Transistor Or Resistor
in [Basics]
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From: Chris W on 29 Apr 2010 19:24 Thanks for all the replies. I still have a few questions. First it seams that using only transistors is not a good idea. The main reason I was hoping to get away from using all the resistors is the cumbersome way of adjusting the load by switching in various numbers of resistors and the fact that the resistors are only going to be able to be used to dissipate the maximum amount of energy at one voltage. Some one suggested using transistors as switches to the resistors. This could make it a bit easier because I could then use a single small switch to add several resistors to the load. However that doesn't really do much to make the interface to adjust the load any more elegant. Using that method the best idea I have come up with to adjust the load is to configure it so my first switch added 1 resistor to the load, the second switch added 2, the third, 4 and so on. Then I would treat the row of switches like a binary number to increment the load. The ideal situation would be to have a single pot that I could use to adjust the load. Alternatively having 4 or 5 pots where I would use the first one to turn the load up to it's max then the second one to add in that load, etc. How hard would it be to use transistors to control the current the resistors are sinking? That seems like it might be the best solution. If I'm not mistaken the transistors wouldn't have to sink very many watts at all. As for dissipating 700 watts, it is closer 1000 watts 14V * 50A + 5V * 25A + 3.3V * 25A = 907.5 watts That is the worst case, and will likely rarely see that high of a load except for a very short time (30 second or less). The most it will ever see for an extend time will be 600 watts. I have a 100 watt RF dummy load. A heat sink 10 times the size of that dummy load would be big, but still manageable. A few good fans would make it even more manageable. Any thoughts? 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
From: David Eather on 29 Apr 2010 19:39 On 29/04/2010 9:14 AM, 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? > > Chris W At 15 volts a 2n3055 will safely pass only 7.5 amps. Unless you use a heatsink to control temperature it will only safely dissipate about 3.5 watts
From: sparky on 29 Apr 2010 20:02 On Apr 29, 11:21 am, ehsjr <eh...(a)nospamverizon.net> 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. > > > Thanks, > > Chris W > > 100 Amp 6 Volt/12 Volt Battery Load Tester > > Item # 90636 at Harbor Freighthttp://www.harborfreight.com/100-amp-6-volt-12-volt-battery-load-test... > > 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 $$, burned > out power transistors, large heat sinks etc - and the need > for Joerg to provide sound effects for circuit demise. > > Ed- Hide quoted text - > > - Show quoted text - Don't expect this tester to dissapate heat for any longer than it takes to test a battery. If you leave it connected as a permanent load you WILL have a fire.
From: Chris W on 29 Apr 2010 21:18 sparky wrote: > On Apr 29, 11:21 am, ehsjr <eh...(a)nospamverizon.net> wrote: >> Chris W wrote: >>> I want to make a load center to test power supplies and batteries. I >>> was thinking of using 50 Watt 4 ohm resistors for 12V loads but I will >>> need 15 of them to get the current drain I want. I would also like to >>> load 5V and 3.3V lines and of course that would require different >>> resistors. >>> I was wondering if this wouldn't be a lot easier with a power >>> transistor. The 50 Watt resistors are going to cost a little over $3 >>> each and I will probably need 30 of them to get the loads I want. >>> The goal is to have a variable load of about 3 to 50 amps on as much as >>> 14V and from about 1 to 25 amps on 5V and 3.3V. Can someone recommend a >>> specific transistor that would work good? I am hoping I can do it with >>> fewer transistors. I do plan on using a large heat sink and fan to keep >>> this cool. >>> Thanks, >>> Chris W >> 100 Amp 6 Volt/12 Volt Battery Load Tester >> >> Item # 90636 at Harbor Freighthttp://www.harborfreight.com/100-amp-6-volt-12-volt-battery-load-test... >> >> On sale now for $19.99 >> >> Use as is, or use the element as a load resistor in whatever >> circuit you design. Using it as is will save you $$, burned >> out power transistors, large heat sinks etc - and the need >> for Joerg to provide sound effects for circuit demise. >> >> Ed- Hide quoted text - >> >> - Show quoted text - > > Don't expect this tester to dissapate heat for any longer than it > takes to test a battery. If you leave it connected as a permanent > load you WILL have a fire. > Also it isn't adjustable, and therefor useless for my purpose. Chris W
From: Jasen Betts on 30 Apr 2010 04:15
tOn 2010-04-29, Chris W <1qazse4(a)cox.net> wrote: > The ideal situation would be to have a single pot that I could use to > adjust the load. this can de done using a resistor sized for the largest load and pulse width modulation, but you may need input filtering so that the powersupply sees a more stable load. possibly splitting the load into 4 or more parts and driveing them out-of-phase with each other cuould help too. --- news://freenews.netfront.net/ - complaints: news(a)netfront.net --- |