Transistor Or Resistor
in [Basics]
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From: krw on 28 Apr 2010 20:20 On Wed, 28 Apr 2010 18:14:12 -0500, Chris W <1qazse4(a)cox.net> wrote: >Tim Wescott wrote: >> 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. >> >> 2N3055, if they're still around. You'll spend lots on the transistors >> and heatsinks, though. >> >> Car headlights work well for this, at least for 12 volts, with car tail >> lights to trim the current. They're bulkier than resistors, but loads >> cheaper -- and they'll brighten your day :-). >> >> Or get a space heater that uses resistance wire, chop it up into ten >> equal sections -- viola! ten power resistors. >> > >The data sheet says it will dissipate 115 watts. With an infinite heat sink, perhaps. >And they cost $1.35 at mouser. That will total to a lot less than the >50 watt resistors. What's the cost of your infinite heat sink, though. >The cost isn't as big of an issue as the complexity >of wiring up a bunch of resistors and having to switch them in one at a >time so I have to use lots of switches too. You could use the transistors as switches (let resistors get hot - that's what they're for). >On a side note can this thing. >http://mouser.com/ProductDetail/Fairchild-Semiconductor/FDL100N50F/?qs=GBxGW0xXju923CYRyhG5QQ%3d%3d > >really disapate 2,500 watts? If so I assume you need a very good heat >sink. Is really possible to realistically have enough heat sink to >dissipate enough heat to put anywhere near that much wattage through >that thing for more than a very short time? Sure, enough water could probably put the fire out.
From: Ron M. on 28 Apr 2010 20:36 On Apr 28, 6:36 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 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 You can buy the replacement nichrome coils in varying wattage levels at most electrical supply houses. I used an old hair dryer for an audio dummy load for long term amp testing (BURN IN). Works great and can be easily mod'd for your ap. Handles 1300 watts too. Get's rather toasty though.
From: Tim Wescott on 28 Apr 2010 21:25 Chris W wrote: > Tim Wescott wrote: >> 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. >> >> 2N3055, if they're still around. You'll spend lots on the transistors >> and heatsinks, though. >> >> Car headlights work well for this, at least for 12 volts, with car >> tail lights to trim the current. They're bulkier than resistors, but >> loads cheaper -- and they'll brighten your day :-). >> >> Or get a space heater that uses resistance wire, chop it up into ten >> equal sections -- viola! ten power resistors. >> > > The data sheet says it will dissipate 115 watts. And they cost $1.35 at > mouser. That will total to a lot less than the 50 watt resistors. The > cost isn't as big of an issue as the complexity of wiring up a bunch of > resistors and having to switch them in one at a time so I have to use > lots of switches too. > > On a side note can this thing. > http://mouser.com/ProductDetail/Fairchild-Semiconductor/FDL100N50F/?qs=GBxGW0xXju923CYRyhG5QQ%3d%3d > > > really disapate 2,500 watts? If so I assume you need a very good heat > sink. Is really possible to realistically have enough heat sink to > dissipate enough heat to put anywhere near that much wattage through > that thing for more than a very short time? 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. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Chris W on 28 Apr 2010 23:46 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
From: Jasen Betts on 29 Apr 2010 04:54
On 2010-04-28, Chris W <1qazse4(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. Or you can make your own from nichrome wire (which is free if you have an old toaster, or hair dryer (etc...) handy for parts....) > 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. automotive lamps are another option.... > The goal is to have a variable load of about 3 to 50 amps on as much as > 14V 50A on 14V is 700W 25A on 5V is 125W > Can someone recommend a > specific transistor that would work good? > I do plan on using a large heat sink and fan to keep > this cool. I'd go with nichrome wire and a fan, build a low voltage hair dryer. --- news://freenews.netfront.net/ - complaints: news(a)netfront.net --- |