I have had a single pair version pushing a 12" Peerless XXLS to high excursion levels, with the diodes mounted on seperate, small/minimal heatsinks, and they didn't even get warm. I really don't think most people will be pushing a V2 or V3 hard enough to be a problem. I'll post a pic of the single pair version when I get home. You will get a laugh when you see the diode heatsink.
Here is my proof. I made the magic smoke. I have been running the F5T on smaller heatsinks for over 2 months, with diodes separated from main sink, Tonight i mounted them all on same sink and viola. Same bias as before of 400mV across Rs. Going to mount another and make sure the fet had good heatsink contact, but they had been resting safely at about 360mV before I raised them the final time. The burn happened in just a second or two, so it seems obvious it was thermally based runway. My suspicions are the diode temperature. By bringing the diodes to the sink temp, you leave yourself with less leeway in terms of temp variation. The likelihood of constant pushing into diode territory, except on extreme transients is not high. If they are adequately heatsinked of the main sinks, I don't see them having a problem. I will watch with temp probe this time.
Same board as post 313
Same board as post 313
Attachments
WOW you can join the NoSmoking club,,,
Hi Buzz,
I hate that happened to you,But now you know how to get in the club,lol.
Heatsink temp cutoffs are your friend,I bought mine from a AC/Heating supply,they are cheap and can be adjusted to temp. There is 2 types just like Thermistors negative and positive going,I use the positive going ,too hot and they cut the power,,,,
Cheers,
NoSmoking,
Hi Buzz,
I hate that happened to you,But now you know how to get in the club,lol.
Heatsink temp cutoffs are your friend,I bought mine from a AC/Heating supply,they are cheap and can be adjusted to temp. There is 2 types just like Thermistors negative and positive going,I use the positive going ,too hot and they cut the power,,,,
Cheers,
NoSmoking,
Tomorrow will be a good day!!!!!
Hi Buzz,
Luck has been with me lately not burned up nothing,The new resistors don't smoke much ,lol,you got to put the meter on them to see if their toast,lol,they used to smoke and burn,I have seen some weird caps explode or just speeewww!They are a mess to clean up too,
So on a serious side are the diodes MUR3020wt used as a current source or more like a limiter,I guess I don't understand their benefit,could you explain?
Thanks,
NS
Hi Buzz,
Luck has been with me lately not burned up nothing,The new resistors don't smoke much ,lol,you got to put the meter on them to see if their toast,lol,they used to smoke and burn,I have seen some weird caps explode or just speeewww!They are a mess to clean up too,
So on a serious side are the diodes MUR3020wt used as a current source or more like a limiter,I guess I don't understand their benefit,could you explain?
Thanks,
NS
I am not sure about the thermal runaway theory. The thermal mass of the heatsink would not permit thermal runaway within a second or two. I suspect something else. How are you attaching your thermistors? I burned up fets on my first build due to a thermistor shorting to the drain lead of a MOSFET.
THis is the current path. I had created tension that was lifting the fet slighlty at that lower end. All fets and diodes will be mounted first and then soldered. THe thermistors are underneath the board and free from contact with anything but the heatsink. They are bent out and when the board is placed, it creates a natural spring tension on them. It is still my beief that seperating the diodes from the heat of the sinks is a good idea.
Hi Buzz,
Luck has been with me lately not burned up nothing,The new resistors don't smoke much ,lol,you got to put the meter on them to see if their toast,lol,they used to smoke and burn,I have seen some weird caps explode or just speeewww!They are a mess to clean up too,
So on a serious side are the diodes MUR3020wt used as a current source or more like a limiter,I guess I don't understand their benefit,could you explain?
Thanks,
NS
Simplest explanation is thta they act sort of like a bypass cap you see in tube amps. Shorting the signal to the ground and increasing available gain. Difference is, that they also have an affect on DC bias, since they entire AC signal is not shorted. Sliding bias is a term that has been thrown around. Perhaps a more astute member could give a more thorough explanation.
interesting-nobody asked Papa to give us pic of his F5T ;
that will give us insight how he addressed that issue - diodes on sink , or not
He said he only officially built V1. Probably loosely asssmebled on test sinks or something. I am probably wrong, but even Jacco recommended them on seperate sinks, suggesting the exponential portion of the diode curve is more greatly affectd by the temp than anything else. Adequate heatsinking is hard thing for DIY member.
EDit:
PLease defer to Nelson and ZM, I trust them a great deal more than myself when it comes to this sort of thing.
EDit:
PLease defer to Nelson and ZM, I trust them a great deal more than myself when it comes to this sort of thing.
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Now that ! crossed my mind yesterday while trying to design a separate heatsinking arrangement for my contraption mounting of boards.
I seem to remember reading somewhere on the beginning of the F5T thread that diodes should track thermal working profile of the Transistors they couple with ?!
But apart from not being sure ...this practical evidence also would advise against that ...
Your thoughts on that ZM ?.....and don't say you don't have any !!!
I can see thermal tracking of the fets if the result was bias containment, sort of like thermistors, but if tracking results in bias increase? ZM is absolutely right in one regard. Nelson would not release this unless it was thoroughly tested. I simply suggest very attentative heatsinking. In my V3 build, I will have 4 fets within a 160mm of each other. That is a lot of heat to get rid of.
Agree on all counts, i like to 3d simulate my builds exactly to try to avoid unpleasant surprises even my first " by the book " F5 was done like that and worked like a charm first time...
Lets see what mighty ZM thinks.....( getting that sinking feeling that he's going to tell me that he doesn't ...heheh
).
