DHT Heater using VCCS - PCB or Kit (Possible Group buy)

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izzy wizzy
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#1156 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by izzy wizzy »

I wouldn't install heatsink lower than the caps. I did install some of mine end on but it was in the prototype. The final layout has them board down horizontal. Didn't use terminals to start but have them for the final as it's assembled in modules.

Ultimately it depends on how hot things get and how close they are and how much ventilation there is to remove the build up of heat over time.
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#1157 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

izzy wizzy wrote: Fri Aug 06, 2021 6:50 am I wouldn't install heatsink lower than the caps. I did install some of mine end on but it was in the prototype. The final layout has them board down horizontal. Didn't use terminals to start but have them for the final as it's assembled in modules.

Ultimately it depends on how hot things get and how close they are and how much ventilation there is to remove the build up of heat over time.
OK thanks. I hadn't thought about impact on the caps. I tend to build upside down and turn over in normal use, which means the terminal blocks (and trimmers if in use) are on the wrong side of the board.

I have the required number of boards unmade - I might make them with terminal blocks underneath the board for easy access.
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#1158 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by izzy wizzy »

RhythMick wrote: Fri Aug 06, 2021 8:01 am I have the required number of boards unmade - I might make them with terminal blocks underneath the board for easy access.
Or not make them with terminal blocks and solder to the pads. Could also mount the trimmer on the other side. However IME, it's super hard to remove once it's in.
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#1159 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by RhythMick »

izzy wizzy wrote: Fri Aug 06, 2021 8:58 am
RhythMick wrote: Fri Aug 06, 2021 8:01 am I have the required number of boards unmade - I might make them with terminal blocks underneath the board for easy access.
Or not make them with terminal blocks and solder to the pads. Could also mount the trimmer on the other side. However IME, it's super hard to remove once it's in.
Andrew has long advised against trimmers and I might follow his advice this time. Resistors would be much easier to remove if needed.
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#1160 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Ray P »

RhythMick wrote: Fri Aug 06, 2021 8:01 am OK thanks. I hadn't thought about impact on the caps.
The affect of the heat on the filter caps is one of the main concerns I have about the excess voltage I have on the small version of Andrew's DHT filament supplies in my 300B HPA, that and the potentially shorter life of the regulator ICs. I always look to minimise any heating and part of that is to make sure the heatsink is oriented optimally, i.e vertically.
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#1161 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Nick »

Ray P wrote: Fri Aug 06, 2021 9:16 am
RhythMick wrote: Fri Aug 06, 2021 8:01 am OK thanks. I hadn't thought about impact on the caps.
The affect of the heat on the filter caps is one of the main concerns I have about the excess voltage I have on the small version of Andrew's DHT filament supplies in my 300B HPA, that and the potentially shorter life of the regulator ICs. I always look to minimise any heating and part of that is to make sure the heatsink is oriented optimally, i.e vertically.
I guess the solution is to mount the two power devices off board using a bigger sink. I am not sure that there is any real evidence that the life of the devices will be reduced as long as they are run within their ratings though. The regulator is specced as working from 273K to 393K (I put it in K to remove a hint of anthropomorphic thought), I just checked the sheet of the reg and there is no temp related lifetime de-rating.
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#1162 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

It's ironic, I have never put them in a finished and cased up amp to find out what happens to them.

I have a load of unfinished projects here :roll:

I put some prototypes in one of Colin's amps, the aftermarket fitment resulted in the regs being "on their sides", we eventually swapped them for proper boards, but the bare prototype PCB did show some heating. The caps seemed ok.
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#1163 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Ray P »

Nick wrote: Fri Aug 06, 2021 9:48 am I guess the solution is to mount the two power devices off board using a bigger sink. I am not sure that there is any real evidence that the life of the devices will be reduced as long as they are run within their ratings though. The regulator is specced as working from 273K to 393K (I put it in K to remove a hint of anthropomorphic thought), I just checked the sheet of the reg and there is no temp related lifetime de-rating.
Maybe I'm wrong (I frequently am) but I've always assumed that heating/cooling cycles are more of an issue than the absolute temperature (assuming it's within tolerance, as you say) but I'm also thinking about the deterioration of PCBs with 'too much' heat from the sinks.
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#1164 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Nick »

Maybe but I was thinking about semiconductors in particular.
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#1165 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Ray P »

Nick wrote: Fri Aug 06, 2021 7:10 pm Maybe but I was thinking about semiconductors in particular.
Would semiconductor packages not be subject to the stresses of heating/cooling cycles?
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#1166 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Nick »

I would have thought not in the sort of environments we work in.
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#1167 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Ali Tait »

Andrew wrote: Fri Aug 06, 2021 2:35 pm It's ironic, I have never put them in a finished and cased up amp to find out what happens to them.

I have a load of unfinished projects here :roll:

I put some prototypes in one of Colin's amps, the aftermarket fitment resulted in the regs being "on their sides", we eventually swapped them for proper boards, but the bare prototype PCB did show some heating. The caps seemed ok.
You might recall us talking about the regs I used in the 300b OTL's. They were getting a little too toasty (was measuring over 90 degrees on the heatsinks). Four in each monoblock all mounted on their sides. Was going to chop some holes in the bases and mount some quiet fans. Must get round to it sometime..
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#1168 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

I don't know if you need the fans, Ali, just get a bit more ventilation and see what they run at.
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#1169 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Andrew »

I don't know if you need the fans, Ali, just get a bit more ventilation and see what they run at.
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#1170 Re: DHT Heater using VCCS - PCB or Kit (Possible Group buy)

Post by Ray P »

Apologies but duplicating information from my 300B HPA thread but thought it might be useful here.

I have two of Andrews 'small' DHT heater supplies (the small one have the onboard heatsinks) in the headphone amp for the 300B filaments. I am using 30VA 9VAC encapsulated transformers. I've been experiencing 'hot' heatsinks because the transforers are actually delivering over 10VAC to the DHT heater supplies.

I've been experimenting with dropper resistors this morning - looking to reduce the input voltage to the DHT filament supplies in order to reduce the heatsink temperature a bit as it's high enough to have an affect on the filter caps located next to the sinks.

With no dropper resistors in place the input voltage this morning was 10.1VAC

Three 0.47R 10W resistors in series dropped the input voltage to about 7.8VAC however, when I listened there was a low level buzz on the channel I had the dropper resistors on.

Reduced it to two 0.47R resistors and the input voltage is about 8.5VAC and the buzz has gone. I'm assuming the buzz was the result of insufficient headroom in the DHT supply as nothing else has changed.

After about half an hour of playing the heatsink temperature of the module without the dropper resistors is 89°C amd the one with is 70°C and those look stable.

I'll get some suitable chassis mountable resistors and work out how I can do a permanent installation.
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