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  1. The circuit is different from Sanyo's. The trimpot is an ordinary one, nothing special. I could easily find a replacement if I had to. The output stage seems to be a triple Darlington - all TOSHIBA. I'll find a way to fix this. Thanks anyway.
  2. You can use a step-down converter for the 5V part. It's more efficient so it doesn't require a large heatsink. At 3A the IC will need an external transistor. The value of the inductor can be calculated with the formulas in the Application Note. MC34063_ST.pdf TL494_AppNote_-_slva001d.pdf
  3. Because it's a rare amp I don't have a schematic. It's hard to guess it by looking at the PCB tracks because there's also a circuit that detects if there's DC at the output. I think the value of the trimpot is too large. The trimpot connects to a transistor. If this doesn't work I'll use a couple diodes.
  4. I replaced a STK3062 in a Marantz PM-52SE power amp with another module. The output DC offset voltage is 0 but the quiscent current in 700mA. :o What should I do to reduce it to 70mA? Here's a list with the values of the components of the replacement module: R1______________47K 1/4W Resistor R2_______________4K7 1/4W Resistor R3______________22K 1/4W Resistor R4_______________1K 1/4W Resistor R5_____________330R 1/4W Resistors R6_______________1K5 1/4W Resistor R7______________15K 1/4W Resistor R8______________33K 1/4W Resistor R9_____________150K 1/4W Resistor R11_____________39R 1/4W Resistor C1_____________470nF 63V Polyester Capacitor C2_____________470pF 63V Polystyrene or ceramic Capacitor C3______________100uF 63V Electrolytic Capacitor C4_____________100nF 63V Polyester Capacitors C5______________10pF 63V Polystyrene or ceramic Capacitor Q1,Q2,Q3,Q4,Q5_____MPSA42 300V 500mA NPN Transistors Q6___________MJE340 200V 500mA NPN Transistor Q7___________MJE350 200V 500mA PNP Transistor STK_3042-3062-3082-3102-3122-3152_III_-_Sanyo.pdf
  5. So, would 30cm2 of heatsink surface be enough for each watt of dissipated power? Even if the heatsink calculated with the above formula is a little bit oversized it doesn't matter. In fact, the cooler a semiconductor runs the longer it lasts. I could build a unit for testing heatsinks because I have a lot of them ;D, most coming from broken computer power supplies, but I need a simpler way to determine how much power a heatsink could handle so the junction of the semiconductor attached to it doesn't exceed about 100 degrees Celsius.
  6. Here's the circuit. I know it will work but I'm developing a better version which I'm not sure how well it works...
  7. Radiation matters more if the heatsink is painted in matte black, but it's a lot less than the heat transferred to the ambient by convection. I could measure the thermal resistance if I have a contact thermometer... and apply a known voltage to a known value resistor stuck to the heatsink. I don't need a precise formula. I took a look at some circuits that mentioned the dissipated power and the minimum surface area required for the heatsink. After some very simple math I got this result: A = 10 to 20cm2 per watt, where A is the surface area of the heatsink. Here are some examples: For 30 watts I'll need a heatsink with a surface area of 600cm2. For 70 watts I'll need a 1400cm2 heatsink.
  8. I used the LM358, which is the same thing except that it has two opamps instead of four and it works well. I built a preamp and a stereo width controller (which is actually a channel mixer) using this IC. If you want lower noise then use the TL074. The power supply must be regulated and within the limits specified in the datasheet. It's not suitable for Hi-Fi because of the THD and the noise. The TL074 is cheap too, but not as cheap as the LM324. Of course... if you have a schematic that works. Just avoid ground loops or you'll get weird noises. You can find the datasheets and download them here http://www.datasheetcatalog.com
  9. :o No. If the voltage across the transistor increases, the SCR, which is placed between the bridge rectifier and the filter capacitor, will stop conducting until the voltage decreases under a value set by a Zener diode. The variable voltage linear power supply will be connected after the SCR. Regardless of what supply I'm going to use (regulated or unregulated) it's going to have an adjustable output voltage. ::) I'll have to find a way to post the schematic of the circuit... :(
  10. The LM723 does have a circuit with foldback current limiting... How about using the other way to limit the dissipated power - the one with the SCR (thyristor)?
  11. How can I determine the approximate thermal resistance of a heatsink if I know its surface area? ::) I searched the web and the most useful thing I found was this http://www.jaycar.com.au/images_uploaded/heatsink.pdf but it doesn't answer my question.
  12. Hi guys, Foldback current limiting would be a solution... I know this method of current limiting from a book about voltage regulators. It has all the formulas for calculating the output short citcuit current... How can I adapt it to this circuit http://electronics-lab.com/projects/power/011/index.html? Will it work with a variable voltage power supply? ??? I doubt it. If the output voltage is 2V and the current limit set to 2.2A and the circuit draws 2A the current limiting circuit won't be active so the dissipation will be 40W. This situation is unlikely to be achieved but it can happen. 40W is still too much. Another soultion would be to keep the voltage drop across the series pass transistor constant so the dissipation would be constant too at a given current. One way to do this is to use a SCR after the rectifier but before the filter capacitor. The SCR will conduct only when the voltage across the series pass transistor falls under a certain value set by a Zener diode. Besides the SCR and the Zener diode the circuit has a low power transistor and three more resistors. I like the way the circuit works but I don't like the frequency at which it works - 100Hz. I already thought of something better... ;D
  13. Wishing to build a variable DC power supply with current limiting I came across this problem: when the output is short circuited to ground and with the current limit set to maximum the series pass element (in this case a transistor) has to dissipate all the power so it requires a big heatsink. But I don't have room for such a large piece of aluminium in the case in which I'm going to build the power supply. For example, my supply will deliver a voltage up to 24V at a maximum current of about 2.5A so if the output is shorted to ground the power dissipated in the series pass transistor will be 60W. I need a circuit that limits the voltage drop across the series pass transistor. Can anyone help me with this? :-\
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