Transistor Or Resistor
in [Basics]
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From: baron on 29 Apr 2010 08:10 Chris W Inscribed thus: > Tim Wescott wrote: >> >> 2500 watts if you maintain the case at 25C, based on a thermal >> resistance of 0.05 C/W junction to case, and a maximum junction >> temperature of 150C. >> >> Of course, they claim a _minimum_ of 0.1 C/W case to sink, so that >> drops you down to 833 watts, and that's _if_ you maintain the heat >> sink >> temperature at 25C. You'll spend a lot more time with the thermal >> engineering of the heat sink than you would wiring up a few switches >> and resistors, that's for sure. >> > > So how many watts could it realistically dissipate? Even if it is only > 200 that is still 4 times more than a 50 watt resistor. Then with > some kind of circuit to regulate the current and maybe 5 or 10 of > those I > could use that at any of the voltages I want to test at. Couldn't I? > > > Thanks again for the help > > Chris W Which ever way you go you are going to have to get rid of around 700 watts of heat. Water cooling is starting to look good. :-) -- Best Regards: Baron.
From: default on 29 Apr 2010 09:29 On Thu, 29 Apr 2010 13:10:17 +0100, baron <baron.nospam(a)linuxmaniac.nospam.net> wrote: >Which ever way you go you are going to have to get rid of around 700 >watts of heat. Water cooling is starting to look good. :-) You are on to something there. Use resistors to heat water? I used to work at a PS manufacturer - we used transistor resistor loads, carbon piles, and some old theater lamp dimmer rheostats, to test and set the over current circuits. For burn-in, fixed resistors are hard to beat for fiddle free operation, cost and reliability. --
From: Bob Eld on 29 Apr 2010 10:34 "Chris W" <1qazse4(a)cox.net> wrote in message news:ML2Cn.162708$EE6.87845(a)newsfe23.iad... > I want to make a load center to test power supplies and batteries. I > was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will > need 15 of them to get the current drain I want. I would also like to > load 5V and 3.3V lines and of course that would require different > resistors. > > I was wondering if this wouldn't be a lot easier with a power > transistor. The 50 Watt resistors are going to cost a little over $3 > each and I will probably need 30 of them to get the loads I want. > > The goal is to have a variable load of about 3 to 50 amps on as much as > 14V and from about 1 to 25 amps on 5V and 3.3V. Can someone recommend a > specific transistor that would work good? I am hoping I can do it with > fewer transistors. I do plan on using a large heat sink and fan to keep > this cool. > > Thanks, > Chris W Lamps and nichrome wire have been proposed but any element that gets hot enough will have a significant change in resistance making the load non-linear which might be a problem. Power transistors will have this problem unless in an active circuit is used to keep resistance, current or load constant or controlled. Be ware of second breakdown if using power transistors. This occurs when both voltage and current are present at the same time even though neither individual rating is exceeded. Check the data sheets. One thing I've done for high power loads is to put power resistors under water in a can or bowl. This keeps them cool, the resistance constant and allows them to dissipate many times their rated power. A single 4 ohm 50 watt wire wound resistor will probably dissipate 500 watts when under water without a problem. An array of resistors can be used to make an adjustable load. Of course water is corrosive over time so the resistors are removed from the water and dried off when not in use.
From: ehsjr on 29 Apr 2010 11:21 Chris W wrote: > I want to make a load center to test power supplies and batteries. I > was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will > need 15 of them to get the current drain I want. I would also like to > load 5V and 3.3V lines and of course that would require different > resistors. > > I was wondering if this wouldn't be a lot easier with a power > transistor. The 50 Watt resistors are going to cost a little over $3 > each and I will probably need 30 of them to get the loads I want. > > The goal is to have a variable load of about 3 to 50 amps on as much as > 14V and from about 1 to 25 amps on 5V and 3.3V. Can someone recommend a > specific transistor that would work good? I am hoping I can do it with > fewer transistors. I do plan on using a large heat sink and fan to keep > this cool. > > Thanks, > Chris W 100 Amp 6 Volt/12 Volt Battery Load Tester Item # 90636 at Harbor Freight http://www.harborfreight.com/100-amp-6-volt-12-volt-battery-load-tester-90636.html On sale now for $19.99 Use as is, or use the element as a load resistor in whatever circuit you design. Using it as is will save you $$, burned out power transistors, large heat sinks etc - and the need for Joerg to provide sound effects for circuit demise. Ed
From: whit3rd on 29 Apr 2010 18:17
On Apr 28, 3:36 pm, Chris W <1qaz...(a)cox.net> wrote: > I want to make a load center to test power supplies and batteries. I > was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will > need 15 of them to get the current drain I want. I would also like to > load 5V and 3.3V lines and of course that would require different > resistors. > > I was wondering if this wouldn't be a lot easier with a power > transistor. The water heater in your home has two 5kW resistors, something in the range of 10 ohms; these are cheap enough as replacement items (and if your municipality recycles steel water heaters, used ones might be easily available, too). If you have long tests in mind, it might pay to circulate water and/or use a radiator core and airflow. Immersible design and cooling water are the key elements. Jumpering for parallel or series connection is tedious, but flexible enough for LOTS of load values. I've seen (for laboratory magnets) arrays of dozens of 2N3055 transistors, on copper plate heatsink with a soldered-on tube for cooling water... NOT pretty. |