Thermal dissipation.  - Page 1
Forum section: electronics / beginners

author: sureshot
author_rank: Regular Contributor
timestamp: June 04, 2018, 12:14:58 am
content: I'm putting together a resistive load for testing purposes. I want to incorporate a crowbar for any likely over voltage scenario that could accrue. What I'm stuck with is the heatsink for the SCR or it's size to be more specific. My maximum load testing won't exceed 22 Amps at a maximum of 14 Volts. More topically testing around 220 Watts at 12 Volts. I know the SCR only heats up if it's fired (I think) So was hoping to get some idea of what size heatsink I might need. The SCR I'm thinking of using is the VS-40TPS12APBF 1200 Volts 35 Amps. This might be overkill I'm not sure yet.Any help appreciated and thanks for reading.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 04, 2018, 12:34:25 am
content: That seems like a lot more risk to your DUT than to your resistors? Resistors are chunks of metal, they hardly need protection!Tim

author: David Hess
author_rank: Super Contributor
timestamp: June 04, 2018, 12:52:42 am
content: SCRs used for crowbar circuits do not usually require any heat sink. The SCR is suppose to fire and blow a fuse and while the peak power is high, the total energy is low and the event happens so quickly that the heat sink is too far away from the SCR's junction to matter.

author: sureshot
author_rank: Regular Contributor
timestamp: June 04, 2018, 02:18:56 am
content: Thanks for the reply. Yes it is a power resistor construction, I was only looking to save the unit if a power supply went high for some fault reason. As you say the power dissipated is brief, so I might be able to get a modest heatsink in the case. It would save the meters and unit also. It's only for 12 - 14 Volts power supply testing.

author: sureshot
author_rank: Regular Contributor
timestamp: June 04, 2018, 02:21:22 am
content: It's these inside the case. The crowbar would save these also, dam expensive.

author: sureshot
author_rank: Regular Contributor
timestamp: June 04, 2018, 11:26:07 am
content: Just had a thought...The input psu under test load requires a relay to switch on before any load is switched in, its this way so i can cut the input even if under full load if i need to. In theory could i incorporate the crowbar at the input wires to this relay coil, there by being able to use a less high current thyristor ? Just a thought. Be handy if that's a logical option.

author: David Hess
author_rank: Super Contributor
timestamp: June 04, 2018, 04:08:03 pm
content: SCR crowbar circuits are sometimes used that way in which case the SCR serves the function of an inexpensive latching switch. I did something similar in a lead acid battery charger to switch from charge to float mode.Controlling the power with a relay is not destructive as with an SCR crowbar but is slower and in some applications risks the relay contacts welding together.

author: sureshot
author_rank: Regular Contributor
timestamp: June 04, 2018, 06:30:04 pm
content: Yes I could see contact weld maybe an issue, I wander if a suppression capacitor might be worthwhile. What I meant was the crowbar turning off the first relays coil in the event of over voltage. I only put a relay on the input to the load, as most switches can't handle much over 10 Amps. At the full load I might need to switch the possible maximum of 300 watts in or out. Although I will wire it up so I can switch in each load resistor one at a time.

author: strawberry
author_rank: Super Contributor
timestamp: June 04, 2018, 06:45:54 pm
content: thermal fuse or thermal switch

author: james_s
author_rank: Super Contributor
timestamp: June 04, 2018, 06:58:39 pm
content: A crowbar is not what you want here. They are good for power supplies where a fault that causes the output voltage to rise will trigger the crowbar and blow the fuse. For protecting a load you should use a relay connected to a thermal switch to open the circuit if the temperature rises too high.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 04, 2018, 07:04:26 pm
content: This is rather after the fact, I guess, but FYI -- metal-case resistors are typically a false economy, once you include the massive heatsink required, and machining and mounting steps.Vitreous enameled types are only slightly more expensive (sometimes cheaper, almost always comparable), somewhat easier to mount, take up less space than the resistor-heatsink combo, and operate at much higher temperatures (= more power density, less airflow required -- assuming a high exhaust temperature is acceptable).They're also much more tolerant of overload: they'll happily glow red hot for minutes on end before expiring. The metal-case types can't tolerate much for long (e.g., 2.5 or 5x power for 5s), and overheating leads to cracking and voids in the epoxy fill, or complete blowouts. That is to say: they are manufactured with magic smoke, while the other kind is not.So, for modest overloads of 2-3x and longer durations, or higher overloads and shorter durations (<= 1s), I wouldn't worry about it. I'd also rather have a series limiter than a shunt limiter, since supplies are usually CV-output (imagine what happens to the poor thing(s) when a beefy unregulated linear supply is connected?).I've made a couple kinds of series limiter (protected switch), battery powered with reasonable life (>6 weeks continuous operation from a 9V battery), let me know if that sounds interesting.Tim

author: sureshot
author_rank: Regular Contributor
timestamp: June 05, 2018, 10:33:42 am
content: Interesting to know about the enamel resistors, although I've not come across any high power variants. I will implement two thermal switches contacts NC opening on temperature rise. Just incase of a fan out peoblem, they will cut the input relay coil. Over voltage is unlikely to happen, but its just being sure its covered if it did happen. The main use for this unit is testing converted server power supplys, whilst also connected to HF radio gear and amplifier, this is to see how the host psu behaves under heavy loads. As in, as if i was using the equivelent of a high power amplifier with radio. I've a modest amplifier and radios. But want to see in real time if these power supplys can handle high current use with HF equipment. So as radio gear is connected to the same host psu, that's also the need to protect from over voltage conditions. That unit will also see lighter testing of linear power supplys. I'm hoping to see some wave forms of different units under different load conditions.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: June 05, 2018, 05:23:15 pm
content: Something like this perhaps:https://www.digikey.com/product-detail/en/ohmite/C300KR63E/C300KR63E-ND/1124873

author: JS
author_rank: Frequent Contributor
timestamp: June 05, 2018, 07:55:19 pm
content: Quote from: sureshot on June 05, 2018, 10:33:42 amThey are resistors, over voltage and over current go together, use a fuse for that case when something funny gets to its inputs and save the world.JS

author: sureshot
author_rank: Regular Contributor
timestamp: June 06, 2018, 09:46:48 am
content: But if HF radio equipment was connected to the loaded host power supply, it could be damaged if the psu went above 14.4 Volts.