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Hi all for many years now I have needed a reliable high current power supply but it wasn't until a fuw years ago when I finally got myself a descent transformer that I finally gave it a go.
My first attempt was a circuit that used 6 2n3055 and an lm317 v-reg and it was supposed to be able to handle 30 amps continues but the best I could get was short runs at 15 amps and it really disliked my high voltage experiments so i decided to start over
My new design uses both a PWM for very large inductive loads (its also wicked when running an ignition coil) and a v-reg simmilar to my first except it uses 10 tip35's
And that's where my problem is I thought thay would be far better then the 3055's but so far the best i have been able to get out of it is about 10 amps and I have already blown 3 of my tip35c's~:-( I really need to find out what is gowing on here any help would be much appreciated
Hear are some pics of what i have done so far and a circuit (it uses tip3055's just remember I am using 10 now 7 tip35c's) and the pic of the heat sink was taken before i added the extra 4 transistors
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
Isn't your circuit basically a high power voltage regulator? I've seen similar setups before in literature as an alternative to using lots of paralleled regulators or a more expensive one.
If you're transformer can only output 24v and 10A, then where's the extra transformation come from to give you a higher current?
Edit: ah ok, your transformer is not the one pictured. In that case, you should use a transformer with multiple taps and switch through them as you vary the voltage so that you don't have to waste so much power in your semiconductors (of course sulaiman's already got that covered.) Not sure if this will help, but it might give you an idea or two (such as the short circuit protection) to implement into your design
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
- If you have a transformer winding rated at 24V 10A a.c. (260 VA) when you full-wave rectify it the current rating will be about 6.2 A dc if you draw more than 6.2 A dc continuously the transformer windings will overheat. Putting the transformer in a box with poor airflow will make it worse.
- at 6.2 Adc output I would expect the voltage across the 22,000 uF capacitor to be about 29V +/- 1V ripple
- an LM317 needs about 1.2V minimum from input to output, the power transistors will probably drop about 0.9 V base-emitter voltage, so I doubt that this design would give more than 26 V @ 7A dc continuous (182 W) Short term you could supply 10A or more but that would reduce the regulated output voltage to about 24 V @ 10A dc.
- At low output voltage high current settings (e.g. 5V @ 5 A dc) the power dissipated in the transistors will be high (e.g. (29V - 5V) x 5A = 120W) your heatsink does not look large enough to dissipate this much power without allowing the transistors to overheat, unless it has a fan.
With a 'scope you can see the voltage ripple on the 22,000 uf capacitor etc. with a dmm you can measure the average dc voltage across the cap (e.g. 29 V) then using the dmm ac voltage range measure the voltage across the capacitor (e.g. 0.5 V) Then measure the ac voltage ripple across the output, as soon as it exceeds a few 10's of mV you know that the output is no longer fully regulated.
At high output currents monitor the temperature of the transformer, bridge rectifier and transistors/heatsink.
Sorry that circuit was just one I got off the net my transformer is 40v (when rectified) at 20 or so amps I'm considering rewinding an MOT just depends on weather or not I get it working properly
Edit: my capacitance is 3 x 5000uf so 15000uf and i wasn't shore wether a bigger voltage difference between the input and output would wean more power through the transistors so thank-you for clearing that up
Another option I thought of if i use a rewound an MOT I could us multiple tapes and have it automatically switch between them depending on the output voltage
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The same principles apply, 40 V @ 20 A is 800 Watts, - at low output voltage, high current most of that 800 W will need to be dissipated by the transistors/heatsink, e.g. 5V @ 20A, power dissipated = (40-5) x 20 = 700W TIP35 max junction temp = 150C, thermal resistance junction-to-case = 1 C/W add about 0.25 C/W case-to-heatsink, Each transistor dissipates 700/10 = 70W so max heatsink temp = 150 - (70 x 1.25) = 62.5 C IF ambient temperature = 22.5 C the heatsink would need to be 40/700 C/W a 0.057 C/W heatsink would be enormous if no fan/forced-air is used.
Let's assume your heatsink is 0.5 C/W (quite large heatsink) and ambient temperature = 25 C
Thermal resistance junction-to-ambient = 0.5 + (1.25/10) = 0.625 C/W Maximum continuous dissipation = (150-25)/0.625 = 200 Watts. That would allow maximum continuous outputs of 0V @ 5A 10V @ 6.7A 20V @ 10A 30V and above @ 20A
If you know the thermal efficiency of your heatsink you can do more accurate calculations. For these power levels you should consider fan-assisted cooling. I like to use the fan to suck air through the enclosure (to cool internal components) then blow it through/across the heatsink.
'Rule of thumb' : If your heatsink can boil water it's probably too hot !
P.S. Most LM317 have a maximum 40V rating, at low output voltage and current this may be exceeded in your setup.
What did you think of my idea of using a rewound MOT with multiple taps I was thinking of using two mot's with each with 5v taps all the way up to 30v if i can get 900 watts each that should be about 30a and I will have it set up so the regulator never sees more then 5v difference between the input and output I will also use a pot with a cople of transistors instead of the lm317
Here is a circuit of the power electronics I will have to finish the control section tomorrow after school
Sorry for the poor image I had to take a pic of my screen
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
if you are making a emitter follower like before you should use 3055 for a linear power supply. if you are doing a pwm set up then you should be using mosfets with the lowest on resistance as pausible. a lm317 can only drive about a amp and the beta on each of those 2n3055 is about 30 so you will need to do a darlington setup to be a linear ps. a re configured mot may not be the right way to go. if you can find a large step down transformer or industrial controle transformer that is comershal say 1kva you may be better off. a good surplus place to look. mot's are generaly very inefishent. a switching power supply is much more complicated and if I wanted to build something basic and quick it would be linear
if you are making a emitter follower like before you should use 3055 for a linear power supply. if you are doing a pwm set up then you should be using mosfets with the lowest on resistance as pausible. a lm317 can only drive about a amp and the beta on each of those 2n3055 is about 30 so you will need to do a darlington setup to be a linear ps. a re configured mot may not be the right way to go. if you can find a large step down transformer or industrial controle transformer that is comershal say 1kva you may be better off. a good surplus place to look. mot's are generaly very inefishent. a switching power supply is much more complicated and if I wanted to build something basic and quick it would be linear
,r:13,s:221,i:89&tx=105&ty=54
that's basically what i am trying to build the 5v taps are just so the transistors will never have to disipate more then 5v and therefore shouldnt be so overloaded and the PWM is compleatly seperate from the v-reg it will just be another fetur on the finished power supply
edit:ok i have finished both schematics and atached them below hopfully you can understand them i am not very good at designing circuits from scrach so if you notis any mistakes or have any sugestions please let me now if you like you can edit and repost them. ]expresspcb.zip[/file]
B.T.W the transistors in the control are bc546
edit: i would like to hear some thoats on my circuit befor i start construction so if anyone has anythigh to say about it good or bad please do.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Many 'bench' power supplies use a pfc front end with a pwm pre-regulator and a linear output regulator for low noise/ripple etc.
One I repaired a few weeks ago only used 1.9V across the linear regulator !
Your tap-switching idea is ok except you would need several expensive (30A) switches' If you re-wind the secondaries you could have higher current (thicker/more wire) for the lower voltages to match your transistor/heatsink capabilities. I agree with the MOT not being efficient/getting hot comment, they also 'buzz' which would annoy me.
A 1 kW variac feeding a step-down transformer would cost more but allow a lot of design freedom (e.g. a 'raw' high power adjustable dc output)
One psu I worked on had the variac shaft ganged with the linear control potentiometer.
For most high-current work I think that just Variac/Transformer/rectifier/capacitor is adequate without regulation. (plus line filter, miniature circuit breaker, switches, fuses, lamps, meters etc.) and MUCH more reliable/repairable.
I did already now about most of those problems with the MOT's but I won't be using it enough for that to be a problem and as for the switches I have a ton of 40a atomotiv relays that should work fine B.T.W I actually got this idea from the power supplies at school which as far as I can tell work the same way
Edit: Im not shore what you mean by ''pfc front end with a pwm pre-regulator and a linear output regulator" I couldnt find anything on the net ether is there any chance you could post a basic circuit to help me understand.
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high current 0 to 40 volt pwm and v-reg power supply
