Two simple questions - Page 1
Forum section: electronics / beginners

author: wobblycogs
author_rank: Contributor
timestamp: October 08, 2013, 02:31:25 pm
content: Hi,I'm just starting out with my first electronics project: building a simple 70V linear power supply. I've got a couple of questions though...Question 1...I'm planning on using one of thesehttp://uk.rs-online.com/web/p/bridge-rectifiers/6296320/bridge rectifiers and feeding it 48Vac (I don't need exactly 70V just something close). The spec states that the forward voltage is 1.1V but I'm not clear if that is per diode or total? I had assumed that it was per diode for a total drop of 2.2V but someone (that I thought was very knowledgeable) has told me that it's 1.1V total. I'm calculating the expected output voltage like this:Vout = Vin * SQRT(2) + Vdrop = 48 * 1.414 + 2.2 = 66VdcFor the application I'll be using the power supply for it doesn't make any difference but I'd like to make sure I'm reading the spec sheet correctly.Question 2...I'd like to include an LED just after the rectifier to indicate that the secondary 70Vdc side is live. I've done the calculation for an LED and resistor in series and for a typical blue LED it comes up with a 3k9 resistor @ 1.6W. I'd rather not be dumping 1.6W of heat into my power supply if possible especially as I already have about 4W from bleed resistors for the smoothing capacitors. Are there any simple and efficient ways to power an LED indicator in this situation?Cheers.

author: c4757p
author_rank: Super Contributor
timestamp: October 08, 2013, 02:37:56 pm
content: I would assume it's per diode, since that seems reasonable for a PN diode at 35A. You're not going to get 0.55V per diode at that current without using Schottky diodes.As for the LED, you could find some lower voltage point in your circuit and use that (doesn't the control circuitry require a lower rail?). Anywhere you have about 5mA flowing is a candidate to power the LED. Alternately, put a neon lamp (with resistor!) across theprimary.But... a 3k9 resistor on a 70V rail will put 17mA through the LED! That's alot! LEDs are quite a bit more efficient now than they were back in the 80s when everybody decided LEDs needed 15mA... 5mA should be enough. You could use a 15k 1/2W resistor and wouldn't wastetoomuch power.Another method would be to use a capacitor as an AC impedance, and power your LED off the AC output of the secondary (before the rectifier). Put the LED in backwards parallel with another diode so it doesn't get a reverse voltage (they only like up to 5V reverse), and then put that all in series with a capacitor (about 270 nF and rated to 100V).

author: wobblycogs
author_rank: Contributor
timestamp: October 08, 2013, 03:40:57 pm
content: Thanks, that makes sense about the bridge and confirms what I thought.I've actually got a neon lamp drawn on the primary side (although without a resistor, oops). I think I'll just ditch the LED on the secondary side as it's overkill, if there's power on the primary side there's power on the secondary. How do I pick a suitable resistor for a neon lamp?The power supply is for a CNC machine I'm making (trying to make would actually be more accurate), it's providing power to the stepper motors hence the requirement to just be near 70V. I've attached the circuit diagram I have so far. I've just added the relay before the transformer, this will be powered from / controlled by a low voltage (probably 24V) safety circuit.

author: c4757p
author_rank: Super Contributor
timestamp: October 08, 2013, 03:55:01 pm
content: Quote from: wobblycogs on October 08, 2013, 03:40:57 pmThe easiest way is to start at 47k and adjust to your liking.Note that some of them have a resistor built in. (They behave kind of like a diode, in that they have a certain voltage drop, below which they conduct almost no current and above which they conduct as much as necessary to maintain that drop. It'll pop pretty quickly without a resistor, just like an LED.)Edit: not necessarily, as I don't know where you live. I'd do 47k on 120VAC and 150k on 240VAC (assuming 1mA and about a 70V drop).Also, IMHO, if you're trying to power motors on a CNC machine, an extra watt here or there falls solidly in the category of "who cares?".

author: RSTPhysics
author_rank: Newbie
timestamp: October 08, 2013, 04:07:25 pm
content: Just a small thought, it's minus the voltage drop, not plus.Vout = Vin * SQRT(2) - Vdrop. And one has to remember that that is the top-voltage under no load.

author: wobblycogs
author_rank: Contributor
timestamp: October 08, 2013, 04:24:30 pm
content: <cough>Good to see someone spotted the deliberate mistake</cough>. I've checked the spreadsheet I've put together for calculating the various parts and that has a minus so all is good.I'm in the UK so it's 230V with a margin of error you could drive a bus through.One of the problems I've had all along with this CNC build is not having a good feel for what matters and what doesn't. I'm a software developer by trade and I'd consider loosing a byte here and there to be seriously bad. I'll put that in the "stop worrying about it" category and move on.

author: RSTPhysics
author_rank: Newbie
timestamp: October 13, 2013, 09:56:48 pm
content: Yeah on the input side, don't count on voltages beeing exactly something, it's going to differ by a few percent. Even your 230V input will differ.Here in Sweden a maximum allowed voltage difference from the nominal 230 at the switchboard is +6/-10%, that means between 207V and 243.8V, BUT, we also have to consider the allowed voltagedrop inside a house, that is, between the switchboard and the outlet, which if I remember correctly is 4%, so, that would mean a accepted voltage in the outlet would be between 198.72V and 243.8V, a difference of over 45 volts!So, design with tolerances, and don't expect a voltage to be precisely what you calculated from nominal sources and with nominal components.

author: smashedProton
author_rank: Frequent Contributor
timestamp: October 14, 2013, 03:09:46 am
content: Here is a recourse I found for yahttp://www.zen22142.zen.co.uk/Design/dcpsu.htm

author: IanB
author_rank: Super Contributor
timestamp: October 14, 2013, 03:26:31 am
content: Quote from: wobblycogs on October 08, 2013, 04:24:30 pmThere, fixed that for you. The standard distribution voltage in the UK is, was, and always will be 240 V. That's how the system was designed and it's very expensive to change it.Of course you may not actually get 240 V depending how far you are from the substation and how many kW your neighbours are drawing at the time, but in urban areas 240 V is typically what you will measure.The "230 V" is the harmonized European nominal supply voltage. But passing a law stating that the voltage everywhere is 230 V does not change the actual voltage any more than passing a low stating that the value of pi is 3.0 will change the actual value of pi.

author: tszaboo
author_rank: Super Contributor
timestamp: October 14, 2013, 03:48:48 am
content: Quote from: IanB on October 14, 2013, 03:26:31 am"Following voltage harmonization, electricity supplies within the European Union are now nominally 230 V ± 10% at 50 Hz. For a transition period (1995–2008), countries that had previously used 220 V changed to a narrower asymmetric tolerance range of 230 V +6% ?10% and those (like the UK) that had previously used 240 V changed to 230 V +10% ?6%."It is nominal 230V. Equipment breaks, and it is being replaced with standard ones.

author: Zero999
author_rank: Super Contributor
timestamp: October 14, 2013, 11:44:28 am
content: Quote from: NANDBlog on October 14, 2013, 03:48:48 am240V is within tolerance so the actual supply voltage hasn't changed at all.

author: geraldjhg
author_rank: Regular Contributor
timestamp: October 14, 2013, 12:26:33 pm
content: hidont worry about droping a few volts because that is preregulated and it drops and ripplesmuch more from no load to full load, concentrate on some other aspectleds are very good (efficient) nowdays. i am using 220 vac a resistor and an opposing4007 to power a blue led at under 1 ma

author: Neilm
author_rank: Super Contributor
timestamp: October 14, 2013, 05:46:25 pm
content: Quote from: Hero999 on October 14, 2013, 11:44:28 amOf course it is in spec. That is why the tolerance is so big.