Any of the components whose electrical characteristics are expressed as "curves" will involve complex math to get hard numbers.
Rules of thumb and merely looking at the curves can get you pretty close.
If your transistor switching circuit is so touchy regarding parts values, you've done something wrong.
It seems to me, your calculation in reply #5 needs to include the apparent capacitance of the FET gate.
How would you factor that in?
This P-Channel MOSFET is fast and powerful (and well, uhm, costs more):
Parameter FQP47P06 IXTH96P085T Units
Turn-On Delay Time 110 23 ns
Turn-On Rise Time 910 34 ns
Turn-Off Delay Time 210 45 ns
Turn-Off Fall Time 400 22 ns
RDS(on) 26 13 mΩ
ID -23.5 -96 A
VDSS -60 -85 V
Ouch! $6. Not much point in getting a stinking fast MOSFET if the circuit driving it isn't going to switch it faster than 5uS anyway.
I've added more to my thread describing using BJTs for switching, if anyone wants to check the calculations.
polymorph:
Ouch! $6. Not much point in getting a stinking fast MOSFET if the circuit driving it isn't going to switch it faster than 5uS anyway.
Hmm ... I thought it was the slow MOSFETs that begin to "stink"
- high switching speed = high power conversion efficiency
- 50% RDS(on) = 50% less steady state power loss
- Larger safe operating area
- Cost savings on heatsink requirements
- Much lower power loss from DC to 5µs (or faster)