0
\$\begingroup\$

I built a driver for a pulsed laser. This required a low voltage DC source to provide a bias current through the laser and also a high voltage DC source to drive an avalanche transistor to make the pulses for the laser. The original design (which works fine) uses a 3 Volt AC to DC wall transformer plus a voltage regulator for the bias current and a separate 15 Volt AC to AC wall transformer plus a voltage multiplier circuit to get the ~65 Volts for the transistor.

I'm trying to simplify the design so that I can power the whole thing with just one wall transformer. I chose a 9 volt AC to AC transformer. I connected the output of the transformer to a 5-stage cascade multiplier circuit to get the high voltage and I also connected the transformer output to a bridge rectifier, smoothing capacitor and voltage regulator for the low voltage laser bias current.

Here's the problem (I think): One input terminal of the cascade multiplier also serves as the negative terminal of the voltage multiplier (i.e. ground). That means that the negative output of the bridge rectifier (ground) is also connected to one of the bridge's AC inputs. I'm a high school physics teacher, not an EE, but I'm pretty sure this is bad. The rectifier gets hot and I'm not getting the high voltage I need from the multiplier.

What would be a better way to do get both the low voltage (6 Volts 25 mA is plenty) and the high voltage (65 Volts at very low current) from a single wall transformer?

\$\endgroup\$
  • \$\begingroup\$ It sounds like a bit of a messy design. Have you seen Jim Williams's simple solution for an avalanche pulse generator, as Appendix D of Linear Tech's AN47: high-speed amplifier techniques applications note? Using a switch-mode converter and a single-stage voltage multiplier, one can get up to 90V (adjustable) from 3V or less. The same type of switcher can be used as a low-voltage current source. You don't have to use the LT1073--that just lets you run the whole thing from an AA battery if you want. \$\endgroup\$ – Oleksandr R. Feb 13 '16 at 16:42
  • \$\begingroup\$ I'll consider this for the next one I build. Currently my design uses options discussed in this article: elexp.com/Images/Speed_of_light_with_650nm_diode_laser.pdf \$\endgroup\$ – Spiff Feb 15 '16 at 18:14
0
\$\begingroup\$

If you use a single-diode rectifier (diode on Line, no diode on Neu/Gnd) for your low-voltage bias supply you'll have to do a bit more smoothing on it, but you would then retain 'true' grounding & should reduce/eliminate the nuisance heating of your bridge rect. while (assuming no other errors exist) fixing your HV output.

\$\endgroup\$
  • \$\begingroup\$ Simplest possible solution: de-soldered the bridge and replaced it with a single diode. Problem solved. \$\endgroup\$ – Spiff Feb 13 '16 at 4:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.