a bit too much current & voltage ? - Page 1
Forum section: electronics / beginners

author: ninonpas
author_rank: Contributor
timestamp: March 20, 2012, 01:08:23 pm
content: hello therecan please someone help me with an urgent problemsome friends of mine are musicians in spain and they have a gig tonight but they have lost the transformer of an old SIRIUS synthetizer that uses DC 10.5volts at 1.5ampsthe only transformer they have disponible right now is a multi voltage 90w computer transformer that can deliver DC 12v at 1.8ampscan I recommend them to try it?could 1.5v and 300ma be too much an excess?they are affraid to burn something because they don't have backup of their programs...I would think they can but I'd like to know some expert opinionthank you very much

author: SoftwareSamurai
author_rank: Regular Contributor
timestamp: March 20, 2012, 01:21:44 pm
content: Admittedly I don't know much about an old Sirius synth, but I wouldnotrecommend using a higher-voltage supply than what's called for, generally speaking.Some electronic circuits aren't designed with protection against over-voltage for DC-in.The extra voltage could easily (and possibly instantly) fry sensitive chips and e-caps. Or worse, it may work for a while, but end up cooking the chips via excessive heat due to their inability to thermally dissipate the extra watts going through them.The extra current supply doesn't matter. Electronic circuits will use whatever current they need, not more (unless there's a short somewhere).

author: ninonpas
author_rank: Contributor
timestamp: March 20, 2012, 02:42:02 pm
content: i seea bit risky theni'll tell them sothank you very much

author: ninonpas
author_rank: Contributor
timestamp: March 20, 2012, 05:15:12 pm
content: i've calculated a little voltage divider to atenuate the 12v down to 10.5vi hope i've choosen adecuate resistor valuessorry to be such a newby but even ohms laws is not evident for me yet...could someone please correct me if i'm wrong in assuming this would work and that I will not burn my resistors?thanks

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 20, 2012, 05:34:06 pm
content: Quote from: ninonpas on March 20, 2012, 05:15:12 pmThe voltage divider won't work. Its impedance will be much too high.What you can do is use two rectifier diodes in series to drop the voltage. This will drop the voltage by 1.4V to 2V. Use diodes rated for at least 2 amps.Something like the 1N5400 to 1N5408 should work. You should be able to pick these up at your local electronics shop. Radio Shack sells the 1N5402, for example.

author: Unknown
author_rank: Unknown
timestamp: March 20, 2012, 05:47:11 pm
content: Quote from: ninonpas on March 20, 2012, 05:15:12 pmTry to estimate the equivalent resistance of the synth from Ohm's law (you know the voltage and current), put that resistance in parallel with that 750k resistor, and calculate voltage across the synth. Then change the values of the resistor to reduce the loading effect. Now calculate the quiescent dissipation in the resistors. This might give you an idea why resistive dividers are rarely, if ever, used for these applications.

author: ninonpas
author_rank: Contributor
timestamp: March 20, 2012, 05:53:36 pm
content: oh, i see... voltages dividers only for few mili amperes...by now I'll do the diodes thing to solve the urgent problemand I will then study VERY carefully about the quiescent dissipation as you say and try to hopefully get the ohm's law sculpted in my cerebelum once and for all !but at least i had the right tongue angle which i learned from daves wonderfull videos !thank you SO VERY MUCH !

author: DavidDLC
author_rank: Frequent Contributor
timestamp: March 20, 2012, 07:44:50 pm
content: Resistor voltage dividers will work for passive loads, if the load is dynamic you need to use something else, like the diode solution given before.David.

author: Unknown
author_rank: Unknown
timestamp: March 20, 2012, 08:02:11 pm
content: It's not so much dynamic loads, but the fact that the impedance of the voltage divider needs to be low compared to the load. Load regulation will be OK as long as the load impedance remains high compared to the voltage divider.

author: Zeta
author_rank: Contributor
timestamp: March 20, 2012, 08:10:36 pm
content: if its old equipment then it most probably will work ok with voltages in the range 11-9.5 (probably up to 11.5) so droping the voltage with a diode or 2 (one on +12 and the other on GND) is the most practical thing to do.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: March 20, 2012, 08:35:17 pm
content: Quote from: alm on March 20, 2012, 08:02:11 pmIf the current variation is low you can deal with a resistor voltage divider, if the current variation is high then you need to use other methods, like additional voltage regulator, diodes, etc.David.

author: amyk
author_rank: Super Contributor
timestamp: March 21, 2012, 09:27:18 am
content: Quote from: SoftwareSamurai on March 20, 2012, 01:21:44 pm10.5V doesn't look like any common logic family supply voltage, so I think internally there would be a regulator (linear?) to drop it to probably 5V. If there are any electrolytic capacitors across the input voltage they would probably be rated for at least 16V given that 10V is too low and the next highest common rating is 16V, so 12V won't hurt them. The only concern is if there is a linear regulator, which may run a little hotter having to dissipate the extra voltage as heat.The only way to be sure is to open it up and check.(There is a similar situation about laptop power inputs, they actually have a wide acceptable input voltage range usually limited only by the ratings of the input caps -- usually 25V with an adapter in the 16-20V range, and 16V with an adapter in the 9-12V range -- and since they all use switching regulators there isn't the problem of excess heat dissipation when run at higher input voltages.)

author: Psi
author_rank: Super Contributor
timestamp: March 21, 2012, 09:35:57 am
content: heheThis is one of those situations were...Most of us would just connect that up (maybe after a quick look inside) but we're not really willing to tell others to do the same.We're all for talking a little risk, it's a critical part of electronicsbut telling others to do it is a little different.Also being an "old" synthetizer it could be quite valuable/rare

author: SoftwareSamurai
author_rank: Regular Contributor
timestamp: March 21, 2012, 04:20:36 pm
content: Yes, exactly! I'm not about to say "Sure! Go ahead and plug it in! Nothing to lose!" and then get blamed for saying it's okay when their old equipment blows up and kills someone. (And yes, overly dramatic there, but still...)

author: codeboy2k
author_rank: Super Contributor
timestamp: March 21, 2012, 04:56:32 pm
content: Quote from: amyk on March 21, 2012, 09:27:18 amYou are probably right, amyk. But there are two many unknowns, without looking inside. it's such a small current, less than 2A, that the 2-diode drop is the most prudent and safest solution here.

author: amyk
author_rank: Super Contributor
timestamp: March 22, 2012, 11:07:53 am
content: Right, that's why I said you can only be sure by looking inside.With 2 diodes each one could still be dissipating a little over 1W so you will need to find ones that can handle that power. I agree it's definitely safer, and killing a diode isn't a big deal in comparison.

author: T4P
author_rank: Super Contributor
timestamp: March 22, 2012, 12:53:01 pm
content: At 1.5 amps , it's nothing the 1N539X can't handle .Obviously even the 1N400X can do this also .

author: Unknown
author_rank: Unknown
timestamp: March 22, 2012, 04:48:54 pm
content: The 1N400x is rated for 1 A of average current. I would not use them in a 1.5 A application. 1N543x or 1N54xx should be fine.

author: T4P
author_rank: Super Contributor
timestamp: March 22, 2012, 06:03:24 pm
content: Quote from: alm on March 22, 2012, 04:48:54 pmOf course not . but the 1N539X is fine for 1.5amps .I would only use the 1N543X for 1.5amps and above .

author: Unknown
author_rank: Unknown
timestamp: March 22, 2012, 06:44:49 pm
content: You're right about the 1N539x. I meant 1N539x, not 1N543x. Advantage of the 1N540x is that it's an extremely common part, just like the 1N400x and 1N4148, so availability might be better.

author: T4P
author_rank: Super Contributor
timestamp: March 22, 2012, 07:48:52 pm
content: Quote from: alm on March 22, 2012, 06:44:49 pmWho knows , i find 1N539x all over in decent german wall warts .Because i only opened up german wall warts