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600W DC-DC converter questions A few years back, I bought the 600W DC-DC converter kit from Jaycar in Perth and I didn’t have the time to start on it until recently. Before I start, I have some questions: (1) Do I need to upgrade the car’s alternator to a higher capacity? My present car is a 1600cc Toyota Corolla CSI. I think the output of the alternator is about 70A. (2) Will there be a surge that will destroy any of the car’s electronics if I wired the converter to come on as the ignition is switched on? (3) Is there 45/90/180 minutes or three times the values that you arrived at. When I ran the SLA battery described above I had the switch on 30-minute time-out but am sure that it was reasonably longer than this. Am I misinterpreting the chart? (2) Given Rb of 3.3kΩ, Rsense of 0.05Ω and Rref of 27kΩ, I cal­culate an Ifast of 3A rather than the 6A quoted in the article. Am I overlooking something here? (P. J., via email). • Voltage regulation at Vbat (pin 19) is 1.63V for an SLA battery. Since the end-point for a 12V SLA battery is 14.6V, the 0.111 division by the resistors for the 12V position on S5b will give 1.6V for the cut-off voltage. The fact that the voltage on your SLA batteries reaches 1.8V before dropping to 1.65V suggests that either the divider resistors for the 12V position on S5b are incorrect or the bat­teries are high impedance and so faulty. The timeout period is also set by the POD voltage at pin 6. This is actually tied low via a 33kΩ resistor (see over- any requirement for a slow start circuit? (4) Do I need to increase the capacity of my present battery? (A. W., via email). • You will not need to upgrade your alternator unless you are expecting to drive your amplifiers at full power when all the headlights and accessories are on. You can switch the inverter on via the ignition and this was shown on the circuit. The car battery should not need changing since the circuit is intended to operate only when the engine is running, so as to keep the bat­tery topped up. lay dia­gram) for a divide by 1 selection and so the 15-minute timeout is set to about 22 minutes. The timeout is 226 x tosc x POD x PTD. PTD is set to divide by 1, 2 or 4 by the selection at pin 7 via switch S2. The charging current does not follow the formula de­tailed in the data sheets because we are using an un­filt­ ered DC voltage to charge the battery. Problem with TENs unit I recently built one of your TENS units (SILICON CHIP, August 1997) and found it terrific for pain relief for an inoperable spinal injury I suffered about 10 years ago. The problem is it was working fine one day then the next day it just would not work. I am in my first year of a diploma in electronics course in which I have passed my hand soldering with a 97% pass mark. I have checked all resistors, capacitors and diodes and they are fine, so I was just wondering if there LE have been any problems with the units after they had gone into production? Alternatively, could you advise me on where to look for any possible faults? (J. C., via email). • The TENS Unit does not appear to have any particular problems. The problem with your particular unit could be with the transformer (T1) or any other component such as IC1 or IC2, or the Mosfets Q1 and Q2. Or simply, the batteries may be “flat”. We suggest you check the operation of each part of the circuit to isolate the problem. First, check the voltage at the drain of Q1 – you should get a reading of 80V. If this is not present, check the supply to IC1 at pin 6 (6V). If this is OK, the windings in T1 may be shorted or open circuit. If you do measure 80V, check the supply to IC2 which should be around 15V. You should also check the output voltage at the electrodes – this should give readings up to 40V (as measured with a DC-reading multi­ meter), depending on the pulse width setting. Lack of output voltage may mean that IC2 is not operating or that Q1 or Q2 are short circuited. Further checking can be made with an oscilloscope. You should obtain similar displays to those published. Notes & Errata RoomGuard Intruder Alarm, April 2000: the 100kΩ resistor at the junction of D1, D2, D4 and pin 9 of IC1 on the circuit on page 31 should be 10kΩ. The wiring diagram on page 32 is correct. LED Dice, May 2000: LED6 is shown back to front on the PC board component overlay on page 60. Its cathode (the flat side) should go towards the top of the page. The circuit diagram on page 58 is correct. SC WARNING! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable. 92  Silicon Chip